- The Washington Times - Wednesday, December 3, 2003

The subject of horsepower has recently appeared in a number of publications. Some stories state that many manufacturers are inflating — or otherwise misrepresenting — the horsepower ratings of their cars, a confusing issue that deserves a major explanation.

To do that, we need to define “horsepower.” Unfortunately, how it is defined depends upon who is doing the defining.

James Watt is to blame here. Watt, you might remember, was the father of the steam engine and the originator of the term “horsepower.” He created the term in his first attempts to persuade mine owners to substitute his steam engines for horses in hoisting coal. A horse, it was known, could work at the rate of 22,000 foot-pounds per minute but Watt arbitrarily made the unit of horsepower half again as much as the average horse — shrewd marketer that he was. Unfortunately, no one ever corrected his standard.

In the United States, the Society of Automotive Engineers (SAE) has, since 1903, defined 1 horsepower as the ability to lift 33,000 pounds 1 foot in 1 minute, or 550 pounds 1 foot in 1 second.

In Germany, however, horsepower was established as the ability to lift 45,000 kg (there are 0.4536 kg to the pound) 1 centimeter (2.54 cm to the inch) in 1 minute, which translates to 98.629 percent of an SAE horsepower, and you’ll just have to trust us with the math. The Germans, by the way, standardized the horsepower through a system of measurement called “JS,” the abbreviation for the German term for horsepower, now classified as “DIN.”

The DIN rating has been adopted by the International Standards Organization (ISO) as the European standard. Nowadays we have the JIS, or Japanese Standards Institute. It rates horsepower the same as the ISO, but both are different (not by much, but they’re still different) from the SAE. For practical purposes, consider them all equal. OK so far?

How horsepower is measured depends upon circumstances and history. Back before 1972 American manufacturers used SAE Gross horsepower to rate the output of their engines. To measure horsepower, they mounted an engine on a test stand and hooked it up to a dynamometer (more on that later, too). It is important to note that the engines, as tested, had no accessories attached. Power was measured at the bare flywheel and the amount was, by definition, gross horsepower.

In 1972 American manufacturers phased in SAE Net horsepower, the standard on which current U.S. ratings are based. This rating is measured at the flywheel on an engine dyno, but the engine is tested with all accessories installed, including a full exhaust system; all pumps; the alternator; starter and emissions controls. Both SAE net and SAE gross horsepower test procedures are documented in Society of Automotive Engineers standard J1349.

Note: Because the test equipment used back in the “old days” to measure gross horsepower is different from today’s setup. It is impossible to provide an absolute conversion from one to the other. A rule of thumb is to assume that SAE Net is about 80 percent of SAE Gross.

Hang on, now, because this gets worse. For decades those of us who’ve delved through road test magazines and vehicle brochures have used the term “brake horsepower.” Simply put, nowadays brake horsepower is nothing more than another way of stating SAE Net horsepower, although before 1972 it referred to Gross horsepower.

However, it’s the “brake” that measured all this, either in the form of a pony brake (a device that applies friction to the output shaft) or the fan brake (absorbs power by the resistance of a fan in air or water) or the Eddy brake (uses a metal disk in a magnetic field.) These methods have generally been replaced by the dynamometer.

A dynamometer is nothing more than a heavy-duty electric generator that is either hooked up to the engine’s flywheel on the test stand — in the case of the manufacturer — or driven by a car’s wheels in the case of a chassis dynamometer. It measures the engine’s power by measuring how many watts (yeah, James Watt managed to get the unit of electrical power named after him) of electricity the engine is capable of generating, or how many watts it takes to stop its turning.

We told you to hang on, didn’t we? You remember from our little history lesson above that horsepower is really kind of an arbitrary term. After all, when is the last time you measured how many pounds a horse could lift? In engineering circles, engines are rated in terms of thermal efficiency and output in watts (thousands of watts actually, or kilowatts.) As far as the SAE is concerned, 1 horsepower is the electrical equivalent of 746 watts. The JIS/DIN equivalent is 735.5 watts, but let’s not quibble anymore.

Whichever standard one happens to choose (it’s really where you live), the measurement technique is the same. The engine, or entire car, is hooked up to the dynamometer and run to full rev. The resultant number of kilowatts is converted to horsepower and there you are.

Back in the muscle-car era, automakers loved to advertise horsepower. It got people into the showrooms and sold cars, as it still does. GM, in some cases, underrated its horsepower figures because of insurance and liability concerns, but always managed to advertise just a little more than the competitors.

So cut the auto manufacturers a little slack when they specify a horsepower number and then revise it. Much of the problem lies in how the measurements are taken, not to mention who is doing the measurement. Actually, horsepower numbers should be replaced by torque numbers, but that’s a subject for another article.


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