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, and to vaporize fuel sprayed into the engine by fuel injectors. Vacuum is also used to operate various components such as the EGR valve, to pull crankcase vapors through the PCV system, to boost the power brakes and to open and close air control doors in many A/C systems. See Manifold Vacuum.
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