Definition
A cockpit instrument that measures the absolute pressure of the fuel-air mixture inside the engine's intake manifold, displayed in inches of mercury (in. Hg). It indicates the power being produced by a piston engine, and is the primary power-setting reference for engines equipped with a constant-speed propeller or a turbocharger.
Plain English
A gauge that shows how hard the engine is working by measuring the pressure of the air being drawn into it. Higher pressure means more power.
Context Anchor
Seen on the engine instrument panel, especially in airplanes where the pilot sets power by watching both engine speed and manifold pressure.
Derivation
Manifold' comes from Old English meaning 'many-folded' -- here it refers to the branching pipe that distributes air to each cylinder. The gauge reads the pressure inside that pipe.
Why Pilots Care
Proper manifold pressure settings prevent engine over-boost damage and ensure safe, efficient power management, especially in turbocharged engines.
Intuition Check
The manifold pressure gauge does not measure fuel pressure or cylinder compression. It measures pressure in the engine’s intake system before the air reaches the cylinders.
Example Sentence 1
After takeoff, the pilot reduced the manifold pressure to 25 inches and the propeller to 2,500 RPM for the climb.
Example Sentence 2
During the climb the pilot monitored the manifold pressure gauge and reduced throttle to stay within limits as altitude increased.