Understanding Gross Versus Net Horsepower Ratings

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.

1966 Chevrolet Corvette Sting Ray 427 engine air cleaner
This is a 1966 Chevrolet Corvette Sting Ray’s 427 cu. in. (6,996 cc) L72 engine. Early literature credited the L72 with 450 hp (336 kW) at 6,400 rpm, but this was quickly amended to 425 hp (317 kW) at 5,600 rpm — the same horsepower as the previous year’s 396 cu. in. (6,488 cc) 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 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 to clarify and tighten the existing methodology and require 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:

  1. 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.
  2. 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.
  3. 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. In general, DIN or CUNA ratings for older engines are reasonably comparable to modern net figures (with one exception we’ll discuss below), much more so than are vintage gross ratings.

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, which included standards for both gross and net horsepower. Until about the mid-eighties, many Japanese automakers typically quoted gross figures in the domestic market. The switch to net ratings began around 1985, but wasn’t completed until late in the decade. The difference between JIS gross and net ratings for a given engine was generally around 15% — sometimes a little more, sometimes a little less.

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.

MECHANICAL VS. METRIC

There is an additional complication when considering non-U.S. power ratings: the question of units. DIN, JIS, and CUNA standards are typically — but not always — quoted in terms of metric horsepower rather than the 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. For example, 300 metric horsepower would be about 296 mechanical horsepower while 150 PS is about 148 hp.

Unfortunately, many sources are maddeningly inconsistent in their application and even understanding of these units and tend to freely interchange them, always writing “hp” or “bhp” even when talking about PS. Occasionally, some sources will attempt to convert one unit to another, sometimes inconsistently — for example, 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.

# # #

NOTES ON SOURCES

Most of this information is based on decades of reading car magazines: Some of our specific references included Robert Ackerson, Chrysler 300 ‘America’s Most Powerful Car’ (Godmanstone, England: Veloce Publishing Plc., 1996); the Auto Editors of Consumer Guide, Encyclopedia of American Cars: Over 65 Years of Automotive History (Lincolnwood, IL: Publications International, 1996); John R. Bond, “Car of the Year: The 1949 Cadillac,” Motor Trend November 1949, reprinted in Cadillac Automobiles 1949-1959, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., 1985), pp. 8-9; Bernard Cahier, “Road Test/10-63: Mercedes-Benz 230 SL,” Sports Car Graphic May 1963, reprinted in Mercedes 230SL – 250SL – 280SL Ultimate Portfolio 1963-1971, pp. 31-34; “Chevrolet Camaro Z28,” Car and Driver May 1971, reprinted in Camaro Muscle Portfolio 1967-1973, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., 1992), pp. 118-121; “GM: Cadillac,” Motor Trend Buyers’ Guide 1971, reprinted in Cadillac Eldorado 1967-78 Performance Portfolio, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., 2000), pp. 80-83; “GM: Oldsmobile,” Motor Trend Buyers’ Guide 1971, reprinted Oldsmobile Muscle Portfolio 1964-1971, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., 1999), pp. 138-140; Tony Grey, “Olds 4-4-2,” Road Test May 1971, reprinted in Oldsmobile Muscle Portfolio 1964-1971, pp. 132-137; John Gunnell, ed., Standard Catalog of American Cars 1946-1975, rev. 4th ed. (Iola, WI: Krause Publications, 2002); Roger Huntington, “Chevrolet’s New V-8,” Motor Life December 1954, reprinted in Chevrolet 1955-1957, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., 1988), pp. 16-17, 31; Jikayousha [Private Car] Buyer’s Guide Spring ’87 Edition, February 1987; Steve Kelly, “Beware the Quiet Fish,” Hot Rod January 1971, reprinted in Plymouth 1964-1971: Muscle Portfolio, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., 2003), pp. 120-122; John Lamm, “King of the Hill: Cadillac Eldorado vs. Lincoln Continental Mark IV,” Motor Trend July 1972, reprinted in Cadillac Eldorado 1967-78 Performance Portfolio, pp. 96-103; Brian Long, Celica & Supra: The book of Toyota’s sports coupes (Dorchester, England: Veloce Publishing, 2007); “Road Testing Chrysler’s Power Flite,” Speed Age November 1953, pp. 58-62, reprinted in Chrysler Imperial Gold Portfolio 1951-1975, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., 2004): 20-23; “The Judge: Car Life Road Rest,” Car Life March 1969, reprinted in GTO Muscle Portfolio 1964-1974, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., 1998), pp. 91-95; and “Triumph TR4 (Road Test No. 26/62), The Motor 11 July 1962, reprinted in Triumph TR4 – TR5 – TR250 1961-1968 (Brooklands Road Test Series), ed. R.M. Clarke (Cobham, Surrey: Brooklands Books Ltd., 1997), pp. 30-33. We also verified a few details from the Wikipedia® entry on horsepower (en.wikipedia. org/wiki/ Horsepower).


