Definition
The absolute pressure of the fuel/air mixture inside the intake manifold of a piston engine, measured in inches of mercury (in. Hg). In an engine equipped with a constant-speed propeller, manifold pressure is controlled by the throttle and serves as the primary indicator of engine power output, used together with RPM to set a desired power setting.
Plain English
It is the pressure of the air being fed into the engine's cylinders. The more the throttle is opened, the higher this pressure becomes, and the more power the engine produces.
Context Anchor
Seen on the manifold pressure gauge and in power-setting charts or checklists for piston-engine airplanes, especially those with separate throttle and RPM controls.
Derivation
The intake 'manifold' is the branched pipework that distributes the fuel/air mixture from the throttle to each cylinder. 'Manifold pressure' is simply the air pressure inside that pipework. The word 'manifold' comes from Old English meaning 'many-fold' or 'having many parts', which fits — one input branching out to many cylinders.
Why Pilots Care
MP directly indicates available engine power; pilots use it to set and monitor performance while avoiding overboost damage.
Analogy
Think of manifold pressure as a way to see how much air the engine is being allowed to breathe in. Opening the throttle lets the engine breathe more; closing it reduces that intake pressure.
Intuition Check
MP is not just any engine pressure. It is intake-manifold pressure, not oil pressure, fuel pressure, or pressure inside the cylinders.
Example Sentence 1
After takeoff, the pilot reduced manifold pressure to 25 inches and set the propeller to 2,500 RPM for the climb.
Example Sentence 2
In cruise the pilot reduced MP to 21 inches to maintain economical power settings.