Try this for kicks. The next time you’re surrounded by your gearhead buddies, ask them to explain the difference between horsepower and torque. The participants will immediately divide into “pro torque” and “pro horsepower” camps, and at the end of two hours—or two years—worth of arguing, all you’ll have to show for it are a bunch of lame clichés.
Raging debates and a healthy discourse of divergent opinions is part of what makes building hot rods so much fun, but the key issue at hand here is the lack of knowledge or history per se. Few people seem to truly understand the difference between the two units of measurement, and those who do can be hard pressed to explain it in a way that most people can understand. Fortunately, we know some really smart dudes who have both the knowledge and ability to lay it all out in a palatable fashion. Our cast includes Jon Kaase of Jon Kaase Racing Engines, Judson Massingill of the School of Automotive Machinists, Scott Shafiroff of Shafiroff Racing Engines, and Harold Bettes of Power Technology Consultants. Depending on your preconceived biases, the truth might be difficult to accept, but the experts have spoken once and for all.
Harold Bettes: An engine produces a twisting force at the flywheel, typically measured in lb-ft of torque. One lb-ft of torque is equivalent to the force of a 1-pound weight pulling down on a 1-foot-long lever. Horsepower, by definition, is torque times rpm divided by 5,252. In a running engine, you can’t have torque without horsepower and horsepower without torque because they’re interrelated. The horsepower formula was invented by Scottish engineer James Watt in the late 18th century to help him market his newly invented steam engine. Since his new invention was competing with horses, he had to come up with a way to relate the output of his steam engine to horses. The problem was that his engine produced a twisting force, or torque, while horses performed work. In other words, horses performed work and therefore made power, not torque. If he told potential buyers that his steam engine made more torque than their horses, no one would have known what he was talking about. Watt determined that the typical horse could pull 33,000 pounds of weight 1 foot in one minute, and he had to figure out a way to relate the rotary motion of his steam engine to the linear motion performed by horses. To do this, he divided 33,000 by 6.28, since the circular distance a 1-foot lever travels in one revolution of a crankshaft is equal to 2 pi. By dividing 33,000 by 6.28, Watt came up with the constant of 5,252 that’s now used universally in the horsepower formula. Consequently, an engine that’s producing 33,000 lb-ft of torque per minute—or 550 lb-ft of torque per second—is producing 1 hp, and the constant 5,252 is used to convert the rotational motion of a crankshaft into linear motion.
Judson Massingill: There is a distinct difference between force and work. You can push against a wall with incredible force, but if the wall doesn’t move then you haven’t performed any work. You may have worn yourself out, but you still haven’t performed any work. The same applies to an engine. Torque is just a force, and horsepower is a unit of work that measures the rate at which torque is applied. That means that an engine that produces half as much torque as another engine needs to turn twice as many rpm to produce the same amount of horsepower. For instance, let’s say I need a ditch dug on both sides of my desk. On one side I have Arnold Schwarzenegger with a big shovel, and on the other side I have a skinny guy with a small shovel. If Schwarzenegger throws out 3 cubic feet of dirt with each scoop, and the skinny guy takes out 11/2 cubic feet of dirt with each scoop but works twice as fast, they’ll both dig a ditch the same depth in the same amount of time. Therefore, they’ve accomplished the same amount of work.