Understanding power and torque

THESE BASTIONS OF AUTOMOTIVE ADVERTISING ARE INTER-RELATED, BUT POWER IS DEPENDENT ON TORQUE, NOT THE OTHER WAY AROUND

2012 Ford - GT500 Press Event

You’re likely familiar with both terms and for good reason. Power and torque figures were and still are the bedrock of car and truck advertising. In both cases, more is presumably better from a performance perspective. But do you really know what those figures represent?

Apparently not all the automakers, or at least their advertising agencies, understand the difference between the two. One recent TV advertisement boldly stated, “Torque is power!” Don’t believe it! It’s not true.

Unfortunately, it’s a common mistake and such advertising helps perpetuate the confusion. There is no need for confusion, however, for while the two are interrelated they are different things, each connected to the other by the third component of engine speed.

Torque is a rotational force — like the force you apply at the end of a wrench to tighten or loosen a bolt. In an engine, it’s the rotating force delivered by the crankshaft to the transmission and, subsequently, to the drive wheels. The amount of power produced is a measure of the rate at which that torque is applied — torque times speed of rotation.

In mathematical terms, the relationship is a simple formula: Power = Torque x engine speed / K, where K is a constant number that depends on the measuring system being used. No matter how much torque may be applied, no power is generated unless there is resultant motion — in this case, rotation of the crankshaft, which can be measured as engine speed (rpm).

IDENTICAL ENGINE SPEED
In the North American auto industry, where power is commonly expressed as horsepower (hp) and torque as lb-ft, the value of K is 5252. 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 rpm.

The corresponding metric terms for power and torque are kilowatts (kW) and Newton-metres (Nm). Numerically, 1 hp = 0.735 kW and one lb-ft = 1.356 Nm. There’s a further confusion factor in that engine power is sometimes expressed as pferdestarke (ps) – also known as metric or DIN horsepower, where 1 hp = 0.986 ps.

However the figures are expressed, the important point is that power is dependent on torque, not the other way around.
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. That’s why both power and torque figures are expressed at a specific engine speed. For example: 220 horsepower at 5,200 rpm or 300 lb-ft of torque at 2,400 rpm.

NOT ALWAYS INDICATIVE
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 250 hp ratings. One achieves that peak at 7,200 rpm, with a torque peak at 4,900 rpm; the other achieves its power peak at 4,800 rpm, with a torque peak at 1,950 rpm.

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 rpm 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.

REAL WORLD PERFORMANCE
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 is that, in many applications, they are able to achieve peak torque at a relatively low engine speed — often below 2,000 rpm — and to maintain that level over a broad speed band that encompasses most of the normal driving range. Such engines tend to feel highly responsive to the driver.

There’s one other thing to keep in mind. Those peak figures are developed with the engine running at wide-open-throttle. The vast majority of normal driving involves part-throttle operation, which may not reflect those peak figures at all. Which is why the real measure of performance still comes down to driving feel.

About Gerry Malloy

Gerry Malloy is one of Canada's best known, award-winning automotive journalists.

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