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| Understanding Gross vs. Net Horsepower Ratings |
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| Written by Aaron Severson |
| Tuesday, 15 April 2008 14:12 |
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There are a lot of misunderstandings among car enthusiasts and historians about vintage horsepower ratings. It's easy to assume from a casual glance at ads or spec sheets that even quite ordinary American family sedans of the sixties were overwhelmingly powerful, with 300 horsepower or more, and yet by 1975, many of those same cars were down to 150 hp or less. When asked the reason for the huge difference, gearheads tend to shake their heads and mutter about emissions controls and anemic, low-octane unleaded gasoline -- which is true, but only partly. Before 1972, most American engines were rated under the methodology laid out in Society of American Engineers (SAE) standards J245 and J1995, which calculated the output of a 'bare' engine on a test stand with no accessories, optimal ignition timing, free-flowing exhaust headers (no mufflers), with a correction factor for standard atmospheric conditions.
What does all that mean? The engine in your car is burdened with various engine-driven accessories, ranging from the engine's own oil and water pumps and generator/alternator to the power steering pump and air conditioning compressor, each of which consumes a certain amount of power. An engine in a passenger car also has mufflers and an exhaust system designed for quiet operation, rather than low back pressure, ignition is retarded to prevent detonation with pump gasoline, and the carburetor(s) or fuel injection system are aimed at fuel economy and driveability, not maximum power. The gross rating reflects none of these losses; it represents an engine's theoretical maximum output under ideal conditions, not how much power it actually produces when installed in a car. As an example, Chevrolet's original small-block V8, which bowed for 1955, had a gross rating of 162 hp (121 kW) at 4400 rpm with a 8.0 compression ratio and a single two-barrel carburetor. Motor Life magazine reported in December 1954 that the factory quoted a net output of 137 hp (102 kW). Until the mid-fifties, the gap between gross horsepower and as-installed output was not vast, but by the end of the decade advertised horsepower ratings far outstripped usable power. Significant inflation was clearly taking place, sometimes to the tune of 25-30%. Nothing in the SAE standards actually said that the calculated horsepower could be whatever the marketing department wanted it to be, but it might as well have, because that was what happened. If Chevrolet advertised 195 gross horsepower (145 kW) for its standard V8, for example, it was not difficult for Ford engineers to tweak their calculations to justify a rating of 200 hp (149 kW) for their standard engine. By the same token, in the mid- to late sixties, it was also not uncommon for power ratings to be deliberately understated. For example, in 1965, Chevrolet released the 396 cu. in. (6.5 L) TurboJet V8 as an option for Corvettes, rated at 425 gross horsepower (317 kW). The following year, the engine was bored to 427 cubic inches (7.0 L), but its power rating remained suspiciously unchanged. (Indeed, some early GM promotional material credited the 427 with 450 gross horsepower (336 kW).) GM imposed corporate rules limiting all their cars except the Corvette to a maximum of one gross horsepower (0.75 kW) per 10 pounds (4.5 kg) of curb weight, leading to curious non sequiters like rating Pontiac's 3,300-pound (1,500 kg) Firebird at 325 hp (242 kW), while the identical engine in a 3,600-pound (1,635 kg) GTO claimed 360 hp (269 kW).  This is a 1966 Corvette 427 (7.0 L) L72 engine. Early literature credited the L72 with 450 hp (336 kW) at 6,400 rpm, but this as quickly amended to 425 hp (317 kW) at 5,600 -- the same horsepower as the previous year's 396 cu. in. (6.5L) L78. Contemporary reviewers were highly skeptical. Why would a manufacturer underrate their engines? Particularly at GM, the most conservative of the automakers, there was real fear of the growing safety lobby, which already thought the amount of power the auto industry offered in its cars was uneemly. In that climate, advertising a 500-horsepower (373-kW) Corvette or 400-horsepower (298 kW) GTO seemed like asking for trouble. Insurance was also becoming an issue, with a growing number of insurance companies levying prohibitive surcharges on very powerful cars (or simply refusing to offer coverage at all). Another concern was racing. Eligibility for different drag strip classes was based on power-to-weight ratio, calculated using advertised horsepower and shipping weight. If an engine produced more power than its rating, it would have a competitive advantage. This type of underrating was at best an open secret. Testing a Pontiac GTO Judge equipped with the $390 Ram Air IV engine, for example, Car Life magazine noted that the division's own executives freely admitted the 370 hp (276 kW) gross rating was purely a fiction to satisfy insurance companies and their corporate superiors. As a result, racing officials frequently "factored" underrated engines for the purposes of classification; Chrysler's very strong 340 cu. in. (5.6 L) engine, for example, carried a conservative 275 hp (205 kW) rating from the factory, but the NHRA treated it as a 325-hp (242-kW) engine for racing purposes. Between inflation and deliberate underrating, by 1970, the relationship between advertised gross horsepower and actual power was at best nebulous. The gross ratings served a variety of political and marketing purposes, but they were far from useful as a realistic measure of engine output. Starting in 1971, manufacturers began to lower compression ratios and de-tune their engines to prepare for the advent of unleaded gasoline. Both the early emission-control systems (air-injection pumps, exhaust gas recirculation) and the reduced compression ratios made engines perceptibly less powerful, whether those losses were reflected in the gross power ratings or not. Faced with this reality, along with the pressures of the safety and environmental lobby, domestic manufacturers decided it was time to abandon the gross rating system. In its place they adopted the SAE net rating methodology, described by SAE standard J1349. "Net" horsepower ratings are still made with the engine on a test stand, but with stock ignition timing, carburetion, exhaust, and accessories -- in short, a closer approximation of how much power an engine produces as actually installed in the car. (SAE net horespower does NOT, contrary to some assumptions, measure horsepower at the drive wheels; both gross and net ratings are at the flywheel, and don't reflect power losses in the drivetrain.) The result of the new rating system was a dramatic drop in advertised power. The Cadillac Eldorado's mammoth 500 cu. in. (8.2L) V-8, for instance, dropped from 400 gross horsepower (298 kW) in 1970 to 360 gross horsepower (269 kW) in 1971, a drop of about 10%. The rated horsepower of the 1972 version was only 235 net horsepower (175 kW), even though the engine itself was basically unchanged. (Although GM did not quote a net horsepower rating for the higher-compression 1970 engine, it was probably 275-285 hp (205-213 kW).) On paper, though, output had been cut by 35%. Because of the vagaries of the old gross standards, there is no precise formula for converting gross to net or vice versa. Some 1971 engines carried both gross and net ratings, but for earlier or later years, the best you can do is make an approximation based on state of tune and real-world performance testing. Why was this change made? The most obvious reason was as an inexpensive PR gesture; overnight, the carmakers made it clear that they were no longer offering irresponsible levels of horsepower, without making any expensive engineering changes whatsoever. Beyond that, the switch in ratings made it easier for salesmen to obfuscate the actual loss of power caused by reduced compression and smog control hardware -- useful when trying to explain to a customer why the 1972 Cadillac he's looking at seems to have 40% less power than the 1970 he's trading in. By the end of the decade, the big drops in horsepower were no longer just on paper. For example, Pontiac's 455, which as late as 1973 had produced a conservative 310 net horsepower (231 kW), could muster only 200 (149 kW) by the time it faded out in 1976. Ford's 4.9 L V8, which had made as much as 306 gross horsepower (228 kW) in the sixties, had plummeted by 1979 to less than 140 net horsepower (104 kW). While the late sixties were a golden age of horsepower compared to the late seventies or early eighties, the differences weren't quite as vast as they appear at first blush. A '67 Impala with the 396, rated 325 gross horsepower (242 kW), probably had something like 220 net horsepower (164 kW) in pure stock form -- decent, but no muscle car. The net rating system was used until 2005, when the SAE issued standard J2723, eliminating a number of loopholes in the existing methodology and requiring an independent observer be present when the ratings are measured. Under these new "SAE-certified output" guidelines, some engines ended up with lower ratings (Toyota's 1MZ-FE engine, the 3.0L V-6 in the previous-generation Camry, dropped from 210 to 190 hp (157 to 142 kW) under the new system), while a few actually rose (Cadillac's supercharged Northstar went from 440 to 469 hp (328 to 350 kW)). The engines were not actually altered in any way -- the testing methodology had just changed. The new rating method is voluntary, but most, if not all, manufacturers now use it for their U.S. market cars. Most European manufacturers, incidentally, rate power under the DIN (Deustches Institut für Normung, German Institute for Standardization) methodology, sometimes also quoting metric horsepower rather than English. Japanese companies in the home market use JIS (Japanese Industrial Standards) rules, which are similar to DIN. The DIN standards haven't changed that much over the years, so if you find old road tests quoting output in DIN-rated horsepower, those numbers are much more comparable to modern ratings than contemporary SAE numbers. Modern DIN, JIS, and SAE-certified ratings for an identical engine tend to vary slightly, but the distinction is not huge -- perhaps 1-2% -- and owes more to the difference between metric and English horsepower. (A metric horsepower, often abbreviated PS, is about 736 watts, whereas an English horsepower is 746 W). # # #
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i have a car title for a 1969 chevelle malibu that says it has 48 horsepower can you tell me what size engine that is and what the real horsepower is it is a missouri title thanks for any help you can give