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The vacuum is higher at higher RPMS, not lower as someone mistakenly said earlier (it is lower *pressure* at high RPM, which means bigger vacuum).


Um, in a steady state, the vacuum is highest at idle. MAP (Manifold Absolute Pressure) is a good relative indicator of the power beging devoped by an engine, which is why it's such a popular way to measure load by EFI systems and it's a common airplane gauge. More MAP, more power, less vacuum.

Of course vacuum is highest at high engine rpms and closed throttle, but that happens only during deceleration.