Doing it Right

[BrettS]

I am guessing no one did the BMEP for the XS1100 motor? I got 9.1 which seems very respectable. It seems to me that a 1200 kit with some extra compression and a mild cam and have it put together well with blueprinting balancing would make a decently strong  and reliable motor.
Sounds very similar to at least one members bike here. I have seen some of the motor mods listed elsewhere (often have trouble finding) but if anyone wants to chime in with their experience improving the XS motor that would be great.
I know they are a pretty neat package and one thing I am guessing we would not muck around with is the head chamber as I hear that was done very well?.

[BrettS]

It's getting past what I have read so Might post some more of his ramblings later. There is also some discussion from others that may be helpful to others in the original thread.
It seems the Guy that wrote all this is Matt (Sonreir) Teazer and Kop also have a lot to say so if interested make sure to browse the original thread as well
HERE

[BrettS]

One other thing to keep in mind about domed pistons...  There is the definite possibility of having "too much of a good thing".

Looking back to the domed piston pic midway through my previous post, you can see how big of a lump that thing is.  Weight considerations aside, what may not be immediately apparent is how that can affect the filling and purging of the cylinder (though the former is definitely more of a concern than the latter).

With large domes, you're not only slowing down the propagation of the flame front (and the entire combustion event, for that matter), you're also disrupting the air flow into and within your cylinder.  At low lift (near TDC), the piston will almost completely block the air flow coming into cylinder and these low lift events can be crucial.  Filling your cylinder with fresh fuel/air mix is a race and many races can come down to the kind of start you achieve.  What you've gained in compression, through high doming, you can easily lose in volumetric efficiency (because of slowed intake velocities) and reduced swirl (though the last item isn't applicable to engines with only a single intake valve and usually isn't application to vintage engines as a whole).

The ideal piston shape for a performance engine is one that is relatively flat, but as most of our engines have hemispherical chambers, domed pistons are a necessary evil.  It's usually OK to chase a bit of compression through domed pistons, but don't go too crazy.  A big lumpy piston may look cool, but you have to take many things into consideration.

[BrettS]

Applications - Compression

As I mentioned earlier, compression is one of those areas in which you can't really go wrong.  More compression increases the efficiency of your engine and provides a boost in torque, and hence power, throughout the entire RPM band.  For this post, unlike my previous post on compression, I'm going to talk a little bit more about the application and a bit of a "how to" rather than stick mostly with the theory.

The ways in which compression help are many.  The primary reason is that an increase in compression increases combustion temperatures.  Because your engine is basically a heat pump, more heat means more power.  Another important aspect is an increase in expansion ratio.  In order for your engine to be able to reclaim heat energy as mechanical energy, you must have a favorable expansion ratio.  Finally, increasing compression ratio also tends to increase combustion speed.  This increase in combustion speed allows your engine to hit higher peak pressures, sooner, and so reduces the amount of energy that is bled into the walls of the combustion chamber (more on this detail in a future post).  A good rule of thumb is that each full point of compression ratio increase will yield between a 3% and 4% increase in torque.  Initial gains will prove to be better than gains higher in the scale.  So this means going from 8:1 to 9:1 will be better than going from 11:1 to 12:1.

Let's examine a hypothetical engine configuration.  For my examples, we'll assume these engine characteristics:

Bore - 67mm
Stroke - 50.6mm
Total Gasket Thickness (Both Head and Base) - 2mm
Combustion Chamber Volume - 26cc
Piston Dome Volume - 10cc
Piston Deck Clearance - 0.5mm (Most pistons do not come all the way up in the cylinder sleeves.  This is the amount left over at TDC).

These numbers get us a static compression ratio of 8.2:1.  In the following topics, I'll be "massaging" these numbers to illustrate how changes we could make represent a real increase in compression ratio.

There are several ways in which higher compression is usually achieved and a number of significant things to keep in mind when chasing bigger numbers.  I'll deal with these five sub-topics in order.

Compression Method #1 - Shortening the Stack
This is probably the easiest method of chasing a modest compression increase.  The idea, here, is to reduce the length of the cylinders (or head) while maintaining the length of the stroke.  There are a number of approaches to implementing this method and it's possible to use all or just one of them.

The first, and probably easiest, option is to run thinner gaskets.  For most engines, it's possible to forgo a base gasket entirely, and simply use a sealant such as Threebond.  A slightly thinner gasket can have a much greater effect than one would think.  Going back to our example, lets say we skip the base gasket entirely and our total gasket thickness is reduced from 2mm down to 1mm.  Our static compression ratio is increased from 8.2:1 to 9.4:1.  Not bad at all.

The second option is to have some metal milled off of the cylinders and/or head.  This process is commonly known as "decking the head".  It's slightly more expensive, but it is slightly more efficient and comes with some other benefits as well.  The basic concept is that the same goal is accomplished, but you get a few extras thrown in for free.  For example, removing metal from the head will reduce the combustion chamber volume, but it also give you an opportunity to have the head surface reconditioned, providing a better sealing surface.  Likewise, removing some metal from the top of the cylinder jugs can produce the same effect, but it also has the added benefit of reducing the piston deck clearance.  Assuming we take a combination of the two options and remove .020" from both the cylinder jugs and the head, we end up with a static compression ratio of 9.4:1.  This is the same as our first example, but we've also managed to clean up the sealing surfaces.

This is a bit of a corollary, but with both this example and the previous, the piston was effectively raised up within the cylinder.  This tends to increase the turbulence within the cylinder during the compression stroke and helps to keep the fuel evenly distributed within fuel/air mix.  It also helps to keep the fuel droplets smaller, leading to faster combustion.  These things are HIGHLY desirable in a "built" motor and I'll talk a bit more about this in a future post.

Milling metal from just the head or using a thinner head gasket (as opposed to base gasket) will not usually produce this effect and so those two options can be considered less desirable in some builds.

Compression Method #2 - High Compression Pistons
The next most common option for increasing compression is to replace the stock pistons with an aftermarket set.  Almost all aftermarket pistons will have at least a modest increase in compression ratio.  The most common way in which pistons increase the compression ratio is through an increase in the dome height.  Basically speaking, the piston now takes up more room with the combustion chamber.  It's not uncommon for some piston domes to be quite large.  In the following pics, you can see a stock CB450 piston as compared with a high compression piston for the same engine.

 

Going back to our example, using a piston with an increased dome volume of 4cc will result in an increase in compression ratio to 9.6:1.  Replacing pistons in order to increase compression is quite good because it also allows you an opportunity to increase the bore diameter and get yourself a nice boost in displacement at the same time.  It's uncommon to see high compression pistons that are not also larger in diameter than the OEM part and when it does occur it's usually due to displacement restrictions in racing classes.

With pistons, it's also possible to increase compression by lowering with wrist pin location.  More often than not, you'll usually find that pistons with a wrist pin location change opt for higher pins, though.  This almost always indicates that these pistons are for use in engines with increased stroke.  The reason for this is because increasing the stroke also increases compression unless something is done.  An increase in stroke worth chasing will almost always result in too much compression with a stock configuration and so part of increasing the stroke length is finding a way to then decrease compression.

Compression Method #3 - Reducing Combustion Chamber Size
The final way to increase compression is to decrease the size of the combustion chamber through the addition of metal.  This is expensive and has more ways it can go wrong than it can go right, and so I don't recommend it to just anybody.  There are usually some pretty good gains gains to be made here (though not strictly in compression), but it's not an easy undertaking and most folks, myself included, omit it unless they're chasing a specific goal.  Furthermore, its usually very difficult to make significant changes to the combustion chamber unless you have special pistons, anyway, and so my recommendation would be to stick with options #1 and #2 unless you know a good deal about (or don't mind learning the expensive way) what constitutes good chamber design for your application.

Simply because I provided a mathematical example for the other sections, I will do so here, as well.  Furthermore, because changes in combustion chamber volume usually involve using differently shaped piston domes, I'll alter both of those values for this example.  Assume you're able to fill in the sides of the combustion chamber and change your chamber shape to a rough oval, instead of the hemisphere seen on a lot of modern bikes.  This will possibly translate to a new combustion chamber volume of 18cc.  The domes on the pistons will need to be altered as well, and so they're going to be reduced from 12cc down to 8cc (higher, but thinner).  Our static compression ratio is now sitting at a healthy 10.5:1.

Caveat #1 - Clearances
First and foremost, when you reduce the area available in the combustion chamber (which is precisely what increasing compression ratio does), you increase the chance of clearance issues.  Many parts are now closer together than they previously were.  Depending on the route you've taken, some areas will be more likely than others, but all should be checked.  The most commonly affected areas are the clearances between the piston and each of the valves and between the piston and the head.  It's also a good idea to check and make sure your gaskets aren't overhanging into the combustion chamber as well.  This is possible if you've gone with aftermarket gasket options and/or a larger bore.

The common methods for checking these clearances is with modeling clay.  Before assembling the engine, spray the interior of the combustion chamber and the top of the pistons with WD40 and lay down a thin layer of clay.  If your gaskets are the compressible type, you'll want to have several of these on hand, you're about to go through at least one of them.

After getting the clay into place, reassemble the engine and torque the head down to spec, set valve timing and clearances... all that jazz.  Now, SLOWLY rotate the engine in the direction of its normal operation.  DO NOT force it if you feel it binding.  It's quite possible to bend steel components with a lot less force than you'd think would be necessary.  After completing two full rotations of the engine (or if you felt something bind), remove the head from the engine and inspect the clay.

It should end up looking something like this:



This pic was taken during the reassembly of my own engine, for the first time.  At the time, all the clearances checked out OK.  You can just see through the clay on the left side of the piston and this was due to a gasket overhang.  On the right side, you can see the clay was broken, but I suspected this was due to the clay folding over when it was pulled back up by the exhaust valve.  I repeated the check to verify that this was the case.  Unlike in the above pic, it's also a very good idea to put some clay on the sides of the pistons to check the clearance between piston and head.

Now, providing everything looks good, it's time to actually measure.  It's possible to measure the clay if you have a steady and gentle hand, but you can also buy special wax strips (can't remember what they're called) from most auto parts stores.  Simply put the wax down in the same way as you've done with the clay.  Assemble the engine, turn it a few times, and then disassemble again.  Time to break out the micrometer.  Though each engine is different in the tolerances it will allow, the clearance between head and piston shouldn't be less than .020".  The clearance between intake valve and piston should exceed .040" and the clearance between the piston and the exhaust valve should be at least .080".  Tighter clearances are possible, but I don't recommend it unless you've done this a few times already.  When clearances get tight here, they're a lot less forgiving elsewhere, too.  Adjust your tappets without enough spacing and you're just trashed your pistons.  Brilliant.  Skip a tooth the cam gear and now you need new valves.  Sweet.