79 Comments

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  1. 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

    1. That sounds like a taxable horsepower rating, using the old RAC (Royal Auto Club) formula. In Britain (and a few U.S. states, I think), your license tax was based on that formula, which was equal to the square of the cylinder bore times the number of cylinders, divided by 2.5. Assuming this is a V8, that means a bore of 3.875 inches. Chevy only offered one V8 with that bore in 1969, the 307 that was standard in V8 Chevelles.

      As for its real output, Chevrolet rated it at 200 gross horsepower at 4,600 rpm and 300 lb-ft of torque at 2,400 rpm. That was for a bare engine on a test stand, so the real, as-installed output would be less. There’s no precise formula to convert gross ratings to modern net figures, but it would be something in the neighborhood of 130-140 horsepower.

  2. Does anyone really know how many horses the 350 rocket engine 4 barrel have? For the 1972 Cutlass S 350 5.7 L 4 BBl.

  3. The 1972 Olds 350-4V was rated at 180 hp @ 4000 rpm and 275 lb-ft @ 2800 rpm. Both are SAE net ratings (Oldsmobile switched in 1972), so they should be reasonably comparable to modern power figures.

  4. Most drops in horsepower from the revised 2005 standards are not from manufacturers cheating in the testing, but because of engines designed to run on premium gas. Yes, there were upwards of 5 horsepower lost from changes in testing methods, but most came from lower octane gas.

    The 1MZ-FE for example, is still a 205 horsepower engine with the new testing methods. However, run with regular gas it will only make 190. The new standards allow manufactures to use premium gas in testing engines designed for premium, ONLY if the owner’s manual explicitly states premium only.

    For marketing purposes, Toyota wasn’t about to state that the engine in the country’s top selling sedan required premium gas. The manual just states that premium should be used for optimum performance and efficiency. The knock sensors and control module safely allow the engine to run on regular gas at the expense of performance and economy.

    1. jayjay,

      Just read your article on the various HP productions of the Toyota 1MZ-FE engine. I find nothing TECHNICALLY wrong with your analysis but your total statement leaves a distorted impression for the “great unwashed” in the HP bragging rights department.

      That distortion is this, (and I paraphrase) “higher octane gasoline, as a commodity, contains more HP.” This is patently not true and if you were talking about prescription medication the FDA (Food and Drug Administration) would be all over you.

      A gallon of gasoline from any given refined crude product will have “X” amount of btu’s (heat value) generally pegged at 112,000 btu per gallon. HP is directly derived from the production of heat in the combustion chamber.

      From your over simplified comment it would appear that HP gains in any particular engine can be accomplished by the introduction of higher octane gasoline. Generally speaking, and for the most part, this is not a true statement.

