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.
What complicates the issue and makes apples-to-apples comparisons difficult is the fact that those pre-smog horsepower ratings were not calculated in the same way as modern engines. “A horsepower is a horsepower, right?” you say. While a horsepower, pre-smog or post, remains 746 watts (or 736, for metric horsepower), the way that output was measured has changed quite a bit. Let’s explain:
GROSS HORSEPOWER RATINGS
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, free-flowing exhaust headers (no mufflers), and optimal ignition timing, 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, while the ignition is retarded to prevent detonation with pump gasoline. Meanwhile, carburetor jetting and fuel injection calibration 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 265 cu. in. (4,344 cc) 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).
OVERRATING AND UNDERRATING
Until the mid-fifties, the gap between gross horsepower and as-installed output was not vast, but by the end of the decade, some manufacturers’ advertised horsepower ratings far outstripped usable power. Significant inflation was clearly taking place, sometimes to the tune of 25-30%. We’re pretty sure that nothing in the SAE standards said that the calculated horsepower could be whatever the marketing department wanted it to be, but that was more or less 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. (6,488 cc) 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 (6,996 cc), 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 per 10 pounds (1 kW per 6.1 kg) of curb weight, leading to curious non sequiters like rating Pontiac’s 3,300 lb (1,500 kg) Firebird at 325 hp (242 kW) while claiming 360 hp (269 kW) for the identical engine in a 3,600 lb (1,635 kg) GTO.
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 unseemly. In that climate, advertising a 500 hp (373 kW) Corvette or 400 hp (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. (5,567 cc) 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.
NET HORSEPOWER RATINGS
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 horsepower ratings or not.
Faced with this reality, along with the pressures of the safety and environmental lobby, U.S. 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.
The result of the new net rating system was a dramatic drop in advertised power. The rated output of Cadillac Eldorado‘s mammoth 500 cu. in. (8,194 cc) V-8, for instance, fell from 400 gross horsepower (298 kW) in 1970 to 360 gross horsepower (269 kW) in 1971, a drop of about 10%. The engine was basically unchanged for 1971, but the switch to SAE net ratings reduced the rated output to only 235 net horsepower (175 kW). (Although GM did not quote a net horsepower rating for the higher-compression 1970 engine, it was probably 275–285 hp (205-213 kW).) In some cases, actual output did drop during this period, mostly due to emissions-related changes, but the reduction in as-installed power was generally far less drastic than the numbers suggested.
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 cu. in. (7,481 cc) V8, 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 302 cu. in. (4,942 cc) 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). It was not until the widespread proliferation of electronic fuel injection in the 1980s that net power outputs again began to climb.
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 than before, while a few actually increased. In most cases, 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.
SOME COMMON MISCONCEPTIONS
Let’s clear up a couple of common misconceptions about gross and net horsepower ratings:
- Contrary to some assumptions, net horsepower ratings do NOT measure horsepower at the drive wheels. Both gross and net ratings are at the flywheel and don’t reflect power losses in the drivetrain.
- Because of the vagaries of the old gross ratings and the widespread over- and underrating of different engines, there is NO precise formula for converting gross horsepower to net horsepower or vice versa. Sometimes, the difference is as little as 5–10%; sometimes it’s more like 25%. The only way to be certain about the comparative gross and net ratings of a specific engine is if the factory released both gross and net figures for that engine. (Some U.S. automakers did this during the 1971 model year and intermittently before that.) Otherwise, the best you can do is make an educated guess based on state of tune and real-world performance testing — keeping in mind that published road tests didn’t necessarily reflect the performance of cars the average consumer could actually buy.
- The ambiguity of gross horsepower ratings means that many pre-1972 American cars were actually a lot less powerful than the advertised figures would suggest. 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. For example, a 1967 Chevrolet Impala with the 396 cu. in. (6,488 cc) V8, rated at 325 gross horsepower (242 kW), probably had something like 220 net horsepower (164 kW) in pure stock form.
NON-U.S. HORSEPOWER RATINGS
What about cars not built by U.S. automakers? As with many things, the answer is, “It depends.”
German automakers have long rated their engines under the DIN (Deustches Institut für Normung, German Institute for Standardization) standards. DIN horsepower and torque figures are net ratings, similar but not identical to SAE net figures due to minor differences in test methodology. Italian automakers used to use the Comitato unitario dell’autotrasporto (CUNA) standards, whose net ratings again are similar but not identical to SAE net ratings. Other European automakers used DIN or SAE gross figures depending on the time period and model.
Until the early 1970s, some British automakers quoted SAE gross figures for horsepower and torque while others listed net figures; some manufacturers listed both. After the U.K. joined the European Economic Community in 1972, the British auto industry switched to DIN ratings.
Japanese automakers rated the output of home-marked cars under the Japanese Industrial Standards (JIS) rules. JIS provided standards for both gross and net horsepower; until about the mid-eighties, many Japanese automakers typically quoted gross figures.
For cars sold in the U.S. prior to 1971, non-U.S. automakers would generally also provide SAE gross ratings for their engines, at least the ones exported to America. However, for the most part, British and European automakers seldom indulged in the sort of er, creativity their American counterparts displayed in interpreting the gross horsepower standards. For example, the Triumph TR4 carried a gross rating of 105 hp (78 kW) and a net rating of 100 hp (75 kW) while a 1963 Mercedes-Benz 230SL had a gross rating of 170 hp (127 kW) SAE and a net rating of 150 hp (110 kW) DIN.
In general, DIN or CUNA ratings for older engines are reasonably comparable to modern net figures, much more so than are vintage gross ratings. However, there is an additional complication, which is the matter of units. DIN, JIS, and CUNA standards are typically — but not always — quoted in metric rather than mechanical horsepower more familiar to our American and British readers. One metric horsepower (often abbreviated ps) is about 736 watts while one mechanical horsepower is about 746 watts, so 1 ps equals 0.986 hp and 300 metric horsepower would be about 296 mechanical horsepower.
Unfortunately, there is maddeningly little consistency in the use of these units. A lot of sources will freely interchange the two; some will write hp or bhp when they’re talking about ps. A few attempt to convert one to the other and occasionally do so incorrectly (for instance, multiplying by 0.986 a rating that was already in mechanical horsepower) — we’ve almost certainly been guilty of that, although we try to avoid it. Since the difference is less than 2%, it’s not a big deal, but it does frustrate a lot of efforts to be consistent or precise.