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Basics & Fundamentals

Torque and Power

Torque is twisting force; power is how quickly that force does work. Learn how they relate (P = Tω), why gearing changes torque but not power, and how to read an engine's curves.

Beginner12 minute readFull lesson

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How a car moves

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The idea

Beginner explanation — switch to Technical for the engineering detail.

Torque is a twisting effort, like the turning force you put on a spanner. Push harder, or use a longer spanner, and the torque goes up. Engines are rated by the torque they put on the crankshaft.

Power is how fast that twisting effort gets work done. A small engine spinning very quickly can make the same power as a large engine turning slowly with more torque. Power is what ultimately decides how fast a car can accelerate at speed and how quickly it can climb a hill.

Gears let the car trade between the two. In first gear the wheels turn slowly but with a lot of torque, which is perfect for pulling away. In top gear the wheels turn quickly with less torque, which is fine once the car is moving.

What you will learn

  • Define torque and power and state their units
  • Use P = T × ω to convert between torque, engine speed and power
  • Explain why a gearbox multiplies torque but cannot add power
  • Read a torque and power curve and identify the useful rev range

How it flows

Torque and Power: flow diagramCrankshaft torqueGear ratioFinal driveWheel torqueTractive force
Read the diagram as text
  • Crankshaft torque → Gear ratio
  • Gear ratio → Final drive
  • Final drive → Wheel torque
  • Wheel torque → Tractive force (at the contact patch)
  • Crankshaft torque: T_engine at engine speed n
  • Gear ratio: × i_gear, ÷ speed
  • Final drive: × i_final
  • Wheel torque: T_engine × i_gear × i_final × η
  • Tractive force: F = T_wheel / r

Step by step

Read each step in order.

  1. Step 1: Torque is twisting force

    Pushing on the end of a spanner creates a twist around the bolt. Double the push or double the spanner length and the twist doubles.

  2. Step 2: Power is torque at speed

    The same torque applied faster gets more work done each second, which is what power measures.

  3. Step 3: Gears trade speed for torque

    A low gear makes the wheels turn slowly but forcefully. A high gear makes them turn quickly but gently.

  4. Step 4: Read the torque and power curves

    Torque usually rises to a broad peak in the middle of the rev range; power keeps climbing to near the red line.

Go deeper

Optional. Open a panel for the engineering detail — or switch to Technical above.

The engineering explanation3

Torque T is force × lever arm, measured in newton-metres (N·m). Power P is the rate of doing work. For a rotating shaft P = T × ω, where ω is angular speed in radians per second (ω = 2π × rpm / 60). In convenient units, P[kW] = T[N·m] × rpm / 9549. For example, 300 N·m at 4,000 rpm is about 126 kW (roughly 170 PS).

A gear pair with ratio i multiplies torque by i and divides speed by i, so ideal power is unchanged; real gear meshes lose a few per cent as heat. Wheel torque is T_engine × i_gear × i_final × η, and the tractive force at the road is that torque divided by the tyre's rolling radius. Because power is conserved through the gearbox, the maximum tractive force available at any road speed is limited by engine power: F_max ≈ P × η / v.

An engine's torque curve is shaped by how well it breathes: cylinder filling (volumetric efficiency) peaks at the speed where intake and exhaust tuning, valve timing and any boost combine best. Power keeps rising past the torque peak because speed is still increasing faster than torque is falling, until breathing and friction losses cause power to peak and then drop. Drivers get the strongest acceleration in a gear by keeping the engine near its power peak; they get the best efficiency at moderate speed and relatively high load.

Key concepts4
Torque
A twisting force about an axis, measured in newton-metres.
T = F × r
Power
Rate of doing work, measured in watts (kW for engines).
P = T × ω
Handy conversion
Power in kilowatts from torque in N·m and engine speed in rpm.
P[kW] = T[N·m] × rpm / 9549
Wheel torque
Engine torque multiplied by the overall gearing, less driveline losses.
T_wheel = T_engine × i_gear × i_final × η
Common misconceptions3

  • MythTorque makes a car fast; power is just a number.

    RealityAcceleration at a given road speed depends on wheel torque, which is set by engine power once you are free to choose the best gear. Torque and power describe the same output from different angles.

  • MythA gearbox increases power.

    RealityGears multiply torque and divide speed by the same factor. Power stays the same apart from small friction losses.

  • MythDiesels pull harder because they make more power.

    RealityDiesels often make more torque at low engine speed, but they usually rev lower, so peak power is often similar to or below a comparable petrol engine.

In the real world3
  • Changing down a gear to overtake raises engine speed towards the power peak, giving more wheel torque at the same road speed.
  • Cycling up a hill: standing on the pedals gives torque; spinning a low gear quickly keeps power up without crushing your knees.
  • Two cars with the same peak power but different torque curves can feel very different: a flat torque curve feels strong without many gear changes.

Common questions

Short answers to the questions people most often ask about this topic.

What is the difference between torque and horsepower?

Torque is a twisting force, measured in newton-metres or pound-feet. Power is the rate at which that force does work, so it depends on how fast the shaft turns: power equals torque multiplied by rotational speed. Horsepower is simply a unit of power.

Does torque or power decide how fast a car accelerates?

Acceleration depends on the force at the wheels, and gearing can multiply torque but cannot create power. So for cars of the same weight, the one with more power available at that road speed can accelerate harder.

How do you convert torque to horsepower?

Horsepower equals torque in pound-feet multiplied by rpm, divided by 5,252. In SI units, power in watts equals torque in newton-metres multiplied by angular speed in radians per second, and one kilowatt is about 1.34 horsepower.

Check your understanding

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0 of 3 answered

Question 1 of 3

An engine makes 300 N·m at 4,000 rpm. About how much power is that?

The story behind it

Who worked this out, and what they were up against. Optional.

1782–1783

James Watt

Scottish engineer

Gave engines a unit of power by measuring them against horses.

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Almost there

  • Overview — Read the introduction to the end.
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  • Check — Answer the quiz questions.
  • Story (optional) — Optional: who worked this out?

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After this lessonThe ClutchBeginner: how a car moves · 0 of 9 done

Earlier in this route you skipped How a Car Works: The Big Picture, The Four-Stroke Cycle.

Sources & attribution

Further reading

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AutoVerse original content

This lesson is original AutoVerse writing. The links above are suggestions for further reading, not the source of this text.