      The engine mentioned in your article has a fixed compression ratio. All things being equal, a higher compression ratio engine will produce more HP for a given gallon of gasoline than a lower compression one. Ergo, the engine can only produce a fixed amount of HP, (assuming a particular tune and compression ratio)

      In the case of the engine cited (and most modern computer controlled ignition engines) the retarded timing protocol will back off timing when the knock sensor indicates preignition, permitting the engine to operate safely at a lower HP output.

      What is happening here is the REALIZATION, or EXTRACTION of more heat in the higher octane rated gasoline, than in lower octane rated gas. I.E., higher compression ratio engines can extract MORE heat from a gallon of gasoline because they can WITHSTAND the increased compression ratio due to the “anti-knock” characteristic of high octane gasoline, NOT the implied fact that high octane gasoline has more heat (HP) in it.

      The use of lower octane fuel in this scenario causes the timing to be retarded (as you noted) to compensate for preignition. Toyota is right on the edge here with the truth of their engine. The higher CR PERMITS more power production but requires more expensive, higher octane, fuel. Less expensive fuel (lower octane) can be used ONLY because the timing protocol will be invoked. (similar to the old days when an engine was tuned -spark retarded- by the local mechanic to run on poor quality gasoline. (spelled cheaper) NOW, the mechanic is replaced by the computer…instantly.

      It is a “WASTE” of money and efficiency to run higher octane fuel in an engine whose compression ratio is below the knock level (8.5-9.0) of the 87 octane rating posted on the pump. No HP gains are possible when doing this. (assuming a standard tune).

      You buy HP at the dealership, NOT at the pump.

      Darrow…for the Prosecution

  5. so would the 1972 500ci eldorado caddi have around the same net horsepower as the 71 would if they had used SAE ratings in 71?
    and would the torque be the same also?

    1. Yup. Cadillac issued both net and gross figures for 1972, which were 365 hp and 535 lb-ft SAE gross, 235 hp and 385 lb-ft SAE net. They quoted the same figures for both 1971 and 1972, in this instance.

    1. My recollection (without digging through old magazines) is 200 net horsepower. Other GM cars with the normally aspirated 3800 in that era were around 200-205 hp, so it should be in that range.

  6. Not true. While the 71 500cui is still basicly the same proud enegine as it was the year before, the 72 was a victim for the new low-emission/low octane thinking.

    From the outside you couldn`t see any difference, but the compression weas lowered this year by adding soap-cup pistons, and to match the lowered compression they`ve made up a new low-performance camshaft with low lift and extreme slow lobes. On top of that the exhaust-flow was reduced by a the new EGR-system.

    Want to know more, check out Potter Automotive!

    1. The information I have says EGR wasn’t added until 1973 (except maybe in California), and the rated compression ratio was the same from 1971 to 1973, dropping a quarter of a point for 1974. (I do know there can be a significant difference between quoted and actual compression ratios.) I don’t have camshaft specs for those engines — if you do, I’d love to see them.

      The quoted net output remained 235 hp/385 lb-ft through 1973, dropping to 210/380 for 1974 and 190/360 for 1975.

      Please note, I’m not necessarily disagreeing with you that the ’72 engines were less powerful, I’m just saying the figures Cadillac quoted in its official specifications for power, torque, and compression were the same as in 1971.

      1. hi ,

        I am trying to find out the engine in my camaro ( not original)

        What would be the hp on this.

        I think it would be a 350 ci from 1974

        Cast #3970010
        Engine # V1029CKD –
        14R411617 (part vin?)

        Cheers

        1. Based on the engine code (the final three digits of which tell you the specific powertrain configuration), that would be a California LM-1 engine with automatic, which in 1974 had 160 hp and 250 lb-ft of torque, both SAE net ratings.

    1. Well, rated (mechanical) horsepower is a function of torque and engine speed, which is calculated like this:

      Power (hp) = Torque (lb-ft) x 2 x pi x Rotational Speed (rpm) / 33,000

      The mathematical relationship between power and torque remains the same regardless of rating system (although obviously the units are different for systems that use metric figures). The only thing that’s changed is the way you measure the engine’s torque. So, the answer to your question is "proportionately."

