[Kenth]

VACUUM

Definition: The absence or reduction of air pressure. Vacuum is created in the intake manifold by the pumping action of the pistons. Air is pulled out of the manifold into the cylinders faster than it can be replenished by air bypassing the throttle plate. The throttle creates a restriction that allows vacuum to buildup inside the manifold. This is necessary to help pull fuel through a carburetor.

Here is a simple description of the principle, worth pondering, that should make things a little clearer.
The engine is an air pump - it ingests air and compresses it.
It´s often assumed the reason air enters the cylinders is because the piston sucks it in - it creates vacuum and that pulls in the air.
But, atmospheric pressure - about 15 psi, will push air into a void, provided its passage is non-restrictive.
When a carburetor is large and its throttle is wide open, atmospheric pressure helps fill the low-pressure area above a piston on the intake stroke.
Close the throttle to a small opening, and atmospheric pressure forces a partial charge of air past the restriction. The result is less-dense air in the consuming cylinder, therefore less power.
Consequently, a small venturi and throttle opening restrict the amount of air atmospheric pressure can push to the cylinder.
For most street and highway driving, a small venturi carburetor is most efficient.
If your goal is maximum power, however, and low- and mid-range economy, driveability, and torque are secondary, then think "big".
Think "big" all the way.
Bigger intake ports, a higher-lift camshaft with longer duration, bigger intake ports and an efficient exhaust system to properly discharge burned mixture.
Restrictions in the induction path keep atmospheric pressure from driving in a full air charge, so the engine wil not perform to full potential.
The more air that atmospheric pressure can force into the combustion chamber, the more horsepower.
From "Rochester Carburetors" by Doug Roe

FWIW
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