Caveat #2 - Detonation and Preignition
One of the effects of additional compression is additional heat.  This heat is not only what provides the increase in power, but it can also cause two other issues.  These issues are detonation and preignition.  Both of these problems will destroy a motor in short order (especially preignition) and so neither are acceptable.

Detonation is the spontaneous combustion of the remaining fuel/air mix after the normal combustion process is nearing completion.  This is caused through the heat and pressure initiated during the combustion process and as both heat and pressure rise, it will get to a point that the molecules within the mix are pounding into one another so violently that they ignite, themselves.  Death by detonation usually results in broken rings or ring lands.

Preignition differs from detonation in that it's not so much as a spontaneous combustion of the mixture.  Preignition is a begin to the combustion prior to ignition from the spark plug.  Preignition usually occurs when a part or parts of the combustion chamber heat up too much.  This can be anything from an excess of carbon deposits (not usually an issue on a freshly assembled engine), damage to the exhaust valve, or an overheated spark plug.  What happens in this case is that whatever causes the preignition has heated up to a point where it actually starts the combustion of the fuel/air mix before the spark plug fires.  This causes cylinder pressures to rise too early (sometimes when the piston is still approaching TDC) and so peak cylinder pressures occur too early in the cycle.  This causes greatly increased stress on engine components and will usually kill an engine a lot earlier than detonation will.  Engine failure due to preignition will almost always result from holes in a piston.

Though there are many ways to combat detonation and preignition, those will be saved for a later post.  For now, just be aware they can be potential problems and the most common method of dealing with these issues is to use high octane fuel.  Consider premium gas to be the only acceptable fuel for a high-compression engine.  Better to push the bike home than fill it with regular.  Also, it's important to note that high compression engines are MUCH LESS tolerant to running lean than the stock factory offering.  Most stock engines will run all day long on a lean mix, but a high compression engine at WOT will blow up right in your face as soon as the float bowls start to get even a little shallow.

Caveat #3 - Combustion Speeds and Timing
This particular issue can be hit or miss depending on how you've achieved your increase in compression, but it's unlikely you'll be able to avoid these effects all together.

First up, it should be known that increasing compression increases the combustion speed of the mixture within the cylinder.  This is generally considered a good thing.  Furthermore, increasing piston dome volume will generally reduce the combustion speed (especially at low RPM).  In theory, this results in a need to change the ignition timing of your engine.  In practice, no change may be necessary or you may not have the data available to make a change that is beneficial.

So what is it we're chasing when we change the ignition timing?  We're changing the point at which Peak Cylinder Pressures (PCPs) are attained.  In four strokes, this PCP should occur at 14° ATDC.  If ignition comes on too early, then PCP occurs too close to TDC.  This places engine components under undue stress and robs the engine of power.  If ignition comes on too late then less power is generated because the expansion of the gases, due to heat, are no longer allowed to follow the sinusoidal pattern allowed by a healthy expansion ratio.  In plain English, the volume of the cylinder (due to the descending piston) is expanding more than is desirable when compared to the expansion of the combustion gases and so pressures never build to what would be otherwise possible.

So what does it come down to, in practice?  Running high-compression pistons will likely lead to a slight advancement of the ignition timing in the lower RPM ranges.  Without increasing the ignition advance, the bike may stutter when blipping the throttle, despite having an appropriate fuel mixture ratio.  Instead of initial timing at 14° BTDC, you may find that 18° provides the throttle response you're after.

As for the increased combustion speed, this is something that many folks just choose to accept and deal with.  In reality, the increased combustion speed that is achieved with a couple of points of compression is not something that needs to be dialed out.  The perfectionist or a person chasing a maximum effort engine will likely pursue some dyno time at this point.  Ideal total timing can be achieved through the measuring of exhaust gas temperatures, and in some cases, cylinder pressures.  For the garage builder and home enthusiast, slightly advanced timing usually results only in a concern for the increased heat and this is usually handled through increased octane or manipulation of the thermostat (in liquid cooled operations).

Now aside from ignition timing, valve timing may also be affected by a compression change.  Shortening the stack in any way (thinner gaskets, decked head, etc) will retard the cam timing.  This causes all of the valve events to happen later.  At the very least, this causes a drop in peak horsepower as all of the valve events rob your engine of a bit of volumetric efficiency.  At lower RPMs, there may be a slight increase in torque.  In more serious situations, this can cause clearance issues as the exhaust valve is now closing later in the rotational cycle and so it is being held open too long while the piston is approaching TDC.

You'll not likely need to correct this issue unless you've taken more than .020" from the total height of the stack (or if you're making other modifications to the engine).  In order to fix this problem, you're going to need a degree wheel, timing information about your camshaft, and a few other basic machinists tools.  The idea is to assemble everything according to spec and then use the degree wheel to determine how far our the valve timing is.  You then either slot the existing cam gear or buy an aftermarket cam gear in order to correct the timing.  By mounting the cam gear at a different rotation than what is called for by the OEM part, your valve timing stays where it is meant to be.

General Considerations and Extra Info
Not much rhyme or reason to this last part, but just a few extra thoughts on compression.
•High compression engines are usually easier to start from a thermodynamic standpoint, but are not as easy to kick over
•Aluminum heads and/or liquid cooling will usually withstand one full compression point higher than air cooled or iron headed engines can endure
•Undersquare engines can handle more compression than oversquare
•A good rule of thumb for an engine running on pump gas (premium, of course) is not to exceed 200 PSI on a compression tester
•High compression engines have an AWESOME feel.  They want to run and rev and are much more responsive to changes in throttle position.
•If you plan on running a hot cam, an increase in compression will almost always be required.  This helps to gain some of the bottom end power back
•If a change in gasket is required, copper is the most commonly used material.  It's a bitch to seal against aluminum though, so skimming a bit of metal from the sealing surfaces is highly recommended
•If you've gone too far on your compression it is entirely possible to then use thicker gaskets to help lower the compression a little bit.  Retarding the ignition timing and running rich on the fueling are also options for combating too much compression, but neither are very desirable.  Thicker gaskets can help when clearances become to tight, as well.

Conclusion
When it comes down to it, high compression is probably one of the defining characteristics of a "built" motor.  In moderate cases, it can be completed by folks who know very little about engines and I recommend this alteration to anyone who plans on taking their engine apart.  In its simplest form, just replacing some gaskets with thinner material is all that is necessary in order to achieve a bit of a compression increase.  Yes, it can get complicated fast, but doesn't everything?

[BrettS]

Applications - Camshaft

OK... lets talk a little less about theory and a little more about application.  We won't be going completely away from the academics, but we will be talking a little bit more about specific parts and changes and how they affect the running of your engine.

I want to talk about camshafts because this is probably the first performance modification made after an increase in displacement and compression has already been completed.  Hopefully I explained displacement and compression in a satisfactory manner in my earlier posts but just to sum it up again:

Displacement - Bigger engine = more power.  No brainer.
Compression - More compression = more power.  Increase in compression causes an increase in heat.  This heat is what makes more power, but too much of an increase in heat and your engine explodes.

The nice thing about displacement and compression is that they benefit your engine no matter the speed at which it operates.  This is often the reason they are among the first modifications done.  Most of the other modifications you make to your engine will be a trade-off of some sort and the camshaft is no exception.

To understand why selecting a camshaft is a compromise, we need to look at it in more detail.  To do that, here are a few definitions:
Lift - This is how far the valves are held open
Duration - This is how long the valves are held open
Intake Velocity - This is how fast the air, coming into the engine, is traveling

Also important for understanding these concepts is a basic knowledge of fluid dynamics (yes, air is a fluid).  Understand that air has mass (and therefore inertia) and that air is compressible.  Now regardless of engine speed, our goal is to have the cylinder fully filled with fresh air and fuel, when the throttle is fully open.  If you recall from my previous post, this is called Volumetric Efficiency (VE) and we always want 100% or better, though this is rarely possible.

For our initial example, lets examine an engine turning very slowly.  Lets say 1 RPM for argument's sake.  In order to achieve our maximum VE, the intake valve should open very near TDC and then close very near BDC.  This will give us a duration of 180° because the crankshaft has turned 180° during the time in which the intake valve was open.  The reason this level of duration is effective is because the air is able to enter through the intake valve at a rate which matches the descent of the piston.  It would take the piston a full thirty seconds to descend from TDC to BDC and so it is highly unlikely that the pressure inside the cylinder would differ from atmospheric pressure at any point.

When we speed things up, however, the situation changes.  If we increase the engine speed to 6000 RPM, the piston now descends from TDC to BDC in .01 seconds.  That allows precious little time for the air to pass through a relatively small intake opening and into the cylinder.  In order to maximize that time, it is beneficial to open the intake sooner and then close it later.  Not only does this allow for more time in cylinder filling, it is assumed that an increase in engine speed also translates into an increase in intake velocity.  Faster moving air is able to pack itself into the cylinder in less time and so this helps as well, provided we help it.

Opening the intake earlier allows air within the intake to start entering the cylinder earlier and this builds momentum within the intake tract.  By holding the intake valve open longer, we utilize this momentum to compress the mixture further.  Fast moving air will more readily act against the pressure created by the rising piston.  So even though the cylinder may be at 80% of it's total volume due to the rising piston, the fast-moving intake charge is still filling it.  If we were to maintain opening the intake valve at TDC and closing the valve at BDC, we would be losing quite a bit of VE because it takes time to build momentum in the intake and the piston may already be halfway down the cylinder by the time this occurs.  The cylinder would then still be at less than atmospheric pressure when the intake valve closes at BDC.  On the other side of the coin, if we were to close the intake valve well past BDC in our first, slower, example, then the piston has already pushed much of the fresh intake mixture back out of the intake port (this is called reversion).  An ideal opening and closing time exists for a given intake velocity.  Conversely, changing the intake velocity will change the ideal opening and closing times of the valves.

For the exhaust valve, the concepts are similar, but there are some differences due to the desire for the gases to flow out of the cylinder rather than into it.  Furthermore, the gases are usually much higher pressure than atmospheric and so that changes the dynamic as well.  Just like the intake, a performance cam will usually opt to open the exhaust valve sooner and close it later.  Also, just like the intake, this creates a situation where the engine produces less torque at lower RPMs.  By opening the exhaust valve earlier, we are in effect bleeding off the still-expanding gases from the cylinder before they've fully acted upon the piston.  This quite literally sends power out the exhaust port, but there is a good reason for doing so.  Again, the purpose is to build momentum in the gases.  Having a higher cylinder pressure when the exhaust valve opens creates a faster flow of gas, sooner.  The faster the gases move, the higher their momentum.  This leaves a low-pressure zone in their wake and helps to scavenge exhaust gases from the cylinder prior to the intake stroke.  In some extreme cases, it can help alleviate pumping loses, because the low pressure actually pulls up on the ascending piston.  If the intake valve opens while the exhaust valve is still open (this is called overlap) this low-pressure can also help to start the intake mixture moving into the cylinder.

