Hi! I've been wondering about intake manifolds. Correct me if I'm wrong here please. If I were to attach a vacuum gauge between the throttle plate and the cylinders (stock fiero plenum) and at red line , WOT, I still had a small vacuum reading, then I could assume the intake up to the point of the vacuum reading was a restriction. Is this correct? A turbo motor will read boost, a completly unrestricted or oversized NA manifold will read "zero"? Any vacuum should mean the cylinders are trying to pull air that isn't there. So if one were to make small modifications to the manifold he or she could use vacuum reading to judge the amount of change from their work. Am I correct or do I need more coffee?

That is essentially the philosophy behind the many threads on this forum describing modified intake manifolds. Use the SEARCH function and include ARCHIVES to see a variety of solutions.

https://www.fiero.nl/forum/A...130314-2-118780.html

[This message has been edited by Gall757 (edited 04-26-2016).]

I think it would depend on where/how you take the reading. Bernoulli's Principle describes how a fast-moving fluid can cause reduced pressure in a tube, even when there isn't any flow restriction. And air is a fluid. So it stands to reason that even at WOT, the intake manifold might still have a slight vacuum. That said, it should still be close to ambient if the intake flows well.

Personally, I think a better way to gauge flow restriction would be to measure air velocity (at WOT) in the intake, and compare it to your ideal air velocity. If it's too high, you need a higher-flowing intake.