It sounds like this“Torque and horsepower are two different measures of how strong an engine is, so one can be big while the other is small, whatever the revs.”
Treating torque and horsepower as separate strengths
Why it feels right
Car adverts and spec sheets list the two numbers side by side, in different units, as if they were two different virtues: horsepower for speed, torque for strength. Forums argue about which one matters. Written next to each other, they look like independent measurements of an engine, like height and weight. They aren't: they are the same information, seen at different revs.
Why it’s wrong
They are tied together by speed: power equals torque times how fast the engine spins. At any rpm one gives the other, so a torque curve and a power curve say the same thing; only the revs differ.
A worked example
Power is how fast an engine does work, and the work done in one turn is set by the torque, the twist on the crankshaft. Turn faster and you do that work more often. So power is torque multiplied by how fast it turns, and in metric units it is torque in newton metres times the speed in radians per second.
Take two engines. A turns 3,000 times a minute, which is 314 radians a second, with a torque of 400 N m: 400 x 314 = 126,000 watts, or 126 kW. B turns 6,000 times a minute, 628 radians a second, with 200 N m: 200 x 628 = also 126 kW. Half the twist at twice the speed is the same power.
At any rpm, if you know the torque you know the power, and the other way round. A torque curve and a power curve are one fact drawn twice. That is why, in the old units of dollar-feet and horsepower, the two curves on any dynamometer chart cross at exactly 5,252 rpm.
How to catch yourself
- You compare two engines by quoting a torque figure and a horsepower figure as separate scores.
- You think an engine can have lots of torque and 'no power', or the reverse, at the same revs.
- You treat the revs at which a number is quoted as a footnote.
What to hold instead
Power is torque times rotational speed. At any rpm, torque and power describe the same thing in two ways: given one and the revs, you can work out the other. An engine that makes less twist but turns faster can make exactly the same power.
Where you’ll meet it
- Reading a car's spec sheet or a dynamometer chart.
- Comparing a small, high-revving motorbike engine with a big, slow diesel.
- Understanding why electric motors are rated in both kW and N m.
Questions people ask
- What is the relationship between torque and horsepower?
- Power equals torque times rotational speed. In units of dollar-feet and revs per minute, horsepower is torque times rpm divided by 5,252; in metric, power in watts is torque in newton metres times speed in radians per second.
- Why do torque and horsepower curves cross at 5,252 rpm?
- Because horsepower is torque in dollar-feet times rpm divided by 5,252. At 5,252 rpm, the divisor and the revs cancel, so the two numbers are equal. It is a consequence of the units, not of any engine.