If it wasn't clear by now, the lobes of the camshaft are what control the valves.  The lobes (in conjunction with the rocker arms) are what determine the amount of lift and the duration.  Lift and duration are roughly interchangeable.  That is, increasing the lift and decreasing the duration will result in roughly the same VE.  Usually, both lift and duration are increasing on a performance camshaft.  Certain engine configurations will favor lift over duration and vice versa.  A couple of articles I've read seem to be placing more emphasis on lift for high-revving motorcycle engines, but your mileage may vary.  As a corollary, increasing lift will place more stress upon the valve train and increase duration will decrease that stress.  Furthermore, increased lift will almost always require upgraded valve springs, which increase stress even further.

As I mentioned in a previous post, peak VE is usually where peak torque occurs.  By moving peak torque further up in the RPM range, we increase the maximum horsepower of the engine.  By moving the torque further down in the RPM range, we decrease the total horsepower of the engine.  Keeping the torque low in the RPM band isn't such a bad thing, though.  Bikes with low-range torque are easier to ride and respond well to throttle over a larger RPM band.  When it comes time to chase ponies, we usually opt for more power, later in the RPM range.

It would be nice to have the best of both worlds; a low duration cam for low RPMs and a high duration cam for high RPM operations, but unfortunately, most of us will need to choose a cam with set duration.  It will only work best within a narrow RPM range of around 1000-1500 RPMs.  Whether that best range is from 3000-4000 RPM or from 6500-8000 RPM is going to be dependent upon our desires for the bike and this is precisely what we need to keep in mind when making our selection.

It's a common rookie mistake to choose the lumpiest cam with the biggest numbers.  This is almost never the most desirable option.  Yes, it will theoretically create the most horsepower, but you do so at a greatly reduced level of torque early in the RPM range.  This means that you need to slip the clutch like a madman just to get going from a stop and your engine needs to operate in the top third to a quarter of the RPM band.  For a Honda 350, this means keeping your revs about 7000 RPM pretty much all of the time.  If the revs drop below that point, then power drops off VERY quickly.  Choosing too big of a cam is called overcamming an engine and it's surprising easy to do.  Unless your plan is to run the salt flats or race an oval track, stay away from cams that are too big.  For short courses with more turns, cams with less duration are beneficial because they provide more torque down low.  A quick question to ask yourself, "Do I prefer accelerating through the twisties or running flat out?".  If your answer is the former, keep to a milder cam.  If speed and power are your ultimate goal (and you have a course where you can utilize those things) a hotter cam will usually be more beneficial.  One more caveat to big cams:  It is entirely possible to move your peak torque so high up the RPM band that you never hit it.  Your cam never reaches its potential and so you've actually robber yourself of power.  This can occur either by moving the peak VE past your redline or by choosing a cam that is so lumpy that your engine doesn't have enough torque to continue to rev up to the ideal range of operations in the higher gears.

So what is too big?  That will depend on your engine.  A duration of 270° is a hell of a lot for some engines or it might be a mild performance upgrade for others.  If your budget allows, I suggest trying new cams one at a time.  Go for small increases to begin with and decide whether or not it's giving you what you desire.  Going for the biggest cam right out of the box is a sure path to disappointment.  Getting the right cam on your bike is going to give you one huge grin, however.

For my own 360, I've opted for a cam that provides .040" additional lift and an extra 30° duration (absolute lift and duration of .341" and 251°, respectively).  It's a little slow off the line and hill starts are not fun at all, but if I keep the revs above 4000, it screams.  The gear ratios are pretty good on a 360 as well, so this helps and is also something to consider with your own build.

[BrettS]

Power Goal #4 - Brake Mean Effective Pressure

OK... I've saved the best for last.  To me, this is the most interesting portion of building power, but it's also one of the most complicated.  I'll be dedicating more than one post to this topic, because there is a lot to cover, here.  The majority of engine upgrades that take place, do so in order to effect cylinder pressures.  Because so many different types of upgrades exist, it's too much to cover in one post.  To that end, this post will be largely dealing with more theory, but following posts will start to talk a lot more about application.

So far, the other three topic we've covered have been relatively simple, but here's a quick recap just to make sure the point made it across:
•Displacement - A bigger engine creates more power than a smaller one.  Add cubic centimeters to your displacement by increasing the stoke, bore, or both.
•RPMs - Power is force over time and so power is derived from torque.  Spinning the engine faster means more power so long as you can keep the torque up and the losses down.  Probably a losing game in the long run, but it can be fun to chase for revs.
•Reduce Parasitic Losses - Make it lighter, make it smoother, make it balanced, make it slippery.  All things take energy to move and so keep the energy losses to a minimum.

So... what is Brake Mean Effective Pressure?  It's the average pressure within the cylinder during one cycle of the engine.  Like power, BMEP is a calculated value.  However, unlike torque or power, this number is unaffected by the displacement of the engine and so it can be a useful metric in calculating the relative ability of an engine to do work.  This means it is entire possible for a CB160 to put out a higher BMEP value than a CBX.  It's a useful number because it illustrates how well any given engine is working.  A higher number means the engine is producing more torque for its size than another engine.  BMEP is the real yard stick by which engines are measured.

To get BMEP, simply multiply the peak horsepower of an engine (in Kilowatts, as measured at the crank) by 1200 and then divide that total by the product of the engine's displacement (in liters) by the RPM at which peak horsepower is achieved.

For a 1973 Honda CB350 the formula looks like this:

BMEP = (26.85 * 1200) / (.325 * 10500) = 9.44 bar

A fairly respectable number.  Most naturally aspirated, four stroke, gasoline engines will run between 8.5 and 10 bar.  Anything above this is impressive.  Turbo engines usually run in the 14 bar range.

Before we go too much further, though, it's important to have an understanding of how cylinder pressures are created.  Contrary to popular belief, the ignition of the fuel/air mix does not create an explosion, nor does it create additional gases that fill up the cylinder.  What burning gasoline does is to create heat.  This heat is the sole contributor to cylinder pressures.  If you were to cool down the exhaust gases to room temperature, they would take up only slightly more volume than the air that went into the cylinder before ignition.

The reason for this can be explained using the Ideal Gas Law.  The IGL states that all gases will behave in a very similar fashion given the same conditions.  The part with which we are concerned is the increase in volume of a gas due to the influence of heat.  Basically speaking, all gases expand as they heat up.  The more they heat up, the more they expand.  The ratio of expansion can be calculated in a fairly simple matter.  First, you must know the starting temperature of the gas.  Let's assume 100°F.  Next, you must know the ending temperature of the gas and for this we will assume 1600°F.

We will need to convert these two temperatures to the Rankine scale and so our values now become 559.67 and 2059.67, respectively.  By dividing our upper number by our lower number we know have the expansion rate for the gas.  In this scenario, we have about 3.7.  Finally, multiply this number by 1.07 to account for the conversion of the liquid fuel into a gas and our final value is 3.94.

How about if we lower our starting temperature to 80°F and raise our ending temperature to 1700°F?  We get a final expansion ratio of 4.28.  That's almost a 9% increase in pressure over our original number.  By applying this idea to intake and combustion chambers we can quickly see how reducing intake temperatures along with increasing combustion temperatures can easily generate more pressure and more torque within your engine.

They are many different ways to increase BMEP, but my initial focus will be on two topics (which I consider to be the primary methods).

Compression Ratios
An increase in compression ratio is one of the very best things that can be done with an engine.  It provides an increase in torque throughout the RPM band and increases fuel efficiency as well.  I consider an increase in compression to almost be mandatory for any person looking to trick out their engine.  Furthermore, it is usually very simple to accomplish, at least for modest increases.

To understand why high compression ratios are desirable, it's important to know what they do within your engine.  The primary function of an increase in compression is to increase thermal efficiency.  Thermal efficiency is the measure by which your engine converts heat energy into mechanical energy.  The better this efficiency, the more power you're getting from burning gasoline.  Most of this efficiency comes from what is known as the expansion ratio.  Generally speaking, the higher your compression ratio, the higher your expansion ratio.  This happens because with higher compression ratios, the increase in volume occurs faster as the piston descends.  By the time the piston nears BDC, the pressures within the cylinder will be less than they would otherwise be with a lower expansion ratio.  This means that when the exhaust valve opens and starts bleeding out the excess pressure, there is less excess.  The engine has done a better job of reclaiming that heat-generated pressure into mechanical energy.

Let's illustrate this using two hypothetical cylinders.  Both cylinders have the same displacement, but they differ in their compression ratios.  Cylinder one is running high compression at a 12:1 ratio.  Cylinder two is running low compression, at 6:1.  Assuming a displacement of 200cc per cylinder and peak cylinder pressure of 1,000 PSI.  For cylinder one, we have a combustion chamber volume of 15ccs and cylinder two will have a combustion chamber volume of 36ccs.  Following through, we can see that as the piston descends, the pressure in cylinder one drops more quickly and so more of the energy is being reclaimed, rather than expelled out of the exhaust.  This is clearly illustrated by the spreadsheet listed below:



The other benefit (but also detriment, as described in a bit) is the increase in temperature brought about by the increase in compression.  Going back to our calculations on volume and how it relates to temperature, a greater total difference in temperature creates a greater pressure.  That relates to compression because compressing a gas adds heat into the gas and this will create greater temperatures on the top end of our measurements.  Please see the attached YouTube video as an example.  In this quick video, a piece of tissue paper is placed into a test tube and then a rubber plunger is quickly depressed.  The resulting compression creates enough heat to ignite the tissue paper.  In your cylinder, this increased heat prior to ignition will result in a higher final combustion temperature.


<iframe width="420" height="315" src="//www.youtube.com/embed/ADeYiOYGGYY" frameborder="0" allowfullscreen></iframe>



While high temperatures are what net you power, they also need to be kept in check.  Too much temperature prior to ignition will result in detonation.  This is when the energy in your fuel is released instantaneously (or near enough) as kinetic energy in the form of a shockwave, rather than as an increase in pressure due to temperatures.  Because the energy is released so suddenly, this results huge strain on your engine components and parts do not last long when exposed to this environment.  In maximum effort engines, it's not uncommon for any form of detonation to destroy the entire engine with little or no warning.

To keep detonation at bay, higher octane fuels are generally used.  With combustion chamber design and racing fuel being used, compression ratios of 17:1 are not unheard of.  For the street, 11:1 is quite respectable.

Generally speaking, a full point of compression will net you around a three or four percent increase in torque across the entire RPM band.  The best increases come when compression is already low.  For instance, going from 8:1 to 9:1 will be better than going from 10:1 to 11:1.

