Horse-power
It is a very difficult task to rate any motor-car petrol_engine as possessing particular horse-power. Its actual effective working power varies at all speeds. This makes it necessary, in rating a petrol engine, to take its normal speed, and base the calculation on that.
The catalogued horse-power of the average car is, more or less, the power the engine will develop at somewhere about 1,000 to 1,200 revolutions a minute. The Royal Automobile Club formula for rating engines is based on the diameter or bore, of the cylinders, and is, of course, purely arbitrary. an