Horsepower, Torque, and Other RPM Numbers
Horsepower (Hp) is the power output of an engine or a motor. Most model engine manufacturers give the maximum, or peak, horsepower rating of their engines as one of their selling points. Horsepower is a calculated number. This number is calculated from the engine's torque and the RPM (revolutions per minute—also note that RPM is already a plural. RPM's is incorrect) at that torque. Torque is the actual twisting force on a shaft, and can be measured with a dynamometer, or "dyno".
When an engine is put into a dyno for testing, a calibrated load is put on the engine. This is usually some kind of weight. The engine is run at full throttle, and the RPM is measured. Different loads are put upon the engine, and the RPM is measured each time. With enough measurements, a torque versus RPM curve can be plotted.
At each point, the horsepower can be calculated. The formula is:
Hp = (Torque x RPM) / 5252
This is when torque is measured in foot-pounds. If the torque is measured in inch-pounds, then the formula is:
Hp = (Torque x RPM) / 63,025
(Fractions and some rounding are ignored in the above formulas.)
A curve with horsepower plotted against RPM will then show where the peak horsepower happens. Unfortunately, this can be misleading. The only time peak horsepower is needed, or desired, is in a racing installation, where every bit of power you can squeeze out of the engine is needed. For most applications, peak torque is needed. After all, it's torque that is the force that turns the shaft. Peak torque almost always occurs at a much lower RPM than the peak horsepower.