In order to increase the compression on your motor, there are several options.  The best option (in my opinion), though also the most expensive, is to go with replacement pistons.  The crown on the piston can be raised to take up more volume in the combustion chamber.  Though a domed piston can slow down the flame front, you also avoid many of the other problems associated with other methods.  The second best option would be to add more metal to the combustion chamber.  Technically, this is generally preferable to domed pistons, but this is a precise and expensive operation; not something generally done by the casual enthusiast.

The cheapest method, that is available to almost all bike owners, is to run without a base gasket and use case sealant (such as Threebond), instead.  This can often add a full point of compression.  This will result in a slight retardation of the cam timing, but this is not usually an issue for most engines.  Always double-check valve timing and clearances prior to running the engine when reducing the distance between the pistons and the head.

Shaving metal from the head and/or cylinder jugs has the same basic effect as running without the base gasket and is often desirable because it's also an excellent time to smooth out the sealing surfaces of your engine.

Volumetric Efficiency
The other great contributor to BMEP is volumetric efficiency, or VE.  This represents the percentage of fuel/air mix that occupies the volume of the cylinder at BDC.  For instance, if you have 100% VE then your cylinder is fully filled with fresh fuel/air mix.  80% VE may indicate that some exhaust gases remain within the cylinder or that the cylinder didn't have a chance to fully fill before the intake valve closes (or even a combination of those factors).  Peak VE of an engine usually corresponds to peak torque.

The goal of all engines built for power is 100%+ VE, under wide open throttle.  Unfortunately, maximizing VE is a difficult task and as engine conditions change, so does VE.

One of the major contributors to affecting VE over the range of an engine's operation is valve timing and valve lift.  As most of you know, valve timing is controlled largely by the camshaft.  As the lobes of the camshaft move the rocker arms, valves are opened and closed in relation to the crankshaft.  When the valves open, how far they open, and when they close is a major contributor to VE because of our old friend inertia.

At slower engine speeds, the inertia of the air flow in and out of the engine is less and so maximum VE is achieved with a later opening of the valves along with an earlier closing.  As the engine speeds increase, valves should be opened earlier and closed later.  The reason for this is two fold.  First, as the crankshaft spins faster, there is less time for intake mixture to enter the cylinder and less time for exhaust gases to leave it.  By opening valves earlier and closing them later, we can allow more time for gases to flow.  Furthermore, the increase in gas inertia, provided by the increase in velocity of the gases within the intake and exhaust, allows for a "stuffing" and "scavenging" effect.

On the intake side, a faster moving gas will compress itself as it enters a closed area.  So as the intake mixture passes through the intake port and begins to fill the cylinder, the fast moving gases coming in from behind will help to push the gases in front and this will create a higher pressure than could be achieved were the intake gases moving more slowly.  The same holds true for the exhaust system, only we're relying on the low pressure wake as the faster exhaust gases leave the cylinder.

However, holding the valves open longer (or open wider) is only beneficial when the gases are moving quickly.  Generally speaking, these gases are only moving quickly when the engine is spinning quickly and so tuning an engine for maximum VE late in the RPM band will result in very poor VE lower in the RPM band.  This occurs because holding these valves open longer allows unburned fuel/air mix to be pushed back out of the intake or to spill into the exhaust without ever being burned.  This process is called reversion and is not a desirable trait.

The amount of time the valves spend open is known as the duration and is generally listed in degrees.  This number represents the number of degrees, over two full rotations (720°) that a valve is held open.  Cylinder filling is also affect by lift, which is how far a valve is opened before it begins to close.  Both lift and duration affect VE and some engines respond better to more lift whereas others will do better with duration.  Generally speaking, both increased lift and duration will result in better VE later in the RPM band, but one of those two values will provide better VE over a longer RPM range, which is definitely desirable.  Remember, our goal is maximum VE at WOT, not just maximum VE at maximum RPM.

Aside from valve timing, the other major consideration for VE is air flow in general.  There is an ideal air speed (for both intake and exhaust) that your engine configuration will enjoy.  This air speed can be adjusted through a number of factors such as the size of your valves, the length and diameter of the intake, and the configuration of the exhaust system.  Other engine modifications will change the ideal air speed.  You can adjust this air speed through a number of methods, but this must be done intelligently and with a goal in mind.  The velocity of the intake and exhaust gases have a very real effect on your engine's performance and making more power is never as simple as cutting of the muffler and slapping on pod filters.

With almost all VE modifications, they will be a trade off.  Increasing your VE toward the top of the RPM band will almost always decrease it toward the bottom, though keeping VE high throughout the RPM range is the ideal scenario.  The trade offs are not always equal, either.  You may gain 10% more power at redline, but lose 30% power at idle.  For maximum effort engines designed for land speed records and oval track racing, it is usually desirable to stack the volumetric efficiency very high in the RPM band.  For road racers and "souped up" street bikes, peak VE should come in about half way (or a little over half way) in the RPM range for a good compromise.

For increases in VE, common modifications include aftermarket camshafts, valves, tuned intakes and exhaust systems, and porting work.  Not all of these are done as a matter of course and the goals for your bike and engine should dictate which are chosen, if any.

[BrettS]

Power Goal #3 - Decreasing Parasitic Losses

The concept behind this goal is very simple, but the means used to achieve it can be exceeding complex and nearly without limit.  I'll cover some of the more common methods as well as the reasons behind them.  Freeing up power through a reduction in parasitic losses is one of the more elegant approaches to the problem of building power and is one of the few areas where gains tend to be exponential as engine speed increases.  Though this is often a long and difficult path, the rewards are many.  Decreased fuel consumption in combination with better acceleration and improved engine response can't be found just anywhere.

First up, it's very important to understand that all mass has inertia and inertia is what causes momentum.  Inertia is an object's resistance to changes in movement.  This includes an object's resistance to acceleration AND deceleration.  The more mass you have, the more inertia you have.  Quite simply, something that weighs a lot is hard to move and hard to stop and this relationship is inversely proportionate.  For every doubling in mass, you halve the acceleration.  Or, if you wish to maintain the same acceleration while doubling the mass, you must double the force.

Know, also, that this applies to EVERY movement on and within your bike.  The weight of the bike and rider both have an effect on acceleration.  If you are able to cut the weight of both yourself and your bike in half, you have just doubled your acceleration.

A bit of a corollary, here, but don't confuse acceleration and top speed.  You won't hit the ton by shedding weight, but your 0-60 times will be a lot better.  In order to hit the ton, you need power and/or streamlining.  It's important to know that acceleration is a function of power VS weight whereas top speed is a function of power VS friction (of the air).  If you want to be faster, you need more power and less drag.  If you want to be quicker, you need more power and less weight.

Anyway... back to the main point.  Every part on your bike moves, takes energy to do it.  The wheels need energy to rotate.  The piston rings need energy to overcome the friction of rubbing on the cylinder walls.  The cam and rockers need energy to be able to compress the valve springs and actuate the valves.  Your transmission needs energy to spin the gears and your chain needs energy to bend each and every single link as the link rounds your sprockets.  The vibration in your handlebars, when you blip the throttle, takes energy.

Now, you may notice from a few examples above, that not all parasitic losses have to do with inertia and so you need to understand how these different losses occur in order to be able to combat them.  As teazer mentioned earlier, this is an excellent starting point for a lot of engine builds and is THE starting point for professional race builders.  Your engine must be capable of getting as much power to the ground as possible, otherwise you're just throwing good money after bad.  You're not going to invest money in a company that is inefficient, why would you invest money in an engine that is inefficient?

Blueprinting
Blueprinting is the process by which most race engines begin their life.  While the concept is relatively simple, the process is tedious, difficult, and expensive.  Most of us don't have the money, skills, and/or equipment to be able to tackle a full engine blueprint, but it's something nice to consider or, at least, be aware of.

The blueprinting process involves disassembly of most or all of the engine components.  In an ideal world, brand new components are used for this process, often unfinished from the factory.  All components are checked for clearance specifications and then adjusted, if necessary.  Reciprocating and rotating components are also checked for balance.  It's important to note that the blueprinting process doesn't usually involve taking an engine's running specifications out to a different measure, but rather making use of the factory specs and just decreasing the tolerances.

For instance, the top ring end gap on a CB360 should be between .15 and .35 millimeters when it comes from the factory.  If I were building a drag racing bike from a 360 motor I may request that the ring end gap be between .150 and .185 millimeters.  A race bike designed to cover 300 road racing miles may specify a ring end gap of .250 and .350 millimeters.  In both cases, the specification falls within the manufacturer's allowances, but the precision is increased and a bias is given depending on the purpose of the engine.  For a drag bike that sees short bursts of power and frequent rebuilds with only few miles, a smaller gap will help with cylinder sealing and provide a bit more power.  For a road race bike that has to compete at high RPMs over a longer distance, the increased gap allows for more thermal expansion without a significant increase in friction on the cylinder walls.

Frictional Losses
Friction is the resistance faced when the surface of one object rubs up against the surface of another.  These two surfaces can be made up of anything that has mass.  So a bike traveling along the road encounters friction from the air as the air contacts all of the surfaces of the bike.  There is also friction from the tires contacting the road surface.  There is even friction from your metal parts rubbing up against oiled surfaces.

Inside your engine, two thirds of the friction will come from the piston assemblies, with the biggest majority of that being the rubbing of the piston rings against the cylinder walls.  The amount of friction from the rings is great enough to the point where many racing engines often run with only with a single ring, in order to keep friction to a minimum.  Unfortunately, this is not an option for most of us.  A single ring will not only reduce compression (forcing you to make it up elsewhere), but it also requires the use of aluminum cylinders with special coatings.  Single-ringed pistons also require frequent rebuilds; something else that most of us don't want to have to do.

There are modifications that can be done to further reduce the friction from the piston assembly, however.  One of the more common methods is to reduce the length of the piston skirts.  This is a great option because it also reduces the weight of the pistons.  Two birds with one stone.  In order to be able to remove metal from the pistons skirts, tight tolerances are needed.  The piston skirts give a mechanical advantage to the cylinder walls when it comes to keeping your piston from rocking back and forth.  You must use tighter tolerances to prevent this rocking motion if the skirts are to be shortened.  It is definitely possible to go too short on the skirts.  The ideal skirt length on any given bike will be different, based not only on the make and model, but the tolerances which are being employed.  Consult the professionals before making any changes.

In addition to skirt changes, is it not terribly uncommon to slightly offset the wrist pin of the piston to reduce side loading on the thrust side.  As I mentioned the previous post on displacement, the pistons are being pushed up against the side of the cylinder walls by the force of the crankshaft resisting the downward motion of the pistons during the power stroke.  By placing the wrist pin further away from the thrust side, it helps to reduce the mechanical advantage of the conn rod against the cylinder wall.  This directly translates into a savings in friction as the two surfaces are no longer being pushed together with the same force. 

