Skip to main content

Manifold Pressure

On aircraft equipped with a constant-speed propeller, power output is controlled by the throttle and indicated by a manifold pressure gauge. The gauge measures the absolute pressure of the fuel-air mixture inside the intake manifold and is more correctly a measure of manifold absolute pressure (MAP).

Carburetor

  • When the engine is not running, the manifold pressure gauge will read approximately ambient pressure.
  • When the engine is started, the manifold pressure indication decreases to a value less than ambient pressure (i.e., idle at 12 "Hg)
  • Engine failure or power loss is indicated on the manifold gauge as an increase in manifold pressure to a value corresponding to the ambient air pressure at the altitude where the failure occurred.

MAP and RPM​

For any given rpm, there is a manifold pressure that should not be exceeded. If manifold pressure is excessive for a given rpm, the pressure within the cylinders could be exceeded, placing undue stress on the cylinders.

  • When lowering power, reduce throttle first, then adjust propeller pitch to maintain rpm.
  • When increasing power, adjust propeller RPM first, then increase throttle to maintain rpm.

For a given rpm, the manifold pressure should be set to the value specified in the aircraft's operating handbook.

Carburetor