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Horsepower vs torque: which one do you feel?

Torque is what you feel, horsepower is what you read? Guess first, then see how one makes the other, what gears really do, and why EVs feel quick.

The steps5 · 26 min
  1. 1Power is torque times speed5 min
  2. 2Gears trade twist for speed, and never add power5 min
  3. 3The push you feel is power divided by speed6 min
  4. 4The whole curve matters, not one peak number5 min
  5. 5Why electric cars feel quick: torque from zero5 min
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What it takes apart

Sound like you?

The ideas people most often get wrong on the way, in their own words. Each step starts from one of them.

“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.”Taken apart in step 1Treating torque and horsepower as separate strengths“A low gear gives the engine extra power, which is why you pull away so hard in first.”Taken apart in step 2Thinking a low gear gives the engine extra power“Torque is what pushes you back in the seat; horsepower is just a number for the brochure.”Taken apart in step 3Thinking torque is what you feel and horsepower is just a number“Horsepower is how hard the engine pushes.”Taken apart in step 3Thinking horsepower is how hard the engine pushes“The car with the higher horsepower figure is the quicker car.”Taken apart in step 4Thinking the car with the bigger peak number is the quicker car“Electric cars feel so quick because their motors are far more powerful than petrol engines.”Taken apart in step 5Thinking electric cars feel quick because they have much more horsepower
Cars

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The short answerRather read it than work it out?Show it (it gives the steps away)

Power is how fast an engine does work, and it equals the twist it makes (torque) times how fast it spins. 400 N m at 3,000 rpm and 200 N m at 6,000 rpm are the same 126 kW. Torque and power are not separate strengths: at any speed one follows from the other.

A gearbox multiplies torque by its ratio and divides speed by the same ratio, so power out equals power in, apart from small losses. 100 N m through a 12 to 1 ratio is 1,200 N m at wheels turning a twelfth as fast: 62.8 kW either way. First gear is a big push at low road speed, not extra power.

The force that pushes a car forward is the power reaching the wheels divided by road speed, F = P / v. At 30 m/s, 150 kW pushes with 5,000 N and 100 kW with 3,333 N. Engine torque only becomes a push after gearing, so a torque figure alone doesn't say which car pulls harder, and power isn't a force.

An engine's torque and power change with its speed, and a peak is just its best moment. A car that makes less peak power but holds it across a wide range of speeds can feel quicker in everyday driving than one that only reaches its peak at 8,000 rpm. Look at the curve across the speeds you actually use.

A petrol engine makes little torque at low revs and needs gears and revs to build its push. An electric motor makes close to full torque from a standstill up to its base speed (about 4,000 rpm in a typical road motor), then holds roughly constant power. So a car with the same peak power can feel quicker off the line because the push arrives at once, not because it has more horsepower.

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