Mechanic Glossary

Absolute Pressure

Pressure measured relative to a perfect vacuum, combining atmospheric pressure and gauge pressure.

Absolute pressure starts at zero in a vacuum. It is the sum of atmospheric pressure and the pressure shown on a standard gauge. Car engines use Manifold Absolute Pressure (MAP) sensors to measure intake vacuum. This data is critical for the computer to calculate fuel injection.

Standard pressure gauges read zero at sea level. However, absolute pressure at sea level is about 14.7 PSI. Using absolute pressure allows the engine computer to adjust fuel mix for changes in elevation.

Frequently Asked Questions

Gauge pressure reads zero at atmospheric pressure. Absolute pressure reads zero only in a perfect vacuum (adds 14.7 PSI at sea level).

MAP sensors measure absolute pressure in the intake manifold, allowing the ECU to accurately calculate air density regardless of altitude.

It reads local barometric pressure, which is about 14.7 PSI at sea level, rather than zero.

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