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absolute vs gauge

EFI Tuning Fundamentals

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i didn't understand the difference between gauge pressure and absolute pressure any help ?

Absolute pressure is measured relative to a complete vacuum. Gauge pressure is measured relative to the ambient pressure (known as the barometric pressure). Ambient/Barometric pressure changes with weather and altitude. Some sensors that are typically provided as gauge pressure: Oil Pressure, Fuel Pressure, Tire Pressure. Sensors that are typically provided as absolute pressure: Manifold Absolute Pressure (MAP), Barometric Pressure.

Some pressures are relative to other sources, for example Boost Pressure is typically considered pressure above ambient pressure, and Manifold Vacuum is the pressure difference between the MAP and ambient (when MAP is less than ambient, such as when the throttle is closed).

thanks a lot now i got it