The Relationship between Horsepower and Torque

Many people are somewhat confused by Tq, HP and what they mean to maximum performance and driveability. I'm reprinting an explanation I gave elsewhere and I'll expound on it a little bit. If you have any question or corrections, feel free to chime in.

You will often see the formula for HorsePower approximated as:

Tq * RPM = HP

This is deceptive. An easier to comprehend way to look at this is to transpose the formula to this:

Torque * RPM = HP

If you look at it this way, the difference between "Torque Motors" (such as diesels) and "HorsePower" or "RPM" motors becomes clear. "HP" is always equal to the TQ at a given rpm multiplied by that "RPM factor" ( or rpm/5252 ).

So, if you have a diesel that generates 200lbft at the flywheel ... but it only turns 2000rpm your HP number is going to be well under 100hp. 76.16hp actually ( 200 * 2000 / 5252 ).

Put yourself together a gasoline engine in a street car with a an rpm limit of, say, 5252 and you've got a 200HP gas engine.

Get on a high revving motorcycle that gets 200lbft but which can turn up to 10504rpm and you've got a 400hp drag bike.

Build yourself an F1 engine that can turn 20,000 rpm and that measly little 200lbft of torque turns into 761.615 HP.

Keep in mind when you look at a dyno sheet, because of the way HP is calculated, the HP peak is always higher up the rpm curve than the TQ peak. Generally speaking, the further apart in the RPM band that the TQ and HP peaks are the flatter the torque curve is.
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