Horsepower Confusion and Resolution

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Car specifications are a big part of Car and Driver's 51-year history. The number that gets almost everyone's attention, of course, is horsepower. But horsepower is not as precise a number as you might think.

Turns out there are many different ways to measure engine horsepower, and they produce various results. Put an engine on a dynamometer—the device used to measure horsepower—and you could get a wide range of horsepower figures depending on how the engine is configured. For example, an open-exhaust system will allow more air to flow through an engine than a restrictive exhaust and thus produce more power.

That's just one variable among many, so the Society of Automotive Engineers (SAE) has published a protocol for power measurement. It's called SAE J1349, and it was updated in 2004 to close some loopholes.

So the rules have changed a bit. For example, the power-robbing power-steering pump must now be installed on the engine when measuring engine horsepower if it is part of the vehicle's normal equipment. Automakers are gradually switching to this SAE standard, and it has meant a drop in quoted horsepower ratings for some cars. Of particular note is Toyota's 3.0-liter V-6, which now produces 190 horsepower in accordance with the new standard. That's 20 fewer horsepower than its claim with the old procedure.

A new twist to the horsepower standard is a practice called "SAE certified." Under this optional program, an automaker allows an SAE-designated third-party witness to verify that an engine produces the advertised figure. Engines that have undergone this procedure carry the badge "SAE certified."

Since minute differences in the dimensions of engine components that naturally occur during series production—the compression ratio could be a smidge higher in one engine versus another—can result in two identical-looking engines making slightly different horsepower, the SAE allows a one-percent tolerance between the number that the witness observes and the horsepower rating that a manufacturer chooses to publish. For example, according to DaimlerChrysler's senior manager of SRT Powertrain, Pete Gladysz, Dodge rates its Viper V-10 at 510 horsepower, even though a randomly selected V-10 that was plucked from the assembly line and broken in according to DC's usual cycle developed 512 horsepower on the dyno run for the designated witness. Sometimes a bunch of components are at the outer edge of production tolerances, and the combination hurts the engine's output. Sometimes they help it. The 0.4-percent difference between the V-10's observed output and its rating lies well within the SAE's allowable tolerance band. In fact, Dodge could have rated the engine at 517 horsepower and still met the SAE's one-percent tolerance. "But we'd rather deliver more than we promise," said Gladysz.

Interestingly, the new SAE standard is closer to the European procedure, which is EEC 80/1269. There are minor differences. For example, the exhaust configuration and the specified fuel-quality requirements vary, and these can have an effect on horsepower. But we're talking about small differences, possibly accounting for a less-than-one-percent variation.

There's also a unit difference between SAE and EEC. In the U.S. we use the English system of units for things like length, volume, and horsepower. Think foot, cubic inch, etc. In Europe it's the metric system—centimeter and liter. The English unit for power is horsepower, or hp. The metric equivalent to horsepower is PS (for Pferdestärke, which is German for "horsepower").

Because we define units differently, one PS does not equal one hp. One hp describes about 1.4 percent more power than one PS (PS x 0.9863 = hp).

The point of all this is to tell you how we arrive at the horsepower ratings you see in our spec panels. In most cases, we simply print the number the manufacturer gives us. But sometimes—when we get an early look at a European car, for example—the power figure we get from the manufacturer is in PS. A good example is the BMW M5. BMW said the V-10 engine produces 507 PS. For our specification panel, we converted that to 500 hp (507 x 0.9863 = 500). Sometime later, BMW released the SAE figure and quoted 500 hp. In the end everything matched up.

But it wasn't that simple with Porsche's latest 911. When we first previewed the car (Upfront, July 2004), Porsche said the 3.6-liter flat-six made 325 PS. We dutifully did the conversion and printed 321 hp. Then, a couple months later, Porsche released the SAE figure and said the new 911 made 325 hp, 4 hp up on our number.

When asked about this discrepancy, Porsche responded cryptically that its engine delivered 325 hp, suggesting perhaps that the company slightly underrated its engine in a manner similar to what Dodge had done in the V-10 example previously mentioned. Regardless of what Porsche said, we continued printing 321.

Porsche is not unique in this matter. Audi also claims the same figures in PS and hp. However, Audi says it retunes its engines for the U.S. market and that's why its engines have identical PS and hp ratings. Every company has its own procedure. European automakers like BMW, Mercedes, and Ferrari perform the conversion the same way we do (as in the BMW M5 example above).

Since this issue came up a few years ago, we've been noting the procedures of each company and performing the conversion where appropriate. But after a lot of debate, we have now decided to change our system and use the manufacturer-supplied hp ratings. We want to avoid the confusion that can arise when our published hp figure is different from the one seen in car ads or brochures. Knowing that engines can produce slight variations in horsepower (that one-percent tolerance, remember) and that there is a small difference between the SAE and EEC test protocol, we will now resist splitting hairs and creating needless confusion over a few horsepower.

Sure, it's a tough call. But if you can't agree, please try to keep in mind that we also provide information that is far more valuable than the hp figure. Namely, performance results. After all, the only reason to ask how much horsepower a car has is to make a prediction about how it will perform. Predictions tell less than half the story, and that's why we'll continue to rely on our test gear.

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