On Thu, 11 Jan 1996, Dirk de Boer wrote:
> The purpose of the vacuum advance is to get an earlier spark during
> high vacuum conditions, but I cannot remember whether vacuum is higher
> during idle or at high rpm's. Anybody else who does?
> I do recall stories about vacuum operated windshield wipers slowing
> down when overtaking another car in the rain, which would indicate
> lower vacuum during high rpm's.
It is not an issue of the rpms alone, but the combination of revs and
throttle opening. At idle, the vacuum is fairly high, because the engine
is running and the throttle plate is amost closed. Think of it as a fair
amount of sucking, but only a small opening to let in air. When you go
down a mountain and use the engine to brake, vacuum is even higher because
there is more sucking, but the same throttle opening as at idle.
When you put your foot down and open the throttle plate wide, the vacuum
drops because there is a larger air inlet. There is a drop in vacuum
whatever the rpms are when the accelerator is floored, but the lower the
rpms the lower the vacuum goes.
So just about the worst case is when you are trying to pass someone in top
gear and you floor the accelerator. Fairly low rpms, which cannot
increase rapidly, and large throttle opening.
Back in the olden days, vacuum guages were popular to encourage driving
with a small throttle opening for economy. Lot of fun to watch, but I was
young and backing off on the throttle to save money seemed contrary to the
whole point of driving.
Ray Gibbons Dept. of Molecular Physiology & Biophysics
Univ. of Vermont College of Medicine, Burlington, VT
gibbons@northpole.med.uvm.edu (802) 656-8910
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