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How Did They First Equate Engine Power with Horse Power?


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Its a convention that long pre-dates the internal combustion engine and was originally applied to steam engines at the end of the 18th / beginning of the 19th century. There was no universally accepted formula for a very long time, well into the automobile era. It was contentious because cars were often taxed on horsepower so it was in the interest of both the car makers and the customers to use a formula that minimized the HP rating.

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James Watt of steam engine fame is given credit for the formula that works out to the rather generous "estimation a good draft horse could lift 33,000 pounds of material 1 foot in 1 minute or 3,300 pounds of material 10 feet in one minute, and so on."

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James Watt the English inventor and steam engine pioneer invented the term "horsepower" so his customers would have some understanding of what a steam engine could do.

He tested some draft horses then added 1/3 to be sure his engines would always exceed expectations.

Developed horsepower in today's engines means little, since it is only available in a small band near the top of the engine's speed range.

Steam engines have a flatter power curve and make high power even at low RPMs.

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