Definition
The fuel-to-air mixture setting that produces the maximum power output from a piston engine at a given throttle and altitude. It runs slightly richer than the chemically perfect (stoichiometric) mixture, with extra fuel present to ensure every bit of available oxygen is burned for power.
Plain English
The mixture setting that gives the engine the most pulling power. There is a little more fuel than strictly needed, because that extra fuel makes sure the engine produces as much power as it can.
Context Anchor
Seen in aircraft performance charts, especially climb and cruise charts, where the handbook states which engine setting was used to produce the listed numbers.
Why Pilots Care
It provides the highest climb rate or cruise speed available from the engine, though it increases fuel flow and cylinder temperatures compared with leaner settings.
Intuition Check
“Best” does not mean best for every purpose. Here it means best for making power; a different setting may be best for saving fuel.
Example Sentence 1
During the climb to cruise altitude, the pilot leaned the mixture to the best power setting to maintain strong engine performance.
Example Sentence 2
The cruise performance chart lists true airspeed at best power mixture for each altitude and power setting.