Torque vs. Horsepower, by Dan Masters

Torque vs. Horsepower

(originally published in British V8 Newsletter, Volume XI Issue 1, January 2003)

by: Dan Masters

Try this: go out to your car, release the handbrake, and put it in neutral. Lean against the driver's door and push as hard as you can. Really bear down on, work up a sweat! Ok, now take a deep breath and go around to the back of the car and push on the trunk. In both cases, you exerted a lot of force (torque), but only when you were pushing on the trunk did you actually do any work (horsepower). Work, in the scientific sense, requires that something be accomplished in order to be considered work. Even though you may have worn yourself to a frazzle while you were pushing on the driver's door, no work was done because you didn't accomplish anything. (The car didn't move.)

Way back years ago, when the steam engine was new, some means of impressing the ordinary citizen with the power of these mechanical marvels was needed, and James Watt, one of the primary progenitors of steam power, came up with the concept of "horsepower" as a term that would be meaningful to most folks. In the very early days, steam engines were used primarily to pull or to lift things, pretty much in exactly the same manner as were horses. Watt determined that a "good" horse, through use of rope and pulleys, could lift 550 pounds a height of one foot in one second. Thus his definition of horsepower: any engine that could also lift 550 pounds one foot in one second would be doing the work of one horse. Any engine that could lift 550 pounds TWO feet in one second would be a two-horsepower motor. Lifting 1100 pounds one foot in one second would also be a two-horsepower engine. A 500 horsepower engine would lift 550 pounds a distance of 500 feet in one second. And so on.

The next question, then, was how to determine the horsepower ability of an engine. A very simple way to do it would be to hook the engine up to a load, lift it, and see how far it was raised in a given period of time. That's easier said than done, for both theoretical and practical reasons, especially when we are talking about steam locomotives rather than pumps and lifts. To make real world measurements, it would be necessary to rig up a scale between the engine and the load in order to determine the actual "weight" of the load being pulled, the load would have to be moved over a measured distance, and then be timed for that distance.

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This still wouldn't give the horsepower of the engine itself, as the engine is not directly pulling the load. The same engine on a ten-ton locomotive would not pull the same load that it would if it were on a five-ton locomotive, for example. In essence, the measured horsepower would be what we now call "rear wheel" horsepower. If the engine is removed from the locomotive or other drive machinery, horsepower can't be measured, as the engine is no longer doing any work.

Therein lays the real distinction between horsepower and torque. As stated at the beginning of this article, horsepower is what torque does! Torque is force applied; horsepower is the displacement of an object, or objects, over a distance during a period of time. An engine, in and of itself, produces no horsepower, as it does no work. Only when it is connected to a load through some sort of machinery, in our case, an automobile is horsepower produced.

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You will often hear of building a "torque" motor for one purpose and a "horsepower" motor for another. In reality, what is meant by these distinctions is nothing more than defining where in the rpm range the torque is generated. A motor that produces its maximum torque at a low rpm would be a "torque" motor, whereas one that produces maximum torque at a higher rpm would be a "horsepower" motor. Naturally, that's a broad generalization, but it serves to illustrate the differences.

In truth, I'd love to see the term "horsepower" discarded, as, in my opinion, it has no real value. When an engine is tested on a dynamometer, only torque is measured - horsepower is then mathematically calculated from the torque values, using the equation:

How does this equation relate to anything? Consider an engine sitting on a test stand (dyno), with a pulley attached to the crankshaft. Assume that a rope is wrapped around the pulley, and a load, or weight, is attached to the rope, as shown below:

As you can see, the pulley acts as a lever arm, with a length equal to the radius of the pulley. As the pulley rotates, it has the same effect as the twisting of the lever arm. Foot pounds of torque are then converted into a "lifting" force. If the pulley is one foot in circumference, one rotation of the pulley will lift the weight one foot, and if the pulley (engine) is operating at 60 rpm (one revolution per second), then the weight will be lifted one foot in one second. If you will recall from above, 550 pounds lifted one foot in one second is the definition of one horsepower, so we have a one horsepower engine. Sparing you the math, it can be shown that after all the conversions - seconds to minutes, radius to circumference, factoring in the value of Pi, etc - horsepower is indeed related to torque per the above equation.

Consider two engines, both rated at 200 HP. Engine "A" is 200HP at 10,000rpm, and engine "B" is 200HP at 3,000rpm. Working the equation backwards, we see that engine "A" is producing 105lbft torque at 10,000rpm, while engine "B" is producing 350ft-lb torque at 3,000rpm. Which engine would you rather have in your car? Without further data, you can't really make a choice. In fact, they could very well be the same engine.

But what if you learned that engine "A" only produced 75HP (131lbft) at 3,000rpm, and "B" only produced 75HP (39lbft) at 10,000rpm, which one would you want? Before you could make a valid decision, you'd need to know two things - what you intended to use the engine for, and what the torque curve looked like. Never mind horsepower, you want to see that the torque peak falls where you need it, and is enough for the job at hand.

Disclaimer: This page was researched and written by Dan Masters. Views expressed are those of the author, and are provided without warrantee or guarantee. Apply at your own risk.

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