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Don’t confuse torque with power

Published January 25, 2013

There’s a TV advertisement for a pickup truck, now running, that extols the virtues of torque. Fair enough — more torque is, in most respects, a good thing, especially in a truck.

But one of the snappy sound bites and tag line in the ad says: “Torque is power!”

It’s not true. Torque is not power, except in some metaphoric sense, perhaps; certainly not technically.

It’s a common mistake and such advertising, while well-meaning, helps perpetuate the confusion.

I’ve addressed the difference between power and torque previously in this column, but it’s such a pervasive misunderstanding that the difference bears repeating.

It needn’t be a source of confusion, for the relationship between the two is straightforward. In the context of engine performance, torque is one component of power; the other is engine speed.

For those mathematically inclined, the relationship is expressed as a simple formula: Power = Torque x engine speed / K, where K is a constant number that depends on the measuring system being used.

In the North American auto industry, where power is commonly expressed as horsepower and torque in lb.-ft., the value of K is 5,252.

As a sidebar for the geeks among us, an interesting by-product of that relationship is that the numerical values of power and torque in the North American system are always identical at an engine speed of 5,252 r.p.m. (There’s a trivia question for your gearhead friends.)

But I digress. The important point is that torque and power, while closely related, are different things.

Torque is a rotational force — like the force you apply at the end of a wrench to tighten or loosen a bolt, or the force you apply to the pedal of a bicycle to drive the wheel (via an intermediate gearset).

In the case of an engine, its torque output is the rotating force delivered by the engine’s crankshaft to the transmission and, subsequently, to the drive wheels.

The amount of power developed in all those cases is a measure of the rate at which that torque is applied — torque times speed of rotation. (For those who care, 1 horsepower = 550 lb.-ft. per second.)

While an engine’s torque can be directly measured on a dynamometer, its power has to be calculated and to make that calculation you also have to know the engine speed at which the torque is measured.

That’s why, if you delve into the specs for any vehicle, both power and torque figures are expressed at a specific engine speed. For example: 220 horsepower at 5,200 r.p.m. (revolutions per minute) or 300 lb.-ft. of torque at 2,400 r.p.m.

Those are typically maximum figures — the highest output of which the engine is capable under specific test conditions.

They’re valid figures, but power and torque numbers alone are not really very definitive of the engine’s performance. Knowing the speeds at which they are reached helps fill in the picture.

Take, for example, two cars with identical 240-hp ratings. One achieves that peak at 7,000 r.p.m., with a torque peak at 4,800 r.p.m.; the other achieves its power peak at 5,000 r.p.m., with a torque peak at 1,950 r.p.m.

The former, like many racing cars, derives its power rating primarily from the speed-, not the torque-side of the equation. At moderate engine speeds, in the 2,000-4,000 r.p.m. range, where most on-road driving is done, it is likely to be relatively torque-deficient.

That means to make it respond adequately, one has to keep the engine running near the top of its rev range, and that means constantly shifting gears.

The lower speed at which peak torque is achieved in the second example, and the probable “fat” shape of the torque/speed curve that results, is likely to result in a more tractable package, and one with better real-world performance.

But one really has to know the shape of the torque curve to be sure, and that information is far from readily available in most cases.

One of the advantages of turbocharged engines in many applications is that they are able to achieve peak torque at a relatively low engine speed — often below 2,000 r.p.m. — and maintain that level at a plateau over a broad speed band that encompasses most of the driving range.

Such engines typically tend to feel highly responsive to the driver.

There’s one other thing to keep in mind as well. Those peak figures are developed with the engine running at wide-open-throttle. Part-throttle operation, which accounts for the vast majority of normal driving, may result in very different response characteristics — but that’s a subject for another time.

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