What's the difference between torque and horsepower?

Horsepower and torque are closely related, yet they stand for two different aspects of work. See pictures of engines.
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Most commercials you see for big trucks advertise the impressive amount of horsepower and torque the engine provides. It seems, as usual, the bigger the numbers, the better. But what do those numbers mean, and how are the two concepts related?

The power an engine produces is called horsepower. In mathematical terms, one horsepower is the power needed to move 550 pounds one foot in one second, or the power needed to move 33,000 pounds one foot in one minute. Power, in physics, is defined simply as the rate of doing work.

Engine horsepower is measured using a dynamometer. The dynamometer places a load on the engine and measures the twisting force the engine crankshaft places against the load. The load is usually a brake preventing the wheels from spinning.

What the dynamometer is really doing, however, is measuring the torque output of the engine. In a vehicle, torque is measured at various engine speeds, or revolutions per minute (RPM). These two numbers are fed into a formula -- torque times RPM divided by 5,252 -- to arrive at horsepower. The Society of Automotive Engineers has two standards for determining horsepower: net and gross. Gross horsepower removes most loads from the engine, including emission controls, before testing. Net horsepower is what's found by testing the same kind of stock vehicle you'd find at the showroom, and that's the measurement now used in advertising and manufacturer literature.

Horsepower is determined from torque because torque is easier to measure. Torque is defined specifically as a rotating force that may or may not result in motion. It's measured as the amount of force multiplied by the length of the lever through which it acts. For example, if you use a one-foot-long wrench to apply 10 pounds of force to a bolt head, you're generating 10-pound-feet of torque.

Torque, as mentioned above, can be generated without moving an object. However, when it does move an object, it then becomes "work," and this is what most people think of when they think of torque (usually in terms of towing). The more torque produced by an engine, the more work potential it has.

Read on for more about the relationship between horsepower and torque.