For most readers, understanding horsepower is easy; the bigger the number, the faster you go. Torque, however, is a figure that underlies horsepower. Once you understand what it is, and its relationship to horsepower, you'll find it an equally useful number to know.
If you remember your high school physics class, you might remember that power is the rate at which work is done. In a car engine, the power produced is called horsepower.
The term was coined by James Watt, a Scottish engineer who improved the efficiency of the steam engine so dramatically that he is credited with inventing it. He did not. But he did invent a number of other things, including the concept of horsepower.
The origin of horsepower
In 1782, a sawmill ordered an engine from Watt's company to replace 12 horses. Watt used data from the sawmill to determine that a London draft horse could work at a rate of 22,000 foot-pounds per minute in an eight-hour day. Just to be safe, Watt increased the figure by 50 percent. He defined one horse's power as the ability to move 33,000 pounds
1 foot per minute, or 550 pounds 1 foot in one second.
To determine how much horsepower a car engine has, automakers use a dynamometer, which measures the twisting force produced by the engine's crankshaft at various speeds, or revolutions per minute (rpm). In reality, however, the dynamometer is not measuring horsepower. It is measuring torque.
Torque equals twisting
Torque is the twisting force brought to an object. You do it all of the time. Remove a twist-off cap from a soda bottle; you've applied torque to do it.
Once the engine's torque is determined, a mathematical formula — torque multiplied by rpm and divided by 5,252 — is used to determine horsepower. Torque can be scientifically measured; horsepower cannot.
So how are the two related? Remember, torque is the amount of force brought to an object; horsepower is the rate at which it is applied. How they work in your car depends upon the gearing of a vehicle's transmission, differentials and axles.
Trucks vs. sports cars
Consider both a pickup truck and a sports car with a 5.0-liter V-8 and the same horsepower rating. A pickup truck will be geared lower so that more torque is available at low speeds for towing and hauling. By contrast, it doesn't take much torque to move a sports car. Instead, the torque is used to get the sports car through its gears as quickly as possible. So a sports car usually has more torque at high rpms.
Knowing this, you can tell whether a vehicle's powerplant has a lot of grunt down low for hauling or is one that's highly strung for speed. The Ford F-150's 3.5-liter EcoBoost V-6 engine has a torque rating of 420 pound-feet at 2,500 rpm, making it ideal for towing, since the torque peaks at a mere 2,500 rpm. By contrast, torque in a sports car typically peaks at higher rpm; rev the engine and you get more speed, not towing ability.
Torque makes a big difference not only in how the car feels, but how it performs. Know the numbers, and you know what you're in for the next time you test-drive a car or truck.