Manifold, Ported and Venturi Vacuum Explained

[Camo]

A paper by Lars Grimsrud

Overview
The airflow through a carburetor and through an engine’s intake system creates various pressure regions
due to a variety of effects. The low pressure regions are sources for "vacuum" used for signal sources and
power sources for operating accessories.

"Vacuum" in an engine is not truly a "vacuum." Rather it defines a lower-than-atmospheric-pressure area
in the engine or in the carburetor. Lower-than-atmospheric pressure is measured in "inches of Mercury."
Mercury has the chemical symbol "Hg," so the terminology for the measurement becomes "in. Hg." To
visualize how this measurement works, imagine a U-shaped glass tube partially filled with Mercury. The
one end of the tube is exposed to the atmosphere. The other end of the tube is attached to your low
pressure source. The low pressure on the one end of the tube will cause the Mercury to rise up the tube.
The amount that it rises is "in. Hg."

A funny terminology issue arises with this: Most people would describe that the low pressure area is
"sucking" the Mercury up the U-shaped tube. To be technically correct, there is no such thing as "suction." What is actually happening is that the low-pressure area is allowing the high pressure on the other end to
"push" the Mercury up the tube. Thus, when you "suck" on a straw in a milkshake, you are not sucking:
You are creating a low pressure region in your mouth, and atmospheric pressure is "pushing" the milkshake
up the straw and into your mouth.

This tech paper will discuss the concepts of 3 different types of vacuum sources, and will briefly discuss
their potential uses and applications in tuning GM V8 engines.

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