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Subject: Vacuum
From: (Denise Thorpe)
Date: Thu, 11 Jan 96 11:10:02 PST
Dirk asked:

> 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.

There are two kinds of vacuum in a car: manifold and ported.  Since an 
internal combustion engine sucks air from the intake and shoves it out 
the exhaust, the engine creates vacuum in the intake manifold and pressure
in the exhaust system.  The more closed the throttle butterfly is, the 
more vacuum is created in the intake manifold.  This is manifold vacuum
and it's highest (lowest pressure) at idle and lowest (nearer to the 
ambient air pressure) with the throttle wide open.

Ported vacuum on the other hand is caused by air flowing through the carb
venturi (Bernoulli effect).  This is what sucks gas out of the jet into 
the airstream.  Any opening in the venturi walls will have lower pressure 
than the surrounding air.  The more air that's flowing (butterflies more 
open) the more ported vacuum there is.  This is the opposite of manifold 

My father had a Mercury Comet (early sixties) that had vacuum operated 
winshield wipers.  They'd go like stink while sitting at a stop light, but
as soon as my father stepped on the gas, they'd slow to a crawl.  Useful,
huh?  So they obviously ran off of manifold vacuum.  The Opel GT has 
retractable headlights whose retracting mechanism runs off of vacuum.  When
going up a long incline, the headlights slowly flop back over.  These
obviously work off of manifold vacuum too.  If you pay attention, you'll 
notice that most Opel GT's have the headlights permanently mounted in the 
up position.  As car manufacturers got smarter, they started using both 
kinds of vacuum to guarantee that vacuum was constantly available.  One-way 
valves prevent the ported vacuum port from being a vacuum leak for the 
manifold vacuum.

As cars got more sophisticated, all kinds of creature comforts were run off
of vacuum because it's essentially free power.  The Porsches and Audi's I 
worked on had vacuum operated door locks and heating/AC vent flaps.  Since 
the engine only supplies vacuum while running, these cars had vacuum 
reservoirs (lime green bulb things) that would make something work a few 
times after shutting off the engine.  You can tell which parts of your car 
are vacuum operated by operating them without the engine running.  If they  
stop working without being broken, they're vacuum operated.  It got to the 
point where so many things ran off of vacuum that the engine couldn't supply 
enough so some cars came with electric vacuum pumps.  So, the car burns gas 
to run the alternator to make electricity to run a vacuum pump to create 
vacuum to operate levers.  This isn't the most efficient power source I 
could think of.

I've been saving up words.

Denise Thorpe

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