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An engine is a complicated lever. Higher manifold vacuum means a shorter lever. When the volume of air inducted is maximum the "lever" is the longest it can be.
That is why best BSFC is achieved at lower RPM and low vacuum which equates to highest load without enrichment, or about 80% of max load.
It's really simple. if you compress a gas to 10 times its atmospheric pressure and ignite fuel in that gas, its pressure increases by the temp difference between induction and combustion.
The ratio is about 7 times the compression pressure, so if you have 200 pounds of compression you have 7 times that or 1400 pounds of combustion pressure.
If you reduce the compression pressure by restricting the intake air to 50% of atmospheric then you cut the 200 PSI to 100 and the combustion pressure to 700 PSI.
An engine "idles" because you have starved it for air until it can't run any faster than idle speed, at which point your efficiency is 0 since you are using fuel just to spin the engine over, which is the most extreme example of manifold vacuum. You would not have enough power to move your car unless you reduce the manifold vacuum by increasing the throttle position.
Most power per unit of fuel consumed 1300-2000 RPM with no enrichment and no manifold vacuum. You can achieve 0 manifold vacuum with very little throttle applied at low RPM as long as the load is high enough.
regards
Mech
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