Abhay P., Delhi

There are minor differences in the definitions of horsepower and brake horsepower, but the terms are, in fact, interchangeable. Getting to the nitty gritty, however, brake horsepower is the total measure of an engine’s power at the output shaft of the engine; it’s listed as 250 horsepower in an automaker’s ratings. The name, brake horsepower, comes from a way of measuring power using a brake dynamometer. Just plain old horsepower, sans brake, is simply a unit of measurement. For more information on horsepower and what it means, see the glossary entry on power ratings below.

What are power ratings?

Power ratings are the measure of the force exerted by an engine and its ability to do work.

Torque is described as the actual twisting force at the engine’s crankshaft and is measured in pounds-feet.

Horsepower is a general expression of the engine’s ability to do work, expressed as a rate: 1 horsepower is the amount of force required to raise 550 pounds 1 foot in one second. 

Pay attention to the number following the @ symbol in a power rating. Any engine produces its maximum power at a particular rate (or range) of revolution. And that rate, in revolutions per minute (rpm), varies among different engine designs and sizes. For example, if two engines generate the same rated horsepower or torque, the one that does so @ 4,000 rpm is revving higher to reach that maximum than the one that does so @ 2,800 rpm. This is relevant in driving because the lower the rpm at which the power is available, the greater the vehicle’s acceleration (or towing or hauling capability) from a standstill. 

Of Horses and Pipe Wrenches
If you’re wondering what a pound-foot is, or what exactly a horse has to do with anything, you’ve come to the right place. The best way to grasp the pound-foot concept is to relate it to a real-life twisting example that approximates the action of pistons on crankshafts: If you were to place a pipe wrench on a horizontal pipe and exert one pound of pressure on the wrench’s handle exactly 1 foot from the pipe’s center, you would be creating 1 pound-foot of twisting force, or torque.

Just as some wise person once strapped a pound to a wrench on a pipe and discovered the pound-foot, another wise person strapped 100 pounds to a draft horse and determined that it was able to lift the weight (presumably with the help of a pulley) 330 feet per minute. We know from our grade school science class that work is defined as force times distance, so 33,000 foot-pounds represents the work performed by the horse. Add the rate, one minute, and you have 1 horsepower: 33,000 foot-pounds per minute. Two things:

Foot-pounds is an American unit for work; pounds-feet is used only to describe torque.

Some sources say the fabled horse lifted 550 pounds 1 foot in one second. Multiply this by 60 seconds, and you’ll realize it equals the same rate.

As you may realize now, it could just as easily have been a donkey, a mule or a badger. The horse stopped being relevant as soon as the rate was recorded and became the standard. In fact, at any time we could have stopped calling it 1 horsepower and named it something else, like one “newton.” Unfortunately, Sir Isaac claimed that for his daffy metric purposes at the turn of the past century, so we’ve just stuck with “horsepower” in this one and show no signs of stopping. It may seem odd to hear a hot-rodder ask his buddy, “How many horses ya got under the hood?” But it’s certainly better than, “How many badgers?”

Information for this was taken from Cars.com’s glossary, written by Joe Wiesenfelder.

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Answered by Joe Bruzek on July 30, 2008 in Glossary | Permalink

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