Finally, further friction can be saved in the piston assembly by shortening the stroke.  Less distance for the rings and skirts to move against the cylinders means less friction.  This option requires careful consideration, however, as it will decrease displacement and compression, both.  It may also lead to an increased likelihood of detonation.  To help illustrate (side-loading, especially), I jacked this picture off the Internet.  Pay special attention to the upwards pointing arrow coming through the conn rod.  This is the force that is pushing your piston against the cylinder wall.

The next largest source of friction within the engine is the valve train.  Most of this friction comes from the followers on the cam, but a good deal also comes from the valves and valve guides.  Reduction of friction between the cam and the follower usually isn't a primary goal because this is one of the very few areas in your engine where friction actually decreases as RPMs increase.  The inertia of the rocker arms and valves tends to work in our favor with this component.  As the speed of the camshaft increases, the rocker arms' resistance to movement means they don't press quite as hard against the camshaft.

The most common solution to the valve friction problem is to ensure everything is well within specifications.  Too little clearance and the increased interference causes friction.  Too much clearance and the valve stem will wobble within the guide and this causes friction, too.  It is also quite common to make materials changes as well.  Bronze offers less friction than aluminum or iron and so it is common for use in valve guides and bushings.  Most aftermarket valves will come with some sort of coating that aids in heat dissipation and/or friction.

Tackling friction in other areas of the engine starts to take some imagination.  Oil viscosity is one prime example.  Using a lower viscosity oil will usually result in less frictional loss because the oil isn't quite as thick.  However, at high RPM operations, a lower viscosity oil may not provide enough lubrication and friction will increase (not to mention wear on engine components).  Replacement of journals with bearings (often of the roller variety) will reduce friction in other areas as well.  Perhaps going to a dry sump and dry clutch are an option for your motor?  This will enable the use synthetic oils, which are generally slipperier.  It also keeps friction down because engine parts don't need to be drug through an oil bath.

When dealing with friction, the goal is to reduce contact as much as possible in as many places as possible.  Where contact is necessary, ensure the surfaces areas are well lubricated with quality oils.

Losses due to Inertia
Inertial losses aren't really losses, per se, but inertia does have an effect on your engine and so I'll discuss it, but briefly.  First off all, let me clarify what I mean by a "loss".  Yes, it takes energy to accelerate your pistons to TDC, stop them, and the reverse their direction.  But this isn't a loss in energy, it's merely changing where the energy is stored.

For instance, as your piston reaches the end of the exhaust stoke and approaches TDC, it must slow down and as it leaves TDC and approaches 72°, it accelerates.  "Ah ha, that takes energy to accelerate the piston and energy to decelerate the piston!", I can hear many of you say.  You are correct.  But your assumption on where that energy is coming from may need some revision.  As the piston is decelerating, it pulls on the crank and that causes the crank to accelerate and store the energy from the decelerating piston.  Next, as the piston passes TDC and begins to accelerate, the energy (for the intake stroke, at least) is coming from the, now decelerating, crankshaft.  The energy to move the pistons up and down isn't lost, it's merely transferred.  Now bear in mind that this energy transfer is not without losses, but they are minor.  The majority of losses during this process come back to our old friend friction.

This same concept applies to your valve train.  Yes, the valve springs take a lot of energy to compress.  But that energy is largely returned to the system as the springs decompress and apply pressure back to the camshaft through the rocker arms.

The biggest concern about inertia is acceleration.  While much of the energy that goes into creating inertia within your engine is reclaimed every or every other rotation, there still must be the initial expenditure of energy.  The energy has to go in before it can come out.  This creates a direction relationship between the inertia in your engine and the rate at which it accelerates.

The more mass and inertia your engine has, the more energy that is required to accelerate it.  But also, more energy is required to decelerate it.  Many production engines that have have areas at which great initial investment of energy is required will make use of heavier flywheels in order to preserve inertia.  Diesel engines, with their compression ratios approaching 20:1, will often make use of heavier flywheels.  This allows for smoother engine operations because the energy stored within the flywheel helps overcome the resistance of the mixture to compression and so keeps the engine turning at a more uniform rate throughout its rotation.

Generally speaking, the more inertia within your engine, the harder it is to start and the slower it will be to accelerate or decelerate.  Furthermore, the increased weight of the components will generally cause an increase in friction in all connecting assemblies.  One possible benefit to increased inertia is a lower idling speed, but this usually translates into a lower redline as well.

In a bike with a sport pedigree, the goal should be to lower inertia as much as possible.  In any moving part, your goal should be lighter without sacrificing strength.  The energy required to get these parts moving is energy that would otherwise go toward accelerating your bike.  On decel, an engine with lower inertia can also make better use of engine braking and provided your rubber holds, your bike will stop better as well.

For those of you that have been following crazypj's 360 build(s), you can see this in effect in the modifications he's made to his rotor and gears.  This is done to reduce inertia.

Also, think outside the box (engine) for combating inertia.  The chances are, anything that moves on your bike got the energy from your engine.  Chain, wheel hubs, wheels, rubber.  All these things have inertia.

Engine Balance
The one final area of parasitic losses I will address is engine balance.  Mainly, this deals with the balance of the crankshaft and that's the area of most concern.  Other rotating components will benefit from being balanced, as well, but all to a lesser degree than the crank and piston assemblies.

So why is it important to ensure your is balanced?  Well, primarily this has to do with wear.  If an engine is out of balance it will wear more rapidly and many of the components will be placed under greater friction and greater stress.  As a more minor problem, the vibrations of an unbalanced engine can be pretty damn annoying.

There are two types of crankshaft balance.  The first, and most important, type is called the primary balance.  Primary balance is pretty simple to understand as it is basically just ensuring that the counterweights on the crankshaft properly balance out the weight of the pistons and conn rods.  You want to ensure that the center of mass for rotation along the crankshaft is as central to the crankshaft as possible.  It is isn't always necessary to change the primary balance of the crankshaft when you change pistons or conn rods, but it certainly is desirable.  This change in balance usually comes in two forms.

In order to "overbalance" a crankshaft, or add counterweight, holes are drilled into the existing counterweights and tungsten plugs are then inserted into the holes.  To "underbalance", or remove counterweight, holes are drilled and then left empty.  It is usually much easier and cheaper to remove weight from the counterweights on a crankshaft and so many aftermarket crank options (where they exist) will be intentionally overbalanced from their maker.

Just about any crankshaft in any configuration can achieve a perfect or near-perfect primary balance, at a given RPM.

Secondary balance has to do with how the assembly balances when rotating under load.  This starts to take into account the kinetic energy of the pistons (which increases as the rotational speeds increase), sideways motions of the counterweights, and any changes in balance due to offset crank pins (in the case of some stroked engines) that would cause the pistons to operate outside of a normal sine wave-like pattern (called "sinusoidal").

The primary means of secondary balancing are the phase of the pistons along the crank (it is very common to rephase 360° twins such as the XS650) and the use of balancing shafts.  These shafts rotate at twice the speed of the crankshaft and work to negate the harmonics of the secondary forces.

As mentioned earlier, nearly any engine can be balanced for primary forces, but it can be very difficult to balance secondary forces.  Rephasing is never a complete solution and at higher speeds, the balancing shafts may need balancing shafts of their own (this is not actually done, to the best of my knowledge, but that is what would be required to remedy the situation).  The configuration of the engine will have a lot to do with how it is to balanced.  Opposed cylinders like you see on many BMWs are naturally balanced for secondary forces and have no need of balancing shafts or rephasing.

This next bit is purely academic, but the best configuration for a balanced engine is a flat eight design (or any number of opposed cylinders evenly divisible by eight).  This is because each bank of pistons is opposed by the piston opposite and the outside pistons of one bank are opposed by the inside pistons on the same bank.  This cancels all major secondary forces.

Conclusion
So... now that you know the idea behind parasitic forces, the solutions should be fairly straight forward.

The primary means of reducing parasitic forces is the reduction in weight of all components.  Anywhere you can shed weight without affecting performance, do it.  Unless you are well aware of the consequences of doing so, a reduction in strength of these components is a bad idea.  Ensure all bearings are in good working order, all clearances are within spec, and your oil is changed regularly.  Keep your chain lubed and your your wheel bearings greased.  Don't over-tighten anything.

For balancing, my advice is to leave this to the pros.  It takes special equipment and special know-how to not make things worse.  The addition or reduction of weight in a crankshaft is a precision operation.  Getting it wrong by just an ounce can add a couple of hundred of pounds in unbalanced forces as crank speeds approach redline.  Rephasing can be undertaken by the garage mechanic, but requires special tools and a custom camshaft.  A daunting task for the first time, but a rewarding experience when done correctly.

[BrettS]

Power Goal #2 - Increased Revs

To first understand why revving an engine produces more power, you must understand that there is a difference between Force and Power.  Force is something that causes an object to undergo a change in direction or movement.  Power is how quickly that force is applied.  Zero force applied over any period of time is always zero power and infinite force applied over any period of time (except zero) is always infinite power.

Since I'm going to mainly talk about horsepower, I'll stick (mostly) with imperial units for this one.  This means we'll be dealing with foot-pounds for our unit of torque and horsepower for our unit of power.  For true correctness, this force unit is actually called a pound-foot, but the two terms are generally interchangeable, so I'll use with ft/lb as it's a bit more common.  A foot-pound is defined as the amount of force, measured at a central pivot point, generated by applying one pound of force at a distance of one foot from that central pivot point.  Because ft/lb is dependent on both direction and magnitude, this makes it a vector unit.  It can also be thought of as a force that "twists" something.  Because torque is a function of both the initial force (one pound) and the initial distance (one foot), it can be increased by either utilizing a larger distance or a larger initial force.  Doubling the distance or doubling the initial force will also double the torque.

Hopefully, that gives a basic idea about torque, because you need to understand torque to understand power.  Power is how much force is applied over time.  The definition of mechanical horsepower (the kind we equate with engines) is 33,000 ft/lb per minute.

If you recall from my first post, horsepower is derived from torque using the following equation:
     HP = (T * rpm) / 5252.

So how does that relate back to 33,000 ft/lb/min?  Well, all that has to do with our inclusion of Revolutions (it's the 'r' part of rpm) into the HP equation.  One revolution is equal to 2pi radians in the math world.  Dividing 33,000 by 2pi gets us 5252 and this is the reason for the use of that constant number in the equation.

Enough math for the moment; the important thing to remember is that without torque, you have no power.  Applying your torque faster gets you more power.  Furthermore, unless you're applying your torque with an appropriate speed, you also have no power.  To get power from an engine it must not only produce force, but apply that force quickly.  While the human body can produce great amounts of torque (especially with mechanical advantages), our inability to apply that torque quickly is what prevents bicycles from breaking the ton.

