electronic ignition

[Christian Raith]

Just enough vac there to keep the firing just a bit above TDC

[pgnz]

Also the  difference on a vac gauge when the throttle is shut on the over-run is mis-leading,  the centrifugal is still holding the timing right up there at 35 deg,  the variation is smaller than what a vac gauge will tell there

[B.Walker]

Okay, so not enough vacuum to operate the advance unit, but a vacuum none the less. If you smoke, then let some smoke waft around the hose and it will suck it up.

I think we both agree that there is no usable vacuum at idle, but vacuum is simply the absence of pressure in relation to a reference pressure and the venturi effect says that it has to be there.

In reference to your first question (if I understood it correctly), the speed of the air flow is not the same on either side of the butterfly, if it was then there would be no vacuum at the venturi. The volume of air on the engine side is much less then the volume on the intake side, and the speed and volume, is controlled by how far the butterfly is opened. It's the resulting low volume, and less pressure, of air on the engine side which creates the velocity, and vacuum, at the butterfly.

But I think weve reached the point of a futility with this discussion...   

[petejw1966]

ok so i went out side and disconected the vac
advance and stuck a another hose on it,
at 800rpm there is no vac, with the guage disconected
and the hose on my tongue i could feel a very slight vacuum
but not enuff for the hose to  stick to my tongue.
with the vac guage conencted (no vac showing) and the throttle slowly opened
the guage  moved slightly and steady  when the rpm hit 1000, as it climbed to 1800rpm
the needle moved more gradually before going berserk,i had it unrestriced.

[petejw1966]

this is going over my head   

so im not arguing just discussing 

if the speed of the air flow is the same,
that is the speed of air flow is the same
on either side of the butterfly would this make any difference,
compared to a ventury where the speed would be faster past
the throttle plate creating a vacuum?

 

[B.Walker]

hey brian,
the above was quoted from a txt book,   
but in saying that, i agree that there is still
air being passed through the intake  while the slight angle of the butterfly is open
but there isnt a lot and its not at a  high velocity,

ive connected a vac guage to the the carb nipple before and it doesnt
show any vacuum at idle, until u crack the throttle.

Sorry, don't agree. The volume of air that 4 lungs require at say 800 rpm is 880 litres per minute or 14.6 litres per second. While not a lot in air conditioning terms that much air being drawn through such a small opening has to create high velocity at the point of restriction, which in turn has to create a low pressure on any port in the air stream. We use this phenomen all the time to introduce fresh air into ducted aircon systems without using extra fans. I'm not arguing that there is no vacuum advance at idle but simple physics says there has to be vacuum at idle. Obvioulsy the engineers decided/realised it wasnt needed and adjusted the spring rate of the vacuum advance unit is set to overcome it.

[Christian Raith]

Put a tube onto the advance nipple and put it in your mouth, I bet your cheeks get sucked in.....hmmmm that is sounding a bit kinky I think

[petejw1966]

hi chris,
theres no vac at idle from the carb nipple, but theres
manifold vac on  the intake boot nipple. 

id be up for one of those boyer systems let us know
how you end up going with it.

[Christian Raith]

Hmmmm should have vac at idle or there is no air through.
No air=no fuel

If there is no vac at idle how do you balance the butterflies??

[petejw1966]

hey brian,
the above was quoted from a txt book,   
but in saying that, i agree that there is still
air being passed through the intake  while the slight angle of the butterfly is open
but there isnt a lot and its not at a  high velocity,

ive connected a vac guage to the the carb nipple before and it doesnt
show any vacuum at idle, until u crack the throttle.

[B.Walker]

vac advance unit

Under part throttle, light load conditions a partial vacuum will develop in the intake manifold. This means that less air and fuel will be admitted to the cylinders.  Thus the mixture will be less highly comnpressed and will burn slower when ignited.  In order to realise full power  from the air fuel charge in the cylinder, the spark should be somewhat advanced when the engine is operating under these light load conditions. To obtain this  a vac adv mechanism is used.

