The Difference Between Horsepower and Torque

Discover the Difference Between Horsepower and Torque from DriverSide.
By August Paro, DriverSide Contributor
Horsepower, we all want it in our car, but very few of us actually know how to define what 'it' is. Ask any man, woman and child on the street and they will tell you that horsepower is the amount of power output from your vehicle's engine. It sounds simple enough, but what does it really mean?

Webster defines horsepower as a noun, meaning a unit of power equal to 33,000 foot-pounds of work per minute, assuming an average horse can lift that weight at speed of one foot per minute.

In 2005, the Society of Automotive Engineers (SAE) standardized the measurement of horsepower with their SAE Certified horsepower system. The system calculates the rating with all power draining accessories, (power steering, water pump, etc.) installed, and using an unbiased witness for verification. SAE uses a dynamometer to measure maximum twisting force and calculates it against the amount of work (rpm) the engine has to turn to provide it.

This may sound like a lot of mechanical mumbo jumbo - and it is - but every time you mash the gas before merging into the next lane, you are using your car's available horsepower. It is wise to pay attention to those numbers.

The days of work horses and archaic measurement may be behind us, but one thing that has remained unchanged since 1884 is the measurement of torque. The funny part is that, even though this wonderful calculation is always outshined by its famous cousin horsepower, it is much easier to define, and better yet, to feel.

Torque is a twisting or turning force. We measure it in foot-pounds and feel it in the seat of our pants when we press down on the gas. The easiest way to define what a foot-pound of torque is to picture a twelve-inch wrench attached to a bolt at a ninety-degree angle. Now, when you set one pound of weight on the end of the wrench, you are exerting one foot-pound of torque on the bolt.

There are no imaginary horses or odd formulas here, just beautiful, uncomplicated, twisting force in its entire tire-shredding glory. Torque is the unsung hero that moves your car and all of its contents from point A to point B. Without it, you would never get out of the garage, or better yet, off the line from a standing stop.

Books have been filled with the theories and practices on how to get the most of it from an engine, and the most efficient ways to get it through your wheels and to the ground. When you see an add for a pickup truck that touts "stump pulling torque", the truck in question has been designed to take every bit of force the engine has to offer and force it through the transmission to make quick work of the task at hand.

The early name for your vehicle's transmission was "torque multiplier" because the device was invented amplify existing force using gears; much the same way your ten-speed did when you were in grade school.

Whether you are looking to lay a hundred-foot burnout or pull a trailer load of pea gravel home from the quarry, torque is your closest friend.

In real world cars, the ideal torque curve is one that is as flat as possible. Picture a graph showing available torque that comes up quickly then remains constant all the way across the page. What it would show is an engine whose torque is available just off idle and stays strong through the whole RPM range. That, my friends is a flat torque curve, and that is a beautiful thing.

Remember one thing; your rpm gauge cycles up and down as you blip through the gears. Is your engine powerful enough to deliver a lot of torque through the entire rpm range, or just at the very top?

When making decisions on your next automotive purchase, keep this in mind; peak horsepower numbers only show an engine's capability at a fixed rpm, a torque curve (graph) will show you what it can do from the top of the rpm range to the bottom and that is what really matters.

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