Manifold vacuum

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Manifold vacuum, or engine vacuum in a petrol engine is the difference in air pressure between the engine's intake manifold and Earth's atmosphere.

Manifold vacuum is an effect of a piston's movement on the induction stroke and the airflow 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 and for the crankcase ventilation system.

Manifold vacuums should not be confused with venturi vacuums, which are an effect exploited in carburetors to establish a pressure difference roughly proportional to mass airflow and to maintain a somewhat constant air/fuel ratio.

It is also used in light airplanes to provide airflow for pneumatic gyroscopic instruments.^{[citation needed]}