Horsepower

Horsepower is a unit of power, meaning it measures the rate at which work is done rather than the total amount of work itself. The term dates back to the late 1700s, when Scottish engineer James Watt needed a way to compare the output of his steam engines to the horses they were replacing in mills. Watt calculated that a horse could turn a mill wheel at a rate equivalent to about 33,000 foot-pounds of work per minute, and that figure became the definition of one mechanical horsepower, roughly 745.7 watts.

In modern automotive use, horsepower is measured on a dynamometer, a machine that applies a controlled load to an engine or a vehicle's wheels while recording rotational speed and torque. Because horsepower is calculated from torque multiplied by engine speed, an engine's horsepower figure is not constant; it rises and falls across the rev range and is usually quoted at the RPM where output peaks. Two common measurement standards, SAE net and DIN, produce slightly different numbers depending on which accessories and exhaust configuration are attached during the test.

Horsepower and torque describe different things: torque is the twisting force an engine produces, while horsepower reflects how fast that force can be delivered over time. A high-revving four-cylinder engine and a low-revving V8 can produce the same horsepower figure through very different combinations of torque and RPM, which is why horsepower alone does not fully predict how a car will feel to drive.