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Manifold pressure, also known as manifold vacuum, refers to the lower pressure inside an intake manifold as compared to the ambient atmospheric pressure outside of it. In automotive applications, the intake manifold is a component that is bolted to the intake ports of an engine with an airtight seal, which allows the tightly-controlled introduction of air to the combustion chamber. When the intake valves are opened during the induction stroke of an internal combustion engine, this airtight seal and the closed or partially closed throttle causes a vacuum to be created in the manifold.
This manifold vacuum is instrumental in the proper operation of an internal combustion engine. For instance, one way optimal fuel efficiency is achieved is by using a Mass Air Flow (MAF) sensor or similar device to closely monitor how much air the throttle allows to enter the manifold. A Manifold Absolute Pressure (MAP) sensor can also be used to monitor the actual pressure inside the manifold. This makes it is possible to track the amount of air introduced when the intake valves of each cylinder are opened, and thus adjust the fuel mix accordingly.
The manifold pressure in an engine can be used to determine many things, from optimal driving habits to worn components or even incorrect timing. Manifold pressure is most often measured with a vacuum gauge, which is hooked up to a port in the manifold in order to directly measure the pressure. These gauges are delineated in inches of mercury (Hg) or millimeters of mercury (mmHG). Atmospheric pressure at ocean level is about 30 Hg (760 mmHg), while the manifold pressure in most properly working engines will run somewhere between 15 and 22 Hg (381 and 559 mmHg).
Vacuum readings above or below the optimal range can be indicative of a number of problems. Poorly seated or burned valves, for example, can manifest in a vacuum which drops several inches or millimeters at regular intervals. A permanently lower vacuum, on the other hand, may indicate a vacuum leak or late valve timing. Vacuum pressure measured at the low end of the normal spectrum can merely indicate retarded ignition timing, which in many cases can be rectified by simply rotating the distributor.
In addition to being an essential component in the way an internal combustion engine operates and an excellent indicator of many common problems, manifold pressure has also been used as an accessory power source. By exploiting the manifold vacuum to power various devices, it is possible to take some of the load off the electrical system. Many automotive manufacturers utilize, or have at some point in the past utilized, vacuum pressure to power systems as diverse as climate controls, cruise control actuators, windshield wiper motors and windshield washer fluid pumps.
Ever had a car with windshield wipers powered by manifold vacuum? A 1955 Chevrolet Bel-Air, for example, has such a system and those wipers will completely stop when the driver stomps hard on the gas. Driving in the rain in one of those things can be a real problem for people who are in too much of a hurry or don't tailor their driving styles to account for rain.