Manifold vacuum, or engine
vacuum in an internal combustion engine is the difference in air pressure between the engine's
intake manifold and
Earth's atmosphere. Manifold vacuum is an effect of
choked flow through a
throttle in the intake manifold of an engine. It is a measure of the amount of restriction of airflow through the engine, and hence of the unused power capacity in the engine. In some engines, the manifold vacuum is also used as an
auxiliary power source to drive engine accessories. Manifold vacuum should not be confused with
venturi vacuum, which is an effect that is exploited in
carburetors to achieve a fixed mix ratio between fuel and air.
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