>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
>vacuum.
>Denise Thorpe
>thorpe@kegs.saic.com
>
>
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
|