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NEW YORK (The New York Times)--Proud new owners were once greeted with a universal request for some measure of their car's performance.
''What'll she do?'' curious neighbors would ask.

Crowded highways and police radar guns, along with electronic speed controls built into engine computers, discourage today's drivers from exploring their cars' limits. Instead, buyers try to gauge performance by comparing horsepower ratings -- usually without realizing that the numbers have not necessarily been comparable.

A recent change in the procedures for measuring horsepower is leveling the field, and causing the horsepower figures of some vehicles to be revised. The rating of the Toyota Camry's 3-liter V-6 engine shrank nearly 10 percent -- to 190 horsepower, from 210 -- though the engine's specifications and the car's performance are unchanged.

The power ratings of other models came in higher than expected: Cadillac originally estimated the output of the supercharged V-8 in the 2006 STS-V sport sedan would be 440 horsepower, but testing to the new standard produced a rating of 469.

The idea of horsepower came from James Watt, the 18th-century inventor who developed the unit of measure as a promotional device to equate the work that could be done by his improved steam engines with the number of horses they could replace.

The concept of horsepower is based on a force moving through a distance in a given period of time. Watt decided to define one horsepower as the ability to lift a load of 550 pounds at the rate of one foot every second.

Horsepower is measured by connecting the test engine to a dynamometer, a machine designed to exert a steady, controlled resistance to the engine's spinning crankshaft. Horsepower is computed from the measured twisting force of the engine -- its torque -- and its rotational speed in revolutions per minute.

The power of European autos is sometimes given in kilowatts; one hundred of James Watt's horsepower are equivalent to about 75 kilowatts.

From an automaker's point of view, the bigger the horsepower number, the more attractive the car will be to performance-conscious shoppers. In the past, automakers logically interpreted test procedures to make their engines seem more muscular.

Rating an engine too optimistically can land an automaker in trouble, though. Last year, Hyundai spent tens of millions of dollars to settle a class-action lawsuit with owners who claimed that the company had overstated power, inflating the cars' value.

In some cases, automakers have chosen to understate horsepower -- when insurance companies threatened to raise rates for high-horsepower models, for instance.

Every industry has its standards organization, and for carmakers it is the Society of Automotive Engineers, now 100 years old. The society's first standards for horsepower measurement were published in 1917. In the 1970's, power ratings advertised by carmakers changed from gross output to net horsepower, reflecting a shift from testing bare engines to measuring them with a full complement of accessories.

The society's new standard is known as J1349. David Lancaster, chairman of the committee that developed the test rules, explained that new technologies open new loopholes in test procedures, which must be periodically plugged. Before 1971, for example, a manufacturer might connect the engine's exhaust system to a vacuum pump, gaining power by eliminating back pressure, the resistance to free flow caused by mufflers. Under the 2006 standard, a complete production exhaust system, or one of equal back pressure, must be used.

What kinds of gray areas are corrected by the new code for testing engine power?

Many modern engine computers electronically adjust ignition timing, either advancing it to take advantage of premium-grade gasoline or retarding it slightly to keep the engine from knocking on regular fuel. Such systems use special sensors to ''listen'' for knock, which can damage the engine.

A manufacturer could therefore accurately say its engine will run satisfactorily on regular gas, yet perform the horsepower test on a higher-octane blend, knowing it will automatically take advantage of that fuel to produce slightly more power. Under the new standard, testing is done with the minimum octane fuel required for that model.

Other loopholes have been closed, too. Power losses caused by friction inside the engine can be reduced by the use of a thinner grade of oil or by using hotter oil (oil flows more freely as its temperature rises); the new testing standards require the oil to be the same grade specified for normal service, at the temperature ''exhibited in service with a fully warmed-up engine.''

Engines today drive an increasing number of accessories, including an air-conditioning compressor and pumps for engine coolant and power-steering fluid, all of which draw some power. Should they be operating when the horsepower is tested? Should the electrical alternator be generating electricity, and if so, how much? The revised standards spell out the answers to questions like these -- and they will surely be revised again as new gray areas emerge.

Adoption of the new certified power standard, which includes a provision for a third-party witness to the testing, is voluntary. General Motors says it expects to apply it to all newly rated engines in the future; some makers will use it for all existing engines.

Horsepower ratings may be impressive to consumers, but the numbers are often meaningless in real-world traffic conditions. Specifications typically state something like ''220 horsepower at 6,000 r.p.m.'' but because this is peak output, less power is produced at all other engine speeds.

In fact, most driving uses just a fraction of the engine's potential; typically, cars need only about 25 horsepower to maintain highway speed on a level road. Drivers never call on the full reserve of available horsepower unless they hold the accelerator fully to the floor all the way to peak r.p.m.

Horsepower numbers are a coded message from the maker; a high number is like bright sporty clothing, suggesting excitement. A moderate number parallels a conservative suit, suggesting traditional virtues like reliability. Such images are important in marketing.

A new performance measure that would take into account how people really drive could be created. It would average the horsepower actually needed in most daily driving.

The most direct way for drivers to determine the right amount of horsepower for their driving needs requires no calculations at all -- just a test-drive.

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