Ignition advance, was Re: alternative engines, WARPED

[G. Scott Ponton]

Everyone has had good points. Put them all together and you about have all
the different ways portd vacumn was/is used. Yes there are 3 different
"types" and they all work differently.

1. Transfer slot.  nothing at idle, large amounts at limited throttle
openings, nothing at WOT unless the carb is small for the engine in which
case you start getting vacumn at higher engine speeds.

2. Above throttle. Nothing at idle, small amounts at minimal throttle ,
larger amounts at mid throttle settings, nothing at WOT again size
dependant.

3. Venturi. Increasing amounts as air flow increases has vacumn at all
throttle openings even WOT.

ever wonder why we have spred bore carbs? and why they have better throttle
responce etc.? Part of it has to do with avalable vacumn for the ported
systems. A simple view. At idle manifold vacumn is confined below the
throttle plates. As the throttle is openned manifold vacumn "climbs" to the
base of the venturi. Add the chike plate as the last "restriction" to
manifold vacumn.

Actually ported vacumn was originly used to combat idle instability with a
big cam. As the big cam has so little manifold vacumn at idle if a vacumn
advance has a low enough spring rate to allow it to stay engaged at idle you
started getting it at WOT with single carb apps before the advent of very
large single 4bbls. Increase the spring rate to eliminate it at WOT and
usually you would run into a situation where the idle vacumn was just enough
to give you some advance but not consistantly. As we all know all else being
equal, and if the amount of advance isn't excessive, the idle will climb. So
sometimes the idle would climb enough to effect even more vacumn raising the
idle even further. The opposite was also true less vacumn lower idle etc. I
think you can see the point here.

As carbs got larg enough to overcome this problem ported vacumn became the
norm for both emissions and drivability. See above for part of it. The rest
has to do with getting the advance when it is needed the most. Ported
systems tend to give you the best of both worlds. Stable, low emissions,
idles with propotional vacumn signal at part throttle. Nothing at WOT.

So is everyone as confused as I am by my explaination?? LOL

Scott