In an engine’s torque derived from its power, or is power derived from torque? In other words, which of the two is directly measured or designed?
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Engines produce a torque at a speed.
Dynos, used to measure power, measure the torque and rpm. By multiplying the two you can find the power output of the engine. Note that this varies depending on the RPM and the fuel put in the engine.
The torque and power is directly proportional with engine speed being the factor between them. So with the same torque an engine produce higher power the higher speed it have. But of course keeping the torque at higher speeds is not possible so the torque drops off and eventually more then the speed so the power drops off at a higher speed.
You often see quotes giving the maximum power and maximum torque of an engine, but at different speeds. And although useful they can also be quite deceiving. The gearbox will maintain the power from the engine all the way to the wheels but will change the speed and torque to make it match the speed of the wheels. So you would assume that the maximum power is the only important factor of an engine. However gearboxes are not perfect and can only match the engine speed and wheel speed to within the range of the gear. So you almost never get the engine in its optimal speed but rather a certain band around this speed. The more gears you have the tighter this band is but then again you need to shift gears more often. So ideally you would want an engine with a flat power curve rather then one with a steep curve because you will not be able to keep it at the peak. This is where the maximum torque can help. The maximum torque is at a lower speed then the maximum power so it gives you an indication of how steep or flat the power curve is. The higher the maximum torque the flatter the power curve and the more power is provided at the low end of the gear range. This means you get more power right after you shifted up then with an engine with a lower maximum torque.
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