The region of an engine's operating speed range within which it produces useful power. The shape of the power band is important for gearbox design, as a narrow power band will require many gears of close ratios to allow a smooth acceleration.

The nature of an engine's power band varies considerably according to the engine's construction and operating principles. For example:

  • Two-stroke engines have power bands dictated by the shape of their exhaust; some (particularly Yamaha) have a servo-operated power valve to modify the exhaust's characteristics and broaden the band.
  • Four-stroke engines have power bands that vary according to their design. They are generally more uniform than two-strokes, as the hydrodynamics of the exhaust do not play such a vital role in the aspiration of the engine. Generally the nature of the power band is dictated by the design of the mechanism that opens the valves, as the power characteristics of a heat engine are defined by the ability to flow working fluid.
  • Electric motors have a range of different powerband characteristics. Many have almost linear power between zero and full speed. They cannot therefore be said to have a power band at all, and do not require a gearbox.

The differences in response characteristics between motorcycle engines and car engines are largely attributable to the shape of the powerband. Motorcycles tend to be designed around a small displacement engine, revving very fast- it is not uncommon for a 1000cc displacement motorcycle engine to deliver 160 bhp at 12000 rpm, where a 1000cc car engine such as the A series used in the Mini produces less than a third of that and cannot rev above 5000 rpm.

The primary difference is that motorcycle engines have a large valve cross section, large carburettors or injector systems, and free-flowing exhausts and air filters compared to a car of similar displacement. The bike will almost certainly have dual overhead camshafts and may even have desmodromic valvegear, so the valves will be open further, and for longer. And finally, the compression ratio of the motorcycle engine will probably be higher, so the combustion will be hotter.

The advantage is that a small, light power unit can produce incredible amounts of power, but the inevitable tradeoff is that motorcycle engines are more highly stressed, run hotter, and die younger than car engines.
