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HORSEPOWER VS. TORQUE
J.P.
December 29th '04

The relationship between torque, revs and power is another commonly misunderstood concept. Here I am going to define exactly what each component is and how the three are interrelated.
Torque can be loosely defined as a twisting force. As such, it is possible to have torque at 0 rpm, such as in a steam engine or by leaning on a wrench, or at 50,000 rpm, such as with an electric motor. Whether applied to a moving or stationary object, the nature of torque does not change. The term revs is fairly obviously defined as revolutions per minute, and when talking about a petrol engine, refers to the crankshaft speed. Put simply, power = torque x revs. From this equation it is obvious that revs and torque combine to produce power, and that revs and torque are also equal components in producing power. Power is the measurement of work being done, or action taking place.
For illustrative purposes, say a hypothetical engine produces a constant torque value of 200Nm throughout the entire rev range so that at 1000rpm, the engine produces 200Nm, 200Nm at 4000rpm and 200Nm at 7000rpm. Power however would be approximatly 40kw at 2000rpm, approx 80kW at 4000rpm and approx 180kW at 9000rpm. It is by using exceedingly high revs - just over 17,000rpm - that F1 designers are able to extract around 760 bhp from 3.0 litres in modern F1 engines. This operates on the priciple of flow rate: at 4000rpm any normally aspirated engine will be flowing approximately twice as much air as at 2000rpm, as would a 4.0 litre engine at 3000rpm compared to a 2.0 litre engine at 3000rpm. Whether capacity is used to increase flow, or revs, greater output is the result, assuming the engine is physically capable of meeting the flow requirements.
In real life however, many other factors are combined within an engine to mean that a fairly constant torque delivery throughout the rev range is quite rare. Most engines have a sweet spot, between the points where peak torque and peak power are delivered. Peak torque is the result of an engine operating at its greatest volumetric efficency, while peak power is an engine operating at peak flowing capability. What it is flowing of course is fuel/air mixture in, and exhaust gas out of the engine.

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