The spring loaded side of the diaghram is connected thru a vacuum line to an opening in the carby.  This opening is on the atmospheric side of the throttle valve when the throttle is in the idling position. (no vacuum)
As soon as the throttle is opened however the edge of the throttle valve moves past the opening of the vacuum passage.

The intake manifold vacuum is thus connected by the vacuum line to the air tight chamber in the vac ad unit, this causes the diaghram to move against the spring and rotate  the breaker plate. this movement carries the contact points around so that the cam opens (and closes) the points earlier in the cylcle. The spark consequently appears at the spark plug earlier in the compression stroke.
as the throttle is opened wider there will be less vac adv.  At wide open throttle under heavy load conditions there will be no vac adv at all, the spark advance under this condition will be provided entirely by the centrifigul adv mech.

ive got way to much time on my hands  lol

Hi Pete

I agree with most of your description, but I wonder if the highlighted bit is correct ? It is true that the vacuum port is on the intake side of the throttle plate, but at no time is the plate completely closed because if it was you would have no air and no idle. At idle the plate is held open by the idle adjustment screw and a small amount of air is passing top and bottom of the plate. This air is travelling at high velocity through the small opening and will create a low pressure in the vacuum port at the botton of butterfly in the venturi. This must have some effect on the advance unit, but maybe it's not enough to overcome spring pressure. This is where I think the yanks have it wrong in their argument about ported and manifold vacuum. They seem to be overlooking the fact that the throttle is never closed and the port is exposed to vacuum at all times when the engine is running. For my money, that is manifold vacuum.

However, in saying that I have never studied it in any great depth (not enough time on my hands ) and my knowledge of air flow and resulting pressures come from many years in HVAC.

[Christian Raith]

I have just spoken to MadBill about getting the Boyers in at a good price.
Long discussion about other systems
Advantages with Boyer is that they have a 5 year wty and they have been doing it for about 30 years.
The XS doesn't need to much fiddling with curves etc
If anyone wants a chat about it send me your phone number and I will call you back and let you in on the gossip (my calls are free, MyNetfone.com.au)

I expect to make a few calls on Sunday 

[steptoe]

that's all well and good if you have a spring loaded adv unit 

and by the way, yep,,, to much time 

[petejw1966]

vac advance unit

Under part throttle, light load conditions a partial vacuum will develop in the intake manifold. This means that less air and fuel will be admitted to the cylinders.  Thus the mixture will be less highly comnpressed and will burn slower when ignited.  In order to realise full power  from the air fuel charge in the cylinder, the spark should be somewhat advanced when the engine is operating under these light load conditions. To obtain this  a vac adv mechanism is used.

The spring loaded side of the diaghram is connected thru a vacuum line to an opening in the carby.  This opening is on the atmospheric side of the throttle valve when the throttle is in the idling position. (no vacuum)
As soon as the throttle is opened however the edge of the throttle valve moves past the opening of the vacuum passage.

The intake manifold vacuum is thus connected by the vacuum line to the air tight chamber in the vac ad unit, this causes the diaghram to move against the spring and rotate  the breaker plate. this movement carries the contact points around so that the cam opens (and closes) the points earlier in the cylcle. The spark consequently appears at the spark plug earlier in the compression stroke.
as the throttle is opened wider there will be less vac adv.  At wide open throttle under heavy load conditions there will be no vac adv at all, the spark advance under this condition will be provided entirely by the centrifigul adv mech.

ive got way to much time on my hands  lol

[petejw1966]

with the boyer system both the mechancical advance
and vac advance are removed, the timing curve of
the boyer system is preprogrammed into the computor,
the advance curve lines arent  necesarrily the same as the stock curve
line, so you need to choose one that suits best. i think they had
9 advance curves to choose from.


theres no ignition  advance at idle if the tube is connected to the no 2 carby, (no vac)
but it does advance (max)  when its connected to manifold vac, one of the problems i had
cause theres no adjustment on the ignition timing compared to the earlier models,
the adv ignition caused me pinging problems.

hey pg, isnt the 5 deg adv your talking about, thats the static ignition timing, which
would be set with the vac adv disconnected.