So... nitty-gritty time.  How does one go about revving an engine faster?  Well, it's not quite as easy as changing gears later in the rev range, though this is a basic gist of it.  There are several main factors in preventing your engine from operating well at high RPMs and each of these issues must be dealt with in order to create an engine lives up to its potential as it approaches five figures.

Mechanical Considerations
Though most everything on an engine can be qualified as a mechanical consideration, for this section I mean mechanical failure considerations.  If you hold the throttle open on your bike, in neutral, you will probably destroy your engine because of a mechanical failure.  I say probably, because some engines will fail past that point at which they stop making power and they don't make enough power to get to the immediate point of failure (I'll get to that in a second).  On race bikes, over revving an engine is a common cause of failure.  When you live on the edge, it's easy to fall off.

When a four stroke engine suffers a mechanical failure it almost always occurs at the end of the exhaust stroke / beginning of the intake stroke.  Why this occurs is quite simple and can be reduced to a few key points:

1.)  Almost all metals (including those used in the construction of your conn rods, crank, etc) have a higher strength rating when comparing compressive (pushing) to tensile (pulling) forces.
2.)  The tensile forces on the crank, piston, and conn rods are the greatest at the end of the exhaust stroke for two reasons:  First, the piston is the furthest from the crankshaft centerline at TDC than it is at any other point in the rotation and if rotational speed is held constant, the most force is exerted at the furthest point from the center of rotation.  Second, at the end of the compression stroke (the other time the piston is at TDC), there is less stress because the compression of the fuel/air mix combined with the increasing pressure of the ignition of the mixture reduces the tensile loading of the components and so they are less likely to fail.  In short, the compressed/ignited mixture is acting like a pillow for the piston and helping to cushion it.
3.)  For top end failures (due to valve float, etc), the exhaust stroke remains the most common because the speed at which the piston chases the exhaust valve is greater than the speed at which the intake valve chases the piston.  During the compression stroke, the exhaust valve is closed, and so the end of the exhaust stroke remains the critical time.  If you float a valve, it's going to be the exhaust valve that eats it first.

So how do you prevent mechanical failures?  Think of two words: Stronger and Lighter.  Chromoly steel is the weapon of choice for crankshafts, connecting rods, and rocker arms.  Titanium makes for excellent valves, though stainless steel is also a good option.  For pistons, forged aluminum is generally the best option, though hypereutectic aluminum will usually work for most applications where weight is more important than strength.

Remember, as speeds double, forces quadruple.  Pistons weighing in at 200 grams will produce 10 joules of energy at 10 m/s but 40 joules at 20 m/s.  Dropping the weight of your pistons by five grams will save you a joule based on mean piston speeds (it'll actually save you more on peak stresses, though).  That may not sound like a lot, but on a Honda 360, that's an extra 150 RPM you gain on the redline before a potential bottom end failure.  Again, 150 RPM may not sound like a lot, but lets say you can hold a steady 15ft/lb of torque from 11,850 RPM to 12,000 RPM.  For the only 150 RPMs, you've gained almost half of a horsepower.  Tacking on a thousand RPM instead of just 150 would net you closer to three horsepower, or an 8% increase (assuming that torque holds steady, which it won't).

When it comes to building a maximum effort engine, the little things don't mean a lot.  They mean EVERYTHING.  A gram here, a gram there frees up a joule here or a joule there, which adds a few more revs to your redline, which adds a few more ponies to your bike.

Pumping Losses and Parasitic Forces
Most people (it seems to me, at least) tend to think only of the mechanical failure portion of chasing RPMs, an equal consideration would be pumping losses and frictional losses.  While not sending rods through your crankcase is important, it's also useful to realize that the work your engine must do increases exponentially with the speed at which it operates.

For instance, as the rotational speed of the crankshaft increases, this causes more side-loading on the pistons and the friction between pistons/rings against the cylinder walls increases because of this.  Furthermore, it requires more energy to keep your pistons in motion because they are not only traveling faster through the cylinder, they're stopping and accelerating more often.  This change in direction, called reciprocation, requires more and more energy as rotational speeds increase.  Rocker arms, too, reciprocate and energy is required to move them.

Engines built to withstand high RPMs must also be able to deal with a phenomenon called, "valve float".  Valve float occurs when the valves springs are unable to provide enough force to close the valves in the time required.  To combat valve float, stronger springs are used in the valve train.  Unfortunately, strong springs means the engine has to use more force in opening the springs than previously required.  Force over time is power and so by strengthening the springs for more revs, you now require more power to open them.

Pumping losses are a big concern as well.  At high speeds, up to 20% of the power your engine creates goes directly into overcoming pumping losses (both above and below piston, though losses above the piston tend to accumulate more quickly in the very high RPM range as compared to below piston losses).  And that's even before you factor in friction, decreases in BMEP, or other concerns.

I won't spend too much time on pumping losses as it will be covered on more detail once I start attacking the BMEP section, but I will at least give a definition and a few examples.

Because ICEs are basically air/heat pumps, they all suffer from pumping losses of some sort.  And since air is a fluid, frictional fluid laws apply (Newton's third law of motion, again).  Think of this in the same way you would think of the the wind while you're riding.  There is a bigger difference in wind resistance between 60mph and 90mph than there is between 30mph and 60mph.

Air has inertia (and hence, momentum) and so accelerating it faster to fill a cylinder in less time requires more energy.  As your piston is descending and the intake valve remains open, a low pressure zone is created within the cylinder.  This low pressure is not only pulling air into the cylinder, it's also pulling up on the piston as it's trying to descend.  Likewise, as your piston starts the compression stroke, the mixture within the cylinder must be compressed at a more rapid rate.  The relationship between force and acceleration means that as greater acceleration is required, the greater the force must be.  These losses occur on the exhaust stroke, too, but in reverse.  The spent exhaust gases will initially rush to the lower pressure areas of the headers, but as the pressures within the cylinder and the exhaust system begin to equalize, the force of the piston is required to help expel the gases from the cylinder.  As with other fluid dynamics, the force required from the piston to expel the exhaust gases increases as engine speed increases.

Combatting above piston pumping losses can be achieved in a number of ways, but on older engines our usual plan is the same as increasing BMEP, which I will address in a future post.  It's also important to note that pumping losses are a byproduct of all ICEs.  You will never fully eliminate them.

Below piston pumping losses must be considered as well (especially with two strokes, because the crank cases are not open to the atmosphere).  No matter how well your cylinder is sealed, there will always be some blow by.  Some of the exhaust gases make it past the piston rings and into the crankcase.  This represents an increase in pressure within the crankcase and it is usually vented out through the crank case breather.  Though usually not a major concern for 180° twins or inline four-cylinder bikes, below piston pumping losses start becoming a real problem for other configurations.  This is because the crankcase volume varies depending on the angle of the crankshaft.  For 360° twins, both pistons are at TDC or BDC at the same time.  At TDC, the crankcase volume is the greatest and at BDC it is the smallest.  As the pistons approach TDC, a lower crankcase pressure requires more force to overcome it.  Likewise, as the pistons approach BDC, higher crankcase pressures require more force to overcome it.  Regardless of the engine configuration, however, the variations between the cylinder pressure and the crankcase pressure mean that some level of force is being exerted on the pistons as they travel between BDC and TDC.  Below piston pumping losses would only disappear if the crankcase pressure and the cylinder pressures were the same, always, which isn't possible.

For limiting below piston pumping losses, your best friend is going to be decent crankcase ventilation.

Combustion Speeds
Finally, there is one more major consideration that prevents high RPMs from being achievable, and that is combustion speed.  The gasoline being burned must create pressure at a rate which exceeds the increase in volume of the cylinder as the piston descends.  The reason for this is (again) inertia.  The momentum of the crank will want to keep the pistons reciprocating long after the the engine stops receiving fuel, air, and spark.  In a high RPM situation, the reciprocating nature of the pistons "steals" some of the energy from the combustion event because the piston was going back down whether or not that mixture was ignited.  It's the difference in trying to punch someone standing still or punch something that's running away from you.  You're going to get a much better hit on the guy who's standing there waiting for it, and the same is true for your pistons.  If they're running away from the flame, you're not making power.

There are some tried-and-true methods for increasing the combustion speed of your mixture, however.  One common way is ignition advance.  By lighting the mixture sooner, you can actually begin building pressure while the piston is still accelerating toward TDC.  By the time it reaches TDC, you also have a ton (literally) of PSI built up and so it pushes the piston back down with greater force.

The next, is good atomization of the fuel and air mixture.  Smaller droplets of fuel burn faster than bigger droplets and so they release their energy faster.  This occurs naturally as your piston speeds increase, which is why additional ignition advance past 3,000 or 4,000 usually isn't necessary.  Using the correct jets in your carbs, decent squish bands in your combustion chambers, and tumble/swirl from your valves also helps (more on these things in the BMEP section).

Also, increased compression will help combustion speeds.  Due to Brownian motion, an increase of density in the air molecules hammers the fuel droplets into smaller portions and smaller portions means faster burn times.  Increased compression also causes a spike in heat which helps to vaporize the fuel right before ignition.

Finally, it should be noted that low octane fuels burn faster than high octane.  It's a common misconception that octane is some measurement of power.  It's not.  Octane is technically a molecule, but it's commonly used as a word to describe a fuel's resistance to ignition.  That's right; higher octane fuels don't catch fire as well as low octane fuels.  Honda's famous RC166 bike required the use of 87 octane fuel in order to reach its redline of 18,500 RPM.  For more "streetable" motors, it's better to opt with higher compression and higher octane fuel as the increase in torque in the lower RPMs is usually more desirable than a few extra turns of the crankshaft at the redline.

Conclusion
In closing, chasing RPMs is actually a losing game (in the long run).  You have almost everything working against you.  As your RPMs increase, the cost in doing it, while still producing torque, is exponential.  It's OK to add a few extra revs once you've tackled some of the other performance areas, but merely adding components for the sole purpose of allowing more revs is not good value for money (though it can be done).  One thing I didn't mention in detail is that it's very possible to rev an engine past the point of being useful.  As more and more energy is required, that's less and less energy available toward moving you forward.  To add insult to injury, the redline of an engine is usually pretty far past peak torque and so while your power may be increasing (slowly), your torque is quickly dropping off.  The faster your torque drops, the slower your horsepower rises.  A large enough drop in torque and your horsepower begins to fall as well.  Building a high revving motor usually requires a peak torque to occur fairly late in the RPM band.  This may build winning race bikes, but they won't be much use on the street.

[BrettS]

Just a slight caveat to my post about displacement.  I tend to get caught up quite a bit in the minutiae of things and perhaps didn't make my conclusion strong enough in that post.

A friend read over what I wrote and had this to say:


Quote

The longstroke and shortstroke stuff is largely irrevelent in the modern era.  A CX 500 has a very short stroke and produces good power up and down in the rev range.  Rod length/stroke ratio is a very important determiner of power production and output.


