Torque is the rotational force at the crankshaft, measured in newton-metres or pound-feet. Power is the rate of doing work: torque multiplied by rotational speed, expressed in kilowatts or horsepower. BMEP (brake mean effective pressure) normalizes torque for engine size, allowing fair comparison between engines of different displacement; typical values range from about 8–10 bar for naturally aspirated gasoline engines to well over 20 bar for modern turbocharged designs.
BSFC (brake-specific fuel consumption) expresses fuel use per unit of work, in grams per kilowatt-hour, and is the standard yardstick for engine efficiency. Volumetric efficiency describes how completely the cylinders fill with fresh charge relative to their displacement. Knock is abnormal combustion in which the unburned mixture ignites spontaneously, and octane rating measures a fuel's resistance to it. The stoichiometric ratio is the chemically exact air–fuel mixture — about 14.7:1 by mass for gasoline — at which combustion can, in principle, consume both fuel and oxygen completely.