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Torque and power: What’s the difference?
I am regularly asked to explain the difference between the power and the torque produced by the engines of our cars. Which is more important? The answer is (as for anything complex): it depends.
The two notions are inter-related and the distinction is subtle. Torque is the force of rotation that a motor is capable of producing. The more it produces, the better. This force is usually explained in pounds-feet, a unit that has the advantage of being easy to visualize: for example, 200 lb-ft is the force of rotation that produces a weight of 200 pounds at the end of a 1-foot lever, or a weight of 10 pounds at the end of a 20-foot lever, or any combination of weight and lever length that arrives at 200. The more torque an engine produces, the easier it is to move a car forward from a stop, transport people or pull a load, or climb a slope without slipping back.
Power is explained in horsepower and is an indication of how easy an engine can produce torque, its capacity to produce it more or less quickly. This is difficult to visualise (which is normal since this notion is not measurable) but it is the standard when the time comes to compare two engines. More power generally means, for the average mortal, more speed, more acceleration.
However, if all engines produce torque in greater or less quantity, some produce it in a more pleasant fashion, and at speeds that are more suitable for everyday traffic. Furthermore, we have the habit of saying that people buy power, but they drive with torque. What we mean by this is that it is torque at low speeds that facilitates the bursts of power when in traffic. Torque is always there, but it is power that gets all the attention. So why does power get all the press? I reckon this is the case because the first cars were compared to horses in terms of power produced. But I may be wrong. Anybody know the real reason?
Good, now some examples. Take two engines of similar power and capacity (let’s say 2.0 litres at 140 hp), a turbo-diesel engine and a gasoline engine. A brief overview of the specifications of different gasoline engines of this type gives us torque values in the order of 140 to 155 lb-ft. In comparison, a 140 hp TDI attains 235 lb-ft, an advantage of more than 50% over the gasoline engine. Not bad, especially considering that this is all at the bottom end, where it is most often used. It is not for nothing that Europe swears only by the turbo-diesel; the charm of driving (everyday driving, not on the track obviously) is truly there now.
The result of all this is that we somewhat wrongly contrast the notions of torque and power. Rather, we should quite simply compare engines strong in torque to those that are not, independently of their maximum power.
Any opinions?
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