        1. Sorry, that’s not n, it’s pi (the extended ASCII character isn’t terribly easy to read in this font). 3.14159… etc.

          1. I should have looked at that closer, sorry. That equation works for both gross hp and net hp but it would produce different values for torque based on those hp values? As in a car with a gross hp of 300 and a car with 300 net hp would have the same torque based on the equation but different because of the difference between gross and net hp?

          2. Yup. It’s important to understand that horsepower and torque are not separate calculations; horsepower is derived mathematically from torque and engine speed. If you know your engine produces 286 lb-ft of torque at 5,500 rpm, you also know that it has about 300 hp at that speed. The mathematical relationship is the same, regardless of whether the figures are gross or net (or even DIN or JIS, provided that you convert the units correctly).

            Generally, the way you calculate an engine’s output is to hook it to a dynamometer (ideally at the flywheel, so you don’t have to estimate frictional and mechanical losses through the transmission) and measure how much torque it produces at each point in its speed range, from idle to redline; this is known as the torque curve. You then plug those values into the equation I mentioned above and calculate how much horsepower that equals at different points in the rpm range; this is known as the power curve. The power and torque curves are mathematically related, but they usually end up being shaped differently — for example, the highest point on the power curve will usually come at a higher engine speed than the peak value of the torque curve.

            The different rating systems don’t change these calculations; what they change are the conditions under which you take your measurements. For instance, is it with the engine stripped, or with all of its normal accessories (water pump, oil pump, etc.)? Is it with the stock exhaust system and mufflers installed or running straight pipes? What are the air temperature and pressure during the test? (Both affect engine output.) If the temperature and pressure are lower or higher than normal, should you apply a correction factor? All of those factors can have a significant effect on not only the maximum torque and horsepower values, but the shape of the power and torque curves — for example, an engine’s net ratings may be at different rpm than its gross output.

  7. That’s a very good question. The 2005 Corvette had 400 hp and 400 lb-ft of torque, both SAE net figures. The 1966 427 was offered in two forms: the L30, with hydraulic lifters, and the L72, with 11.0 compression and solid lifters. Both had gross torque ratings of 460 lb-ft, but the L30 was rated at 390 gross horsepower @ 5,200 rpm, the L72 at 425 hp @ 5,000 rpm. As mentioned in the text, the L72′s gross rating was widely suspected to be conservative (an engine with a hotter cam and solid lifters with a [i]lower[/i] horsepower peak than its milder brother?), with some observers suggesting that a more accurate peak figure would have been more like 450-460 hp @ 5,600-6,000 rpm.

    To my knowledge, Chevrolet never released factory net horsepower ratings for the L30 and L72 engines, so a lot comes down to guesswork. Since the L30′s gross rating was shy of the C06′s LS2′s, it’s safe to assume its net output is lower, as well — at a guess, 300-320 net horsepower, perhaps less if it has California smog-control equipment. The L72 is trickier. Aside from the possibility that the gross rating was deliberately conservative, making estimates based on performance figures is not easy: the power-to-weight ratio is such that quarter mile performance reflects tires, traction, and launch technique as much as developed horsepower. Still, my guess would be that the L72′s net output is actually quite close to that of the LS2. My rough estimate would be somewhere between 375 and 400 net horsepower.

  8. I have a 67 for galaxy 500 with the standard 289 motor, my issue is when i went to get it licensed the say it has 51 hp, which makes it pricey to keep on the road. I have a 06 Daytona charger that they show is 49 hp, i know they now use a horsepower rating system to come up with those numbers,it just seems like the 67 is not near the hp the 06 is. Using that new system what would the hp be for the 67, it is the stock motor that came in the car back in the day.I don’t know enough about motors to figure this out, if at all possible could you figure the hp for the 67 using this system. I need to know what it is in today’s terms so i can get it changed and save money each time i have to license the car. Thanks a million,JW

    1. I don’t know what state you’re in, but it sounds like they’re using the old Royal Auto Club taxable horsepower formula. That is a very old formula used in the U.K. before the war to calculate road tax.

