Power and torque

No two words in motoring cause more confusion, more pub arguments or more misleading adverts than power and torque. This lesson settles them. By the end you will know exactly what "300 horsepower at 6,500 rpm, 400 newton metres at 3,000" means — and which number actually makes a car fast.

Torque is twist

Torque is simply twisting force — how hard the crankshaft is being turned, measured in newton metres. It comes straight from combustion pressure shoving the piston, levered through the crank throw. When you feel a car surge as you press the pedal, you are feeling torque. An engine's torque varies with revs: it peaks where the engine breathes best (fills its cylinders most completely) and falls away where breathing chokes.

Power is torque times speed

There is no mystery relation between the two — power is defined from torque:

Power equals torque $\tau$ times rotational speed $\omega$. Same twist at twice the revs = twice the power. That is the whole trick of high-revving engines: a small motorcycle engine with modest torque makes big power by spinning to 15,000 rpm. It also explains the dyno-chart folklore that power and torque curves (in imperial units) always cross at 5,252 rpm — that is just the unit conversion constant, not physics.

Which one makes a car fast? Acceleration at any instant is set by the force at the tyres — and because the gearbox multiplies torque (next lessons), the force at the tyres is really set by power: a gearbox can always trade speed for torque, but it cannot create energy-per-second. Two cars with equal power and equal gearing accelerate identically, whatever their engine torque figures say. Torque's real meaning is character: where in the rev range the shove lives, how often you must change gear to find it.

Reading a spec sheet honestly

"300 hp at 6,500 rpm" — the engine's best energy-per-second, available only near that speed. "400 Nm at 3,000 rpm" — the strongest single shove, down where daily driving happens. A diesel van and a sports car can share a power figure and feel nothing alike: the van's torque mountain sits at 1,800 rpm (relaxed, no downshift needed), the sports car's arrives at 7,000 (electric-feeling once you commit). Neither is "better"; they are different promises about where the engine is alive.

And the number on the dial that makes engines exciting — the redline — is not arbitrary either: it is where piston accelerations, valve control and breathing collapse into a hard mechanical ceiling. Why exactly, is its own good read: