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Re: Vacuum

To: (Denise Thorpe)
Subject: Re: Vacuum
From: Craig Wiper <>
Date: Thu, 11 Jan 1996 20:16:00 -0800
>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 

>Denise Thorpe
So, when I use my SU's and manifold on the '74B, the system achieves higher
vaccuum at idle. And when I use the Weber and it's manifold, the vaccuum is
lower or non-existant at idle and higher when I'm revving (because the Weber
uses ported vaccuum)? Is this true? If so, do _both_ these situations
provide optimum power? How can this be? One situation is the opposite of the
other?  Am I confused? (Yes!). Which is better? Please explain.

Craig Wiper
early '74B

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