And I do agree.  There's usually not much of a bad way to go about getting more displacement.  For you newer guys, any way you can get more CCs without adversely affecting your engine, that's what you want to do.  While not all displacement is created equal, stroke and bore are not leagues apart, either.

As for rod length:stroke, that is something I neglected to mention too much about.  A topic for a future post, perhaps...

Also, I totally agree with what swan and teazer have added.  Especially the portion about getting it running and learning your bike before you tear into it.  You MUST, MUST, MUST, have a baseline for your modifications.  If you modify a bike you've never ridden or don't know well, how are you to know whether your changes are improvements or detriments?

Finally, I also agree with teazer about tackling losses and volumetric efficiency, first.  VE, especially, is a topic that will take pages to cover and so I'm leaving it for a bit later.  The order in which I choose to address these topics has more to do with getting the "simple" stuff done first rather than implying that this is the order in which things should be done.  Trimming weight and, to a lesser extent, increases in displacement can be done with a minimum of investment.  When you start talking about significant gains in volumetric efficiency is when the money really starts to matter.  A future post, to be sure.

[steveh]

Good Stuff Bretts!

[BrettS]

Power Goal #1 - Increase Displacement

As mentioned previously, there are four main methods to employ in the quest for more power.  One of the simpler methods is to increase displacement.  As with my previous post, I'll start with simple and build from there.

Displacement is the volume of the area occupied by the pistons (at any time) within the cylinders.  If you were to measure all of the area occupied by a piston as it moves from BDC to TDC (called the "sweep" or "swept volume" of the piston), this would be the displacement.  As the calculation of a cylinder is fairly easy, calculations of displacement are also easy.

The formula for calculating displacement is to first divide the bore (diameter of the cylinder, though diameter of the piston can also used for this calculation) in half.  This will give you the radius.  The radius is then squared and multiplied by PI (3.1514).  The final answer is then multiplied by the stroke (the difference, in distance, between TDC and BDC).  You'll now have your displacement in whatever units you used during the calculation process.  For most of us, this will be cubic millimeters and so you may wish to divide by 1000 to convert to cubic centimeters.  Divide again by 16.387 if you want cubic inches.  Finally, multiply by the number of cylinders in your engine for a final answer.

For example, this is the displacement calculation for my own CJ360.  Bore is 69mm (non-standard) and stroke is 50.6mm.

     R = 69mm / 2 = 34.5mm
     R² = 34.5mm * 34.5mm = 1190.25mm
     A₁ = 1190.25mm * 3.1415 = 3739.17mm²
     A₂ = 3739.17mm² * 50.6mm = 189202.00mm³ = 189.2cc
     Final Displacement = 189.2cc * 2 = 378.4cc

This equation can also be "simplified" to one line in the following manner:

     Displacement = PI/4 * bore² * stroke * #cylinders

The displacement of your engine directly affects how much fuel and air can enter the cylinder and so more displacement will almost always translate into more torque and more power.  All things being equal, a big engine is more powerful than a small engine.

Not all displacement is created equal, however.  Many engine designs opt for a longer stroke or a larger bore.  Engines with a stroke longer than the bore diameter are called, "undersquare".  Engines with a bore diameter wider than the stroke is long are called, "oversquare".  If an engine has the same stroke and bore (or are within 5% of one another) then the engine is "square".  Generally speaking, motorcycle engines follow an oversquare design, though the more displacement an engine has the more likely it will be approaching undersquare.  All HDs of the modern age run undersquare engines and almost all sport bikes will run oversquare.

So lets cut to the chase.  Are oversquare engines better than undersquare engines?  Well that's kind like asking are apples better than oranges.  It'll depend on who you ask and the purposes of the build.  Generally speaking, undersquare engines hit peak torque sooner.  This gives them a feel of very strong acceleration, but it also tapers off quickly.  Oversquare engines take a little while to get up to pace but then will pull harder through the top end.


Undersquare Traits
An oversquare and an undersquare engine will have very different characteristics for the same displacement.  Because undersquare engines have a longer stroke, this means their pistons are moving faster than the oversquare engine for a given RPM.  Longer distance over the same time means more speed.  Engine components can only handle so many forces and as speeds double, forces quadruple. The increase in speed of the pistons directly translates into a need to reduce the engine RPMs before the breaking point of the bottom end is reached.

Piston speed is generally measure using "Mean Piston Speed".  This is usually listed in feet per minute or meters per second.  Mean Piston Speed is precisely the reason why the Triton was born.  The Triumph engine of the day was a better alternative to the Norton because of its ability to rev.  The maximum MPS for an engine is usually right around the 4000ft/min or 20m/s mark.  Some engines can go as high as 4900ft/min or 25ms but unless you've made modifications, I don't recommend it.  To calculate mean piston speed (in ft/min) multiply the stroke times two times rpm and then divide by 60.  For my own CJ360 it becomes:

     2 * 50.6 * 11,000 / 60 = 18,533mm/sec or 18.53 meters per second at 11,000 RPM

If I were to rev to a mean piston speed of 20m/s, then I'd be hitting nearly 12,000 RPM.  Probably doable for short periods of time, but I don't think I'd want to live up there.  For argument's sake, lets say I increased the stroke out to 60mm.  20m/s of MPS now occurs at 10,000 RPM.  I've had to drop my hypothetical redline by 2,000 RPM to accommodate the new stroke.

Furthermore, cylinder filling can become an issue at higher RPMs because the increase in displacement of the cylinder (but not it's diameter) prevents the use of bigger valves.  Next, the increased travel of the pistons creates more friction.  A majority of the friction in your engine comes from the piston rings against the cylinder walls and increasing the distance the pistons needs to travel increases parasitic losses.  Finally, the longer crankshaft arms (or pin offset) necessary to create a longer stroke also causes an increase in sidewall pressures of the cylinder and on the piston skirts.  This is illustrated below this paragraph.



Lets assume this simple image is a side view of your crankshaft with the center of the crankshaft being the point, "C".  The black circle is the path which the big ends of your connecting rod follow during the rotation of the crankshaft and the blue line, "c", represents the conn rod, itself.  The graphic here represents the most extreme scenario; when the piston is halfway between TDC and BDC and the angle between the cylinder centerline ("b") and the conn rod are the greatest.  Let's assume that crankshaft is rotating clockwise and we're halfway through the power stroke.  The natural tendency will be for the expanding gases to push straight down onto the piston which will then push down through the conn rod and to the crankshaft.  The third law of motion tells us that the crankshaft and conn rod must be pushing back through the piston as well.  As this force is not applied directly upward at 0°, then some of the force is directed to the left (in the graphic).  In real world terms, the piston is forced against the side of the cylinder wall and how hard it is pushed against the side is related to the angle A, which is determined by the length of the conn rod, c, and the distance from the crankshaft centerline, a.

But, it's not all doom and gloom on the undersquare front.  Undersquare engines do provide some very distinct advantages.  By increasing the length of the crankshaft arms or by offsetting the pins, we create a greater mechanical advantage.  This mechanical advantage is easily thought of as the handle on a wrench.  It's much easier to turn a bolt with a long-handled wrench than with a short one.  The same holds true for a piston applying its forces onto the crankshaft; things turn easier with long handles.  So with a longer stroke, you're not only getting the torque advantage of more fuel and air, you're also a getting a mechanical advantage.

A further advantage of undersquare engines is the surface area of the cylinder, combustion chamber, and piston that is exposed to the ignition of the mixture.  As an engine gets more and more undersquare, the area exposed to the initial ignition shrinks in relation to the displacement.  This creates thermal efficiencies within combustion that directly translate into a greater BMEP for equal displacement engines.  Because of this greater thermal efficiency, higher compression can be employed without the need to use higher octane fuels (which is also beneficial because higher octane fuels burn more slowly).

Also, the longer stroke of the piston aids in port velocities.  Generally speaking, the greater the port velocities, the better the volumetric efficiencies (we'll cover that in a later post).

Finally, the flame front within an undersquare engine travels faster and the rate of increase in combustion chamber volume as the piston descends more closely matches the natural characteristics of the expansion of gases created by the combustion of gasoline.  This leads to smoother operations and another increase in BMEP.

Oversquare Traits
As you'd expect, the opposite of an undersquare engine is an oversquare engine and so many of their traits are opposite as well.  With a shorter stroke you are not only able to rev an engine higher to get more power, you're actually required to do to.  Oversquare engines will produce their peak torque at a higher RPM than that of its undersquare cousin.  The gives the engine a feeling of wanting to run.  Taken to an extreme, however, many oversquare engines will be feel high strung.

With a larger diameters than an oversquare engine, an undersquare engine can pack in larger valves or even more numerous valves.  This has the effect of increasing engine breathing at higher RPMs, but it does lower intake velocities and lower RPMs.  This is a significant reason why undersquare and oversquare engines achieve peak torque at different spots in the RPM band.  Also, the shorter stroke makes those higher RPM forces more tolerable for the engine components and results in lower frictional losses as well.

On the downside, oversquare engines will have a greater area exposed to the flame front and so will generally run hotter while getting less torque per cubic centimeter.  These engines need to be revved to get their full potential because they rely on speed for power rather than force.

Oversquare engines are most common in applications that require higher levels of power at the cost of efficiencies.  Race cars (yes, even NASCAR) use oversquare engines.  Semi trucks, marine diesels, trains, Toyata Prius, etc, all use oversquare engines when efficient operations are more of a concern.

Square Traits
Square engines, obviously, sit between the two extremes.  A well designed square engine can have the best of both worlds while a poorly designed square engine will have the worst.  Much of this will have to do the selection of peripheral components.

Which to Choose
Well... it's not that simple.  Even after laying out the traits of each motor, you can't just pick one and run with it.  If your starting platform is an 883 EVO engine, you're going to have a hell of a time reaching oversquare.  Likewise, you may encounter some difficulties if you want to stroke out your Ducati 1198.

An increase in bore will require larger diameter pistons.  Unless you spend some dough, these pistons will almost definitely be heavier and this has the adverse effect of adding more tension onto the conn rods and crank.  More tension means lower RPMs before parts start trying to occupy the same space at the same time.  A larger bore will also require the use of a specially made head gasket.  You can't have edges of the gasket poking into the combustion chamber or it won't last very long.  Copper is a common material for custom head gaskets but it's a bitch to seal properly (especially with iron-sleeved aluminum chambers)  and is NEVER as easy as it looks, although I'm not sure it ever looks easy.  Going very overlarge on the bore may require your combustion chambers to be remachined or even your entire head to be replaced.  In some cases, increase in compression or timing will be required to ensure the flame front continues to propagate at a reasonable speed.

