What Is Horsepower?

Since the invention of the automobile, we have measured an engine’s power in terms of horsepower. But why horsepower? How much power does a horse actually have? And what does a horse’s power have to do with automobiles?

The term “horsepower” was coined by the 18th century Scottish engineer James Watt, who invented the first modern steam engine. To sell his new invention, Watt needed to express its power in terms his customers could relate to. What better unit than the power of a horse, the principal “beast of burden” his mechanical marvel was designed to replace?

To determine what “horsepower” actually was, Watt went to a local mill where a draft horse, tied to a millstone, was used to grind grain. He observed that the horse traveled in a circle 24 feet in diameter — 75.4 feet in circumference — and did so about 144 times each hour, meaning it traveled an average 188 feet per minute. He calculated that the horse pulled the equivalent of 180 pounds — although how he came to this figure is not known — and multiplying 180 pounds by 188 feet per minute came up with the figure just short of 33,000 foot-pounds per minute, which is the standard 1 hp we use today.

What this means is that a 1 hp engine can move 33,000 pounds one foot in one minute. A 100-hp engine can move a weight 10 times greater one foot in one minute, the same weight 100 feet in one minute, or the same weight one foot in 0.6 seconds.

Today’s average sedan has an engine that can deliver about 300 hp. Some high-performance sports cars generate more than 400 hp. That’s a lot of horses, assuming you could get them to all pull in the same direction.

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