Manifold Pressure

Manifold pressure is the absolute pressure of the air-fuel mixture in the intake manifold of a reciprocating engine, measured in inches of mercury (inHg) or hectopascals (hPa). It is the primary power control instrument for constant-speed propeller aircraft and turbocharged engines.

Reading Manifold Pressure

With the engine stopped, the manifold pressure gauge reads ambient atmospheric pressure (approximately 29.92 inHg at sea level in standard conditions). At idle, manifold pressure drops below ambient due to the throttle plate restricting airflow. At full throttle, a normally-aspirated engine’s manifold pressure approaches (but cannot exceed) ambient. A turbocharged engine can exceed ambient pressure at full power up to its critical altitude.

Power Settings: MAP and RPM

On a constant-speed propeller aircraft, power is controlled by the combination of manifold pressure (throttle) and RPM (propeller pitch). A general rule for throttle-then-prop management: when increasing power, increase MAP first, then increase RPM. When decreasing power, decrease RPM first, then reduce MAP. This prevents detonation from high MAP combined with low RPM.

Related Terms

See also: Manifold Pressure Gauge, BHP, Full Throttle Altitude.