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Technobabble: December 2001

The Bullshit Filter

Every morning I stumble over the tires, auto parts, and camera gear piled throughout the broom closet I call an office, shove the morning's Fed Ex pile to the floor, and fire up The Bullshit Filter, my most valuable editorial tool. Without The Bullshit Filter, I would believe the press release touting the 25 horsepower spark plugs, entertain the phone call from the guy with the 600-hp naturally aspirated Corolla that runs on jet fuel, and take the OE's horsepower ratings as gospel.

But I don't. In fact, it was The Bullshit Filter that caught the frequent discrepancies we've been finding recently between the horsepower ratings of new cars and our own dyno results.

When we started dyno testing stock cars a few years ago, our intent was not to find and identify exaggerated claims. We simply wanted to learn more about each car's power curve and establish baseline numbers against which we could compare tuner cars. Completely by accident, however, we've stumbled across an alarming number of suspicious results.

Checking a manufacturer's power claim with a chassis dyno is nearly impossible. Rated horsepower is measured on an engine brake dyno, a machine that differs from our Dynojet in two very significant ways.

First, an engine brake dyno measures output straight off the flywheel, whereas our dyno measures output at the tires, after the drivetrain has absorbed some unknown quantity of motive force. Second, these brake dynos measure output at steady state. In other words, power at 6000 rpm is measured by combating the engine's torque with just enough resistance to keep the engine from accelerating past 6000 rpm. Our Dynojet, on the other hand, measures output while the engine is accelerating through its powerband over a period of between 5 and 10 seconds. During our test, this can create subtle differences in output, because some of the engine's power is usedto accelerate the engine itself.

Despite these differences, measuring power at the wheels can still be revealing. Our most notable revelation of late came when testing the 2001 Miata. The 2000 Miata, which had been rated at 138 hp (in California emissions trim) made around 112 hp at the wheels. Since the 2001 car was rated at 155 hp and the rest of the driveline was unchanged (thus, the drivetrain losses would remain consistent) we naturally expected to see around 129 hp at the wheels. Instead, we saw 112. After I, along with several Miata customers, pointed this out to Mazda North America, it re-rated the engine at 142 hp and tried to make nice with the jilted Miata owners.

Another output oddity has stricken the new Sentra SE-R Spec-V. Before its release, output was expected to be 180 hp. (All official pre-launch horsepower figures were wisely offered with precautionary prefixes such as "anticipated output.") Since the Spec-V uses an all-new six-speed gearbox, we have no established yardstick for drivetrain loss. Still, we were surprised to see only 141 hp at the wheels. In comparison, the 1991 to 1993 Sentra SE-R, which was rated at 140 hp, makes around 120 hp at the wheels. Even our own Project Sentra SE-R, a 1991 model with 75,000 miles, made 123 hp at the wheels. It seems highly unlikely that the new transmission could be soaking up 20 more horsepower than the old one.

After seeing our results and re-testing several early-production engines, Nissan dropped the final, official rating to 175 hp, a figure that still seems optimistic.These are the most blatant examples, but there are others. Take the 1994 to 2001 Integra GS-R, for example. Rated at 170 hp, GS-Rs regularly dyno at 150 hp at the wheels. The 1997 and '98 Type R, which was rated 25 hp higher at 195 hp, regularly registers around 161 hp--only 11 hp higher.

If you're starting to smell a conspiracy, it may come as a surprise that not all the inconsistencies we've seen suggest bloated power claims. Volkswagen's 1.8T, for example, as found in the Golf GTI, is rated at 150 hp, but turns the rollers at 140 hp, suggesting either an unlikely level of drivetrain efficiency, or a refreshing bit of modesty on Volkwagen's part.

Toyota is also notoriously modest. The 138-hp MR-2 Spyder registers 122 hp at the wheels, fully 10 more than the Miata with exactly the same rating. Surely the MR-2's transverse drivetrain is slightly more efficient, but some difference in rating strategy is evident as well.

The simple fact is horsepower ratings cannot be trusted. Still, rather than outright deception, both the Nissan and Mazda anomalies look like the result of major communication shortfalls between the U.S. and Japanese arms of the companies, combined with the pressure of increasingly compressed development schedules. The shortened development is especially problematic, because horsepower ratings are almost always published before production begins and very often before the final engine calibration is completed.

Emissions calibrations can happen remarkably late in the development process, especially with some of the newest flash-programmable ECUs, which can be re-programmed in the car. This technology opens the door for after-sale re-calibrations, which can potentially change the output of your car even after you buy it.

Several hundred Acura RSXs are rumored to have been re-programmed after arriving at the port, possibly explaining the discrepancy between the RSX we tested recently (WRX vs. RSX, SCC November 2001) and the test results we've seen elsewhere. There's also evidence the Toyota Celica GT-S may have gained a few horsepower after the first few months of production, the result of an as-yet-unidentified change.So if horsepower ratings aren't to be believed, what are we to do about it? For starters, we've stopped using the term "rated horsepower," replacing it with the more accurate "claimed horsepower." That way you and the manufacturers know we're not taking their word for it.

Sport Compact Car will also put every new car we test on a chassis dyno. Why? Because the power available at the wheels is what actually moves the car. The SE-R's new engine could make 175, 180, or even 500 hp, but the 141 hp at the wheels is still the only number that will affect the driving experience. These at-the-wheels numbers may be smaller than what you are used to, but they'll save your Bullshit Filter a lot of work.

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