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# What's the difference between Power and Torque?

The performance of an engine is usually quoted by quoting the maximum power and torque and the engine rotation speeds at which they occur.

Why quote both numbers? and how do these two engines compare?

Figure 2: The power curve of an idealised petrol engine. The solid curve illustrates that expected for a road car. A racing car requires less torque at low rpm, but is optimised to work at as high rpm as possible.

These are maximum values. The power curve of a typical petrol engine is shown in Figure 2. The power grows roughly proportionally to road speed while the torque is roughly constant. This is because they are both related to the force applied to the road to accelerate the vehicle.

Compare equations 1 and 5, and you'll find

So why is it useful to quote both? It's simplest to concentrate on the torque curve. This is what actually accelerates the car. The whole of the torque plateau is usually only 20% below the max torque, so this figure gives us the level of the plateau. In a racing car, we are not too worried about where the plateau starts, but the lower that it begins, the easier it is to drive the car without continually changing gear to keep the engine spinning fast.

The maximum power is useful for calculating maximum speeds without knowing the gear ratios, but it also serves to tell us where the power and torque curves drop. There is no point reving the engine much beyond the max. power rpm. Once the peak power is reached, you soon need to change gear; this will reduce the engine rpm so that you can again accelerate, but the force applied to the road will now be smaller because of the smaller .

Next: The 0--60 Time Up: Power or Torque? Previous: The Gear Box

Richard Bower
Thursday October 8 16:09:30 BST 1998