The manifold pressure is higher when you apply throttle.

What is meant by the term vacuum is a lower pressure in the intake manifold. The manifold pressure will be low at idle typically below 12" hg and close to ambient at full throttle pushing towards 30" hg man pressure at sea level and a standard day.

So when you step on the gas, the man pressure increases and the vacuum decreases.

It is obviously not a difference of pressure in the manifold itself that makes air fill the cylinders.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record