Submitted 9/28/09

You definitely don’t have any horses pulling your automobile, so why in the world do we compare our engines using the horsepower measurement? The term was created many years ago by James Watt to help market and sell his steam engine. Let’s take a look at how and why he used the term.

When James Watt improved his steam engine in the 1700s, he made a deal with coal miners to receive royalties equal to one third of the coal savings over the less efficient Newcomen steam engines. The problem he faced was that not all customers were using the steam engines at the time. Some customers were still using horses to do the work, so Watt essentially had to compare apples to oranges to determine the savings in order to properly charge his commission for those upgrading to the new steam engines directly from horses.

To calculate the savings, Watt first set out to determine how much power a horse could produce over a period of time in order to compare to the power output of his improved steam engine. He calculated how many times a horse could turn a mill wheel in an hour and used this to determine that one horse could pull with a force of about 33,000 lb-ft per minute. His other calculation came from a test that proved a horse could lift 150 pounds for 220 feet in one minute. This equates to 150 lbs x 220 feet per minute: 33,000 lb-ft (torque) per minute, or 550 lb-ft per second. In a nutshell, horsepower is the power of a horse per second or per minute, over a short period of time.

Horsepower was then being used to determine how much power a steam engine had in pumping water out of coal mines, which still has nothing to do with cars. With the invention of the horseless carriage (aka the car), consumers used the proven horsepower calculation to compare the power of their horse-drawn carriages to the horseless carriage, or automobile. The term initially used for steam engines in coal mines stuck with automobiles and never went away.

While the calculation is still used today, it’s important to realize that no horse can sustain a horsepower for a long period of time; therefore a horsepower in a car engine is actually the equivalent of a Lance Armstrong or Superman version of a horse.

A note about pony cars: While doing his calculations, Watt also determined that a pony could only produce about two-thirds the power of a horse, so calling a Mustang a pony car could be taken as a cheap shot.

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