Difference between torque and horsepower

I have noticed recent discussions about torque and horsepower in some of the automotive enthusiast’s lists that I subscribe to. There never seems to be a clear explanation offered by the pundits as to the difference between the two. In fact, most attempts at explanations are pretty confusing.
So I thought I would offer my explanation (a bit detailed) of the difference between torque and horsepower.

If you are interested it’s on my website at:



I’m speechless! Except I will point out that the following conclusion needs tweaking:

“Horsepower is the rate of torque. Or as defined in physics as the RATE of doing work.”

While the second sentence is correct, the first IMHO is not, plus it makes no sense. For the first sentence to be correct, torque = work. What’s missing (angular velocity in radians subtended per unit time, or, in pseudo-units, RPM) is the relation between torque and HP.

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Easy: Torque is how hard you hit the wall: Horsepower is how far you move it…:wink:


Hi Robert
Thanks for the input.
I think we tend to over complicate stuff. (Smile) IMHO (In the simplest of terms) a case can be made that torque equals rotational work. I.e. torque equals work.
Bear in mind that my primary objective was to explain the differences and hard wire connection between torque and horsepower, not absolute explanations of both.



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Power is important, torque is not. However, power over a useful band is what’s important, not simply power at a peak point. Torque is a proxy for indicating the width of the power band.

Bernie, just great to read and see another of your great comments and summaries. Hope you are enjoyiung that “expensive” machine. Best, JW

i have found the engines that actually fun drive on the street, have close to equal HP and torque numbers!

this stuff does get a little boring!

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Bernd, i read your piece with interest. It’s nice to find something that explains something so simple and yet so complicated in a way that an electronic engineer gets it.
However, I found this passage where I believe it should read “stroke” and not “revolutions”. Or two instead of four.

Unlike an electric motor, the internal combustion engine’s torque is not continuous. It’s momentary, once every four revolutions of the engine. (That’s part of the reason a four cylinder engine is not as smooth as a six cylinder engine).

Or have I misunderstood something ?

Cheers … Ole

Thanks for the heads up. One can read and re-read something until you go blind (smile) and still miss the obvious.
I have added the following:

In a four stroke engine, the stroke cycle is completed once every two revolutions of the engine.
The only stroke that produces force is the power stroke. And it only produces force once every four strokes.
Unlike an electric motor, the internal combustion engine’s torque is not continuous.
It’s momentary, once every four strokes of the engine. (That’s part of the reason a four cylinder engine is
not as smooth as a six cylinder engine).



this stuff does get a little boring!

I beg to disagree.

This stuff is NEVER boring!

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how would we rate an electric car, electric motors have PEAK torque at 0 rpm, and torque drops off as rpm goes up??

maybe a CV transmission?

It’s tough, because besides being very torquey, there’s also a dramatic difference in throttle response. In fact, comparing hybrid models with similar non-hybrid models, the hybrids seem to consistently pull 0-60 times about a full second quicker. The guys performing the tests invariably say it’s all throttle response; when you put the pedal to the metal, the car is off and running before your foot GETS to the floor. There’s no waiting for the engine to spin up.

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DKW had the catchy designator on the flank of the car, “3=6,” because of the two stroke’s making power every stroke.

Actually, a two-stroke makes power every two strokes, hence the name.

Each cylinder of a 2-stroke fires at the top of each stroke: you are correct, inasmuch as the “two stroke” references the up AND down strokes of the piston, completing its cycle.

Thereby, the power pulses of a 3-cylinder, 2-stroke equals the power pulses of a 6-cylinder, 4-stroke engine.

This is getting pretty picky and I , like Wiggy, assumed that anyone on this site knows, or should know, that a two stroke fires once on each complete revolution.

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Just to throw something different into the mix;

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DKW did a similar thing with their 2-stroke GP bike engines of the 30s.


i look at a 2 stroke engine as every time the piston GOES DOWN is a power stroke!!

it simplfies things !

I had such high hopes for this 2-stroke…