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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 intervening carburetor or throttle body leading to the intake manifold. It is a ...
Engine vacuum is the difference between the pressures in the intake manifold and ambient atmospheric pressure. Engine vacuum is a "gauge" pressure, since gauges by nature measure a pressure difference, not an absolute pressure. The engine fundamentally responds to air mass, not vacuum, and absolute pressure is necessary to calculate mass.
The vacuum is only an assist and the brakes can still function, requiring greater force, if the booster vacuum is used up. Many older vehicles used vacuum-powered windshield wipers. Loss of manifold vacuum when the engine was working hard, or at wide open throttle, necessitated using a vacuum booster pump which was usually part of the fuel pump.
The widely used Bourdon gauge is a mechanical device, which both measures and indicates and is probably the best known type of gauge. A vacuum gauge is used to measure pressures lower than the ambient atmospheric pressure, which is set as the zero point, in negative values (for instance, −1 bar or −760 mmHg equals total vacuum). Most gauges ...
The system usually consists of a tube, a one-way valve and a vacuum source (such as the inlet manifold). The unwanted gases, called "blow-by", are gases from the combustion chamber which have leaked past the piston rings. Early engines released these gases to the atmosphere simply by leaking them through the crankcase seals.
Boost gauge on a Ford Focus RS (left) 30 psi Boost gauge Top: Turbo/APC boost gauge in a Saab 900. A boost gauge [1] is a pressure gauge that indicates manifold air pressure or turbocharger or supercharger boost pressure [2] in an internal combustion engine. They are commonly mounted on the dashboard, on the driver's side pillar, or in a radio ...
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However, the inch of mercury is still used today in car performance modification to measure the amount of vacuum or pressure within the engine's intake manifold. This can be seen on "boost gauges (forced induction) or vacuum gauges (natural induction), which give a rough indication of the relative power being produced at any given time.