<P> 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 . </P> <P> Manifold vacuum is an effect of a piston's movement on the induction stroke and the 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 and for the crankcase ventilation system . </P> <P> Manifold vacuum should not be confused with venturi vacuum, which is 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 . </P> <P> The rate of airflow through an internal combustion engine is an important factor determining the amount of power the engine generates . Most gasoline engines are controlled by limiting that flow with a throttle that restricts intake airflow, while a diesel engine is controlled by the amount of fuel supplied to the cylinder, and so has no "throttle" as such . Manifold vacuum is present in all naturally aspirated engines that use throttles (including carbureted and fuel injected gasoline engines using the Otto cycle or the two - stroke cycle; diesel engines do not have throttle plates). </P>

Where does vacuum come from on an engine