An increase in stroke is accomplished with a new crankshaft and/or pin offsets between the crank and the conn rod.  This has the effect of causing the piston to rise higher and drop lower at TDC and BDC, effectively creating a larger circle in which the crankshaft spins.  Obviously, there needs to be room within the crankcase for this new length, but there also needs to be room for the pistons.  Because the pistons descend lower, the skirts are more likely to make contact with the cases or the crank, itself.  On the top end, the piston now might be making contact with the head and so that issue will need to be dealt with as well.  Pistons with the wrist pins situated closer to the ring lands (this lowers the piston at both TDC and BDC) and longer cylinders with higher decks are some examples of solutions, though the latter is usually reserved for big spenders or auto enthusiasts.  It's also not uncommon to use shorter rods in combination with a stroke increase.  This will help to reduce side-loading and usually reduces the weight and hence the forces at play.

So... which to choose?  That is going to depend on many factors.  Generally speaking, stroking an engine is going to give you more torque than an equal increase in displacement coming from bore alone, but the investment in time and money will be a lot greater.  HD engines LOVE a good stroking (don't we all?) and their popularity means there are a lot of aftermarket options available to pursue that avenue.  Aftermarket options for other engines may be more limited and so you'll have to undertake a lot of work yourself or follow the path of those that have come before.

My advice is to shoot for a modest increase in bore and then call it good, unless you're running with an HD engine.  Any increase in displacement is going to be a good thing and so, for this topic, the focus should be more on what's cost effective for your build rather than what provides the exact traits desired.  That said, if anyone feels like stroking out a CB350, please let me know.  That's a thread I want to be following.

[BrettS]

I found what I think is some great information that would help us Laymans understand what the Engine Guru's are on about and maybe there is something it it for the Engine Guru's as well.
Here is the link CLICK
But I will post the info here as well. No harm to skim this first section but I thought I should keep it intact.

Doing it Right" or "How to Build a Functional Café Racer"

OK... admittedly this is a very broad topic and I wasn't quite sure where to put it, so I stuck it in the Engines section because it's my intention to make this more of a technical type of article and also because I'm going to be talking mainly about engines.

I was reading a blog post from a friend of mine and it really struck a chord with me.  It seems that the café racer genre has really taken off these past few years, but a lot of the newcomers seem to be missing some of the basics.  They're immediately drawn to the looks of the café racer, but are perhaps not understanding the function (or even that the form follows the function).  Now don't get me wrong... I'm not hating on the guys.  Fresh faces are what will keep this knowledge alive and well in the years to come, but only if the knowledge is taught and understood.

Please allow me to repost the blog before we go too much further:


Quote from: kopcicle

What is a "Cafe Racer"?
Make it go ...
We all know that there are gains to be had just in the fine details without resorting to pistons , compression , valves , cams , extensive porting , carburetor and exhaust . Well , no , wait . Within reason and budget that is the idea !
Make it stop ...
Disc upgrade or pursue a lost art in tuning up drum brakes .
Make it turn ...
Make what you have the best it can be or bin it and adapt a modern front end entire.
Make it look ...
Like it's more at home carving corners than sitting outside the favorite pub .
Take pride ...
In the fact that each sub assembly is the very best that you can do with the tools , talent , time and budget available .Only then will the bike reflect that it is more than the sum total of it's parts .
Teach ...
What you learn . Without this not only the lesson but the spirit of the lesson dies with you .
Enjoy ...
What you do , what you build , what you ride .

Our bikes are not only a form of personal expression but a loosely defined art form that stems from the enthusiasts of previous decades . Without growth and change we stagnate and die . In a world of posing and posturing for the benefit of who knows who what is the point of modifying a bike to be just like whatever unless it's meant to be a replica . Be original . Be different . Experiment . Let function be your guide and form will follow .

Our legacy to the next  generation of riders and builders is our collective and individual vision . Our passion for that something extra defines our enthusiasm . Our ability to communicate and teach how to learn is our obligation . The definition of a "cafe racer" isn't rooted in our collective or individual past it will be defined by what we choose to do in the future . I've never known a brighter future for the genre in all my years turning a wrench . I can't wait to see what happens next .



Now one other thing... there's a been more than a little drama on forums recently and I think one lesson we could all take away from it is that "you catch more flies with honey than you will with vinegar".  That is, maintain a positive attitude.  Instead of telling someone what they're doing wrong, tell them what they can do right.  TEACH them right and wrong so that they can identify it for themselves.  Be aware that sometimes there is more that one way to be right, but also be aware that there is almost always one BEST way.

With these things in mind, I'd like to get back to the main topic of this post.  "Make it go".  My own personal philosophy about café racers can be boiled down to two concepts.

1.)  Form follow function - It has to work well first, look good second.  Generally speaking, if it works well, it will look good.
2.)  Built not bought - It's easy to take this maxim to an extreme, so please use this in the more moderate sense.  It's your bike, you need to understand what's going on with it.  If something breaks or there's something you want to improve, try it yourself.  It may cost you some extra parts when you screw it up, but you've learned something in the process and knowledge is priceless.

Now that the preamble is over.  Lets get down to it.  This article is an attempt to pass on a bit of the knowledge I've gathered over the past few years.  Undoubtedly, there are guys here with a lot more experience and a lot more knowledge than I and I welcome them to please correct me where I'm wrong and chime in with additions where applicable.  I readily admit that most of my knowledge comes from reading and studying rather than doing and so keep that in mind.  As always, empirical data trumps rhetoric.  Something tested, measured, and replicated is something proven.  Something written is just words on a screen.  If you disagree with something I've said, please post why so we can all learn from it.

A café racer without performance enhancements is just a tractor with a body kit.  Do you really want to be one of those kids in a 1.6L Honda Civic with glowing lights under the body panels racing his gutless wonder from stoplight to stoplight?  If the answer is, "yes", you can probably stop reading now. 

Seriously though, the engine is the heart and soul of your bike and I'm seeing fewer and fewer builds that attempt to improve this key component.  I'm not sure why this is, but I suspect that our culture has come to favor looks over performance or perhaps people are a little wary of cracking open something that has so many parts?  Cost is also a consideration, but if you can afford to drop $500 on fiberglass seats and tanks and another couple of hundred on paint and upholstery, a bit of money for the engine doesn't seem out of line, right?

Building an engine can most certainly be done in stages, but the most important thing to keep in mind is to take a holistic approach.  There are few things you can change that won't also have an effect on something else.  Understand the consequences (both good and bad) of each action before you take it.  Not all parts will work in all circumstances and the final goal of your engine build SHOULD be the deciding factor of which parts go into it.  Building a comfortable long distance cruiser versus building a café racer is more than just adjusting the seat and control locations.  The engine characteristics are the soul of a bike.

So what are engine characteristics and how does the design of the engine affect them?  Well... put simply, the characteristics of the engine can be categorized by the throttle response, revs, acceleration, torque and a host of even more subjective items.  The engine from a semi truck can put out more than 600 horsepower, but you're never going to find one in a sports car (size issues aside).  Sports car drivers want high revs.  The very successful Honda S2000 can redline at 9,000 RPM.  Much higher than most passenger vehicles.  That redline is necessary to create an appropriate feel and power for the vehicle's purpose and equal thought should be given to your own engine.

There is something intangible to engine characteristics, though.  How much of a grin is on your face after your bike pushes you through a tight corner?  Is your bike still egging you on for more throttle even when you're skirting the ton?  Or is your bike telling you it's had enough when you start pushing 80?  The engine on a café bike should pull strong through the mid range and get even better as the rpms climb.  The engine on a café bike should bounce off of the redline after a gear change and actually feel a bit sad that you didn't take it further.  The engine on a café bike wants nothing more than to rev itself to pieces.  It would love the opportunity to see how fast it can spin before parts start flying out.  Is your bike screaming for more or is it screaming "enough"?  Your engine should be willing and your bike should be braver than you are.  Remember, café racers started out at street legal race replicas, mimicking the race bikes of their time.  Would your bike be at home on a race track or did you just build another tractor with a body kit?

So... enough rhetoric already.

Time for some theory.  I'll get into more details in subsequent posts, but for the remainder of this post I'm going to talk about the four general ways in which engine performance can be improved.  Before that, though, lets talk about a few engine basics just to make sure we're all on the same page.

The Workings of a Four Cycle Internal Combustion Engine (ICE)
This may seem a bit basic to many of you, but I'm of the mind that a decent house needs a decent foundation and so I've included the info here.

An ICE is basically a self-powered air pump.  Fuel and air is drawn into each cylinder, compressed, ignited/combusted, and then expelled.  Repeat as frequently as possible.  Those four steps are as follows:
Intake Stroke - Piston starts at Top Dead Center (TDC) and descends toward the crankshaft centerline.  During the entirety of this stroke, the intake valve(s) remain open and the descending piston can be thought of in a similar manner to the plunger on a syringe, drawing fluid into it as it opens.
Compression Stroke - The piston ascends from Bottom Dead Center (BDC) back toward the head as the intake valve begins to close.  The fuel/air mixture that was drawn into the cylinder during the intake stroke is now squished into a smaller and smaller volume.
Power Stroke - The compressed mixture is ignited with a spark from the plug and the ignition releases a great quantity of heat.  This heat is what causes the gases in the cylinder to expand and produce an increase in pressure.  This pressure pushes the piston back toward BDC.  Partway down, the exhaust valve opens and begins bleeding excess pressure out the exhaust headers.
Exhaust Stroke - The piston passes BDC and heads back toward TDC.  Going back to the syringe metaphor, this is the plunger being depressed and expelling all of the fluid back out of the syringe.  During the entirety of this stroke, the exhaust valve remains open.  The intake valve will open during this stroke as well.

An astute reader will have noticed that the piston descends and ascends twice for an entire period of the four cycles.  This means the crankshaft is rotating 720° for each complete period.

How efficiently each of these four strokes can be accomplished is what determines the performance of your engine.  These performance modifications will always go to serve one (or more) of four goals.  These four goals are the same four goals for everyone, everywhere, and your modifications need to answer to these objectives.

The Four Goals of Engine Performance
1.) Increase Displacement - All things being equal, a bigger engine will outperform a smaller one.
2.) Increase Revs - Horsepower is a unit derived from torque.  Torque is what is measured, horsepower is what is calculated.  HP = (T x RPM) / 5252.  If you can keep torque the same and increase your revs, you've just "created" horsepower.
3.) Parasitic Losses - The power generated by your engine goes into a lot more than just turning your rear wheel.  It takes a lot of energy to spin metal as fast as your bike does and that's even without having to contend with friction of all the components necessary to make it happen.  Reduce this friction and inertial losses and your engine will spin faster, sooner, and more of that power will make it to the ground.
4.) Brake Mean Effective Pressure (BMEP) - This is the average pressure within the cylinder, generated by the power stroke.  More BMEP directly translates to more torque, which, of course, means more horsepower.

I'm right around my mental limit of typing for the day, so I'll go into more detail on each of the above four items in a later posts.