Manifold vacuum refers to the low pressure present in the intake manifold of a normally-aspirated internal combustion engine. It is formed in that case by the low pressure created inside a cylinder during the intake stroke of a standard four-stroke engine - as the piston lowers with the intake valve open, the volume of the cylinder increases, and hence the pressure inside the cylinder drops. Since the intake valve typically should open as close to top dead center as possible in order to maximize the amount of fuel/air mix drawn into the cyllinder, the eventual pressure will be quite low. Naturally, the intake manifold itself will therefore be at a lower pressure than ambient, as the operating cylinders constantly draw air (and fuel, if the engine is not a direct-injection type) out of the manifold.

This vacuum is sometimes used elsewhere in the vehicle's systems. Prior to the cheap and easy availability of reliable electric actuators, for example, manifold vacuum was used to serve as a mechanical power source for vehicle brakes (and sometimes steering) which could be distributed throughout the vehicle using tubes or pipes rather than moving parts. In addition, systems intended to move fuel throughout the vehicle (vacuum pumps) can be powered, as can vapor management systems for pollution control. Mechanical distributors in ignition systems are sometimes fitted with vacuum advance, which changes their timing adjustment depending on the stength of the vacuum and hence how fast the motor is running.

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