      RAC horsepower is the square of the engine’s cylinder bore diameter (in inches) times the number of cylinders divided by 2.5. The Ford 289 V-8 had a cylinder bore of 4.00 inches, which means its RAC horsepower racing was 51.2 (rounded down in most cases to 51). I assume your Charger has the 5.7 liter Hemi, which has a bore of 3.92 inches, giving it a RAC horsepower rating of 49.17, which rounds down to 49.

      As you’ve figured out, these ratings have nothing to do with how much actual horsepower the engines develop. (Eighty or 90 years ago, the RAC formula did give a reasonable estimate of actual engine power, but that hasn’t been true since before 1930.) If your Galaxie has the base C-code 289 (with a two-throat carburetor), it was originally rated at 200 gross horsepower, which is probably something like 140 in the modern net rating system, while the Charger Daytona is rated at 350 net horsepower. The only reason the Galaxie has a higher taxable horsepower rating is that its cylinder bores are slightly wider.

      I don’t know what the specific rules are in your state, but I would very much doubt that there’s anything you can do to change these ratings. If they were actually incorrect (for example, if the department of motor vehicles thought your Galaxie had the bigger 428 engine, which would have a RAC rating of 55 HP, instead of the 289), you could probably get them to fix that, but the ratings aren’t [i]wrong[/i], they just don’t have anything to do with the output of the two engines. The formula is probably written into the state law, in which case the department of motor vehicles couldn’t do anything to change it if they wanted to (other than correcting an error, which this is not). Sorry!

    1. The 1987 “eta” engine was rated at 121 net horsepower and 170 lb-ft of torque at the flywheel. On 3-series cars, the axle ratio was 2.79:1, regardless of transmission; I would have to go digging to find the individual gear ratios.

  9. Have thought that net HP was always measured at the rear wheels, untill today. Was it ever the standard or did I misunderstand the whole thing years ago? May of been a misprinted article either way..

    1. Net horsepower (like gross horsepower) has always been measured at the flywheel/flexplate, not at the drive wheels. However, your memory is not wrong: When net ratings were first introduced in 1971-72, a number of automotive magazines reported — incorrectly — that the new ratings reflected drive-wheel horsepower. I think what happened is that some writers and/or editors saw that the new ratings were 20-30% lower than the old and assumed (logically but erroneously) that the drop represented drivetrain losses, without looking up the actual methodology.

      Within a few years, of course, the various magazine writers had corrected their mistake, if only because of the early-70s fad for “dyno-tuning”: putting a car on a chassis dynamometer to optimize ignition timing and carburetion settings. On a dyno, the difference between SAE net ratings and drive-wheel power was readily apparent.

    1. Well, if it’s any consolation, you weren’t the only one confused by it and your memory of what you’d read was accurate — it’s just that the contemporary articles were not necessarily correct on that matter.

  10. Sorry , not sure if i sent last request correctly.

    My Camaro has a replaced engine and i am trying to find out what it is or what the hp is Gross vs nett.

    350 ci (1974 i think from the engine #’s)

    Block casting 3970010

    Engine pad # V1029CKD – 14R411617

    Cheers for any help

    1. Pete,

      See my reply to your earlier comment above. The engine code means the engine is a California LM-1 (350-4V, California emissions), which had net ratings of 160 hp and 250 lb-ft of torque in 1974.

      By ’74, Chevrolet no longer advertised gross power ratings, so the published figures are all SAE net. The LM-1 was not offered before ’74, so there are no published gross ratings for it. However, the LM-1′s net output is similar to that of the 1971 L-65 engine (350-2V), which had a gross rating of 245 hp and 350 lb-ft of torque (165 hp/280 lb-ft net).

