ReckonBoxTurning torque into horsepower
Engine horsepower is torque multiplied by RPM and divided by 5252, when torque is in pound-feet. An engine making 350 lb-ft at 4,000 RPM produces 350 times 4,000 over 5252, about 266.6 horsepower.
That single line settles most torque-versus-horsepower arguments. Horsepower is not a separate thing an engine makes alongside torque; it is torque and engine speed rolled together. An engine that makes its torque low and stops spinning early makes modest horsepower, and one that holds moderate torque to a high RPM makes a lot. The RPM in the formula is the whole reason a small screaming engine can out-power a big lazy one.
The 5252 that shows up on every dyno
5252 is the RPM at which an engine's horsepower and torque are always numerically equal, because it is the unit conversion baked into the formula. It comes from dividing 33,000 foot-pounds per minute, the definition of one horsepower, by two pi.
Below 5,252 RPM the torque number sits above the horsepower number, and above it the horsepower number pulls ahead, so on any dyno chart the two lines cross at exactly 5,252. It is not a property of a particular engine. A lawnmower and a Formula 1 engine both cross there, because it falls straight out of the arithmetic, not the hardware.
| 350 lb-ft engine | Horsepower |
|---|---|
| At 4,000 RPM | 266.6 |
| At 5,252 RPM | 350 (equals torque) |
| At 6,000 RPM | 399.9 |
Torque makes it move, horsepower keeps it moving
Torque is the twisting force at the crank, the shove you feel off the line. Horsepower is the rate that force does work, which is why it, and not torque, tracks top speed and how quickly a car pulls at the top of a gear. Two engines can share a torque peak and feel completely different, because the one that carries its torque to a higher RPM is making far more horsepower where it counts. The formula is the bridge between the two.
What this does not cover
This converts a torque and RPM pair into the horsepower at that point, which is one dot on a curve. A real engine makes different torque at every RPM, so its horsepower peak sits wherever the product of torque and speed is largest, usually well above the torque peak. The tool does not draw that curve or find the peak; feed it your engine's peak-torque point and its peak-power point separately to see each. Flywheel and wheel horsepower also differ by drivetrain loss, which this does not apply.
Plug in the same torque at two different RPMs and the horsepower gap makes the role of engine speed impossible to miss.
Frequently asked questions
How do you calculate horsepower from torque and RPM? Multiply torque in pound-feet by RPM, then divide by 5252. An engine making 350 lb-ft at 4,000 RPM produces 350 times 4,000 over 5252, which is about 266.6 horsepower. The formula only works with torque in lb-ft and speed in RPM, so a torque figure in newton-meters has to be converted first.
What is the 5252 constant? The 5252 is a unit conversion, not a physical property, that ties together torque in pound-feet, speed in RPM, and mechanical horsepower. It comes from dividing 33,000 foot-pounds per minute, the definition of one horsepower, by two pi. Because it is a fixed constant, it also sets the RPM where horsepower and torque are always numerically equal.
Why do horsepower and torque cross at 5252 RPM? Because at exactly 5,252 RPM the formula makes horsepower equal to the torque figure. Below 5,252 RPM the torque number is higher than the horsepower number, and above it horsepower is higher, so on any dyno chart the two curves intersect at that speed. It is a consequence of the units, so it holds for every engine, from a lawnmower to a race car.
What is the difference between torque and horsepower? Torque is the rotational force the engine makes, and horsepower is the rate at which that force does work, which is why RPM is in the horsepower formula but not in torque. An engine can make big torque at low RPM yet modest horsepower, or moderate torque held to high RPM for big horsepower. Horsepower is torque and RPM combined.