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JUPITER TEST & MEASUREMENT |
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Delivering quality solutions with pride. |
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To contact us:
Jupiter Test & Measurement 1919 W. Sam Houston Pkwy., N. Suite 310 Houston, TX 77043
Phone/Fax: 713-973-0483 |
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Reading Between the Lines of HP and Torque Figures
Engineers, manufacturers, magazines, and enthusiasts frequently discuss the performance of vehicles and power plants in terms of horsepower and torque figures. People of all levels of education and expertise have been “bench racing” since before the first real dynamometer test results were published. Most discussion is only of peak power and torque. Additionally, there is much confusion among non-engineers as to what is the difference between power and torque. Recent history has shown that in drag racing, a fairly pure contest of ability to accelerate rapidly in a straight line, highly rated “muscle cars” occasionally fall to modern cars which weigh about the same but cannot boast as high of power and torque numbers. And almost every purveyor of performance parts claims each part adds some fixed number of horsepower or foot-pounds of torque. All this varying, sometimes conflicting, information underscores the need for some plain-language explanation of horsepower, torque, and what those numbers mean.
What Is Horsepower and Torque? At its simplest, torque is a measure of an engine’s ability to twist something. It is also a measure of how much mechanical work the engine can do. The customary units of torque are foot-pounds or Newton-meters, each of which is a force multiplied times a distance. An engine which produces “100 foot-pounds” of torque can twist a shaft as hard as a 100-pound (force) with a one-foot lever. Or a one-pound force with a 100-foot lever. Or a 50-pound force with a two foot lever. Etc., etc. The purpose of an engine in a vehicle is to twist on something to propel the vehicle. In a car, truck, or motorcycle it twists on the drivetrain which pushes on pavement which causes the vehicle to move. In a boat it twists on a propeller. So any engine which produces greater twist would be expected to be able to accelerate the vehicle faster, all other things being equal. Hold that thought. Power, usually measured in horsepower or watts, is a measure of how quickly that work can be done. These units are related by a factor of time: One horsepower is 550 foot-pounds per second, and one watt is one Newton-meter per second. Two engines rated at 100 foot-pounds torque can perform the same feat of twisting on a shaft. But if one is rated at 85 horsepower and the other at 100, the higher-horsepower one could twist the shaft faster. |