      Therefore, we can reasonably guess that if the LM-1 HAD been offered during the gross ratings era, the rated output would have been something between 240 and 250 gross horsepower, with between 325 and 350 lb-ft of torque. Again, that’s a guess, since factory gross output figures aren’t available.

      1. Hi ,

        Thanks for you response makes things very clear.
        One other thing you maybe able to help with. By the #’s i supplied can you tell what sort of vehicle the motor came out of? ie , car , truck etc.
        Cheers

        1. I do know that the first seven digits indicate the plant (Flint), the built date (Oct. 29, 1973), and the specific powertrain (California 350-4V, automatic transmission). The rest is a portion of the VIN. I’d have to do some digging to see if there’s enough there to identify the original model.

  11. So is there a consensus on what percentage of horsepower is lost between the Gross & at the rear wheels (dyno)? I have heard rumors that there could be as much as 100 horsepower lost from Gross to dyno. For example a friend of mine had a 1970 Camaro Z28 LT-1 engine which was Grossly rated at 360 HP, but when he had it dyno’d it was rated at 260 HP. Does this sound reasonable? Thanks!

    1. Richard,

      Nope, there’s no consensus on it and really isn’t ever going to be.

      Now, since [i]net[/i] horsepower ratings reflect an engine’s as-installed output at the clutch/flexplate, you could come up with some kind of reasonable formula for estimating rear-wheel horsepower based on the net rating and the type of transmission. For gross ratings, not so much.

      Why not? The problem is that difference between gross ratings and as-installed power can vary enormously from engine to engine; there is no consistent formula for estimating the relationship between gross and as-installed output. Note that I say there is no [i]consistent [/i]formula: For some engines from some manufacturers, you could probably come up with a reasonable formula; for example, British automakers did not get into the same kind of number games as Detroit did, so the difference between British gross figures and DIN net output is generally a fairly realistic representation of the power consumed by accessories and a stock exhaust system. However, there’s no guarantee that that formula would apply to every engine from every manufacturer.

      Some gross ratings are within 10-15% of the net figures; for other engines, the gap is 30% or more. There’s just no consistency, even between different engines from the same manufacturer.

      To put it another way, if you had actual dyno-confirmed rear-wheel horsepower numbers for a given car with a given engine, you could make a reasonable estimate of the car’s net (at the clutch) horsepower. However, you’d have no way to determine from that what the advertised gross output was. Even if you pulled the engine, stripped it of accessories, muffler, and air cleaner, and connected the engine directly to a dyno, the horsepower and torque figures you recorded wouldn’t necessarily bear any clear or consistent relationship to the advertised gross figures.

  12. I have a stock ’58 Bel Air with the power pack and no power accessories. The book says it should have 230 gross horsepower. It only has 48,000 miles on it and it is very quick. It has a 3.36:1 rear axle ratio says the manual. From zero to maybe 50 it is quick as my Mark VIII, which has 280 net HP. But I won’t take it over 65 MPH. What would be the Chevy’s net horsepower?

    Thanks,
    Dave Kennedy

    1. I strongly doubt that the Super Turbo-Fire 283 has more horsepower than the Mark VIII’s DOHC V8; I have no net figures for the 283-4V, but I’d be surprised if it were more than about 200 net horsepower.

      However, there are other factors here. The Lincoln InTech engine’s displacement is actually a bit smaller than the Chevrolet’s: 4,601 cc to 4,638 cc. I am guessing the Lincoln’s 285 lb-ft of torque is more than the Chevrolet’s net torque peak, but probably not by much, and I’m pretty sure the DOHC peaks at significantly higher engine speeds; its torque peak is at 4,500 rpm. The 283′s gross torque peak is at 3,000, and while the net peak might be somewhat higher or lower than that, that figure gives you a pretty good idea of the torque curve.

