Vacuum at RPMs

[Stallion]

What is it? Higher vacuum at low RPM, and low vacuum as the RPM goes higher? Is that how it is? Or the other way around? I always thought it was highest vacuum at idle, and lower as the revs go up.

Can somebody technically explain this?

 

[Ken]

An engine builds more vacuum at idle when suction in the intake manifold is high. This occurs when the butterflies on the carburetor are almost all the way closed and the engine is still trying to suck air in, thus creating high vacuum. At WOT the blades are open and the engine is ingesting as much air as possible, thereby eliminating any vacuum you may have.

 

[Jack]

Ken's right. And ... if you wind motor up to high rpm and then snap throttle shut ... manifold vacuum will be high momentarily.

Motor is is simply a pump. Imagine it this way ... your lungs are the motor ... and you have a drinking straw betwixt your lips ... the straw is the intake manifold ... and you're holding the other end of straw betwixt your fingers ... your fingers function as a valve as do the throttle blades. So, the vacuum inside the straw is relative to two different things ... how hard your lungs are pulling ... and how tightly you're crimping end of straw.
JACK:gap

 

[Ken]

"betwixt" Good one Jack!

 

[JohnZ]

Manifold vacuum is simply a function of engine load - the lowest loads are at idle and when backing off the throttle from cruise, which is when you'll see the highest vacuum readings. When you press on the throttle, you put the engine under load (and increase the size of the engine's only opening to atmosphere to pull air through), and that's when vacuum drops. You can go from high to low or no vacuum at one mph or 100 mph - revs don't matter - just engine load.