      As a result, the Lincoln engine is most likely producing a little less power than the Chevy at lower engine speeds — say, up to 3,500 rpm or so. (I wouldn’t be able to say more precisely without seeing a net torque curve for the 283-4V, but that’s probably a reasonable guess.) Depending on the body style of the Bel Air, curb weight is probably fairly close, with the Lincoln likely being the heavier of the two by a small margin. The Lincoln has more gears and a shorter first gear, but a taller axle ratio (3.07 was standard) and a big gap between first and second, which allows the engine to lose more RPM on the 1-2 shift and puts you in a thinner part of the torque curve.

      So, just based on that, I’d say it’s entirely plausible that the Bel Air’s low-end torque would put it in the lead at lower speeds, even if peak horsepower is 80-90 hp less. However, if you were to take both cars to a drag strip and put them both through the standing quarter mile, I would expect the Lincoln to catch up with the Bel Air above 50 mph and then leave it behind, reflected in a significantly higher trap speed.

  13. Thank you so much for your rapid reply! You sure know your cars. I totally agree that the Lincoln would pull away after about 50 MPH. The Bel Air is a four door sedan, which is probably heavier than a two door. I didn’t realize the 283 was larger, and I sure didn’t know about the different torque peaks. It does feel as if the Lincoln IS producing less power at lower engine speeds. I’ve owned more than 60 cars in my life, and these are the two fastest and best, and most fun to drive. The Mark VIII has 78,000 miles, and at about 50 MPH if I want to pass a slow-poke, I put it to the floor and it makes a huge jet engine sound that terrifies the folks in the car or cars that I just passed. Over 50 MPH it seems to accelerate faster and faster. Great car!

  14. I find the claim of 50′s to the early 70′s being the Golden Age of the automobile to be slightly exaggerated now. Yes, many cars were on the less crowded roads and all those plusses that came with an automobile culture in it’s infancy, compared to today anyway.

    Yes, you could tell a Pontiac from a Buick a bit more easily (how dated is that reference now) and cars were simple to work on. No 5 mph bumpers clogging the design. Not many acronyms (ABS,TCS,TPMS,etc.) graced our dashboards (now referred to as instrument panels or IP, though they have less “instruments” then they used to), just pure marketing gold for Hydromatics and Select O Drives, things like that.

    Yet, in my opinion, now is the Golden Age of the Car as we know it. Cars produce more power, are more efficient and safer than they’ve ever been. A V6 Mustang now makes 300 hp off the showroom floor and can get 30 mpg if you manage to keep your foot out of it. Much more comfort to be had by those who want it. Basic cars aren’t so basic, many containing power windows standard, if not door locks and keyless entry.

    The downside obviously is that the average transaction price is now 25000 dollars. Of course, I was just reading the article on the 65-71 Riviera. It stated that a fully optioned Riv was $6000 or about $38000 in today’s money. Not cheap then either. Also today’s are very complex and require much more tech to work on, but it can be done, just not as simply as it used to be.

    This site is wonderful. Thanks!

    1. In the main, I would agree except for the instruments — a fair number of American cars of the ’60s and ’70s had no instruments beyond a speedometer, a fuel gauge, and perhaps a clock (which often didn’t work or didn’t work properly). Those cars that did have gauges too often scattered them across the dash in some kind of triumph of styling over function or jammed a tach or accessory gauge cluster on the console. My personal favorite, in the sheer “What the hell?” sense, was the tachometer integrated with the clock…

  15. What would you arrive at for a comparable horsepower rating for a 1968 L88 427cu in engine verses a 2007 LS7 427 cu in engine?

    1. The LS7 was rated at 505 hp and 470 lb-ft of torque. The L88 — that’s a trickier question. The L88′s paper rating (430 gross horsepower, 450 lb-ft of torque) was theoretically lower than the three-carb L71, despite a much higher compression ratio, higher-lift cam, bigger exhaust valves, etc. The explanation I’ve generally heard is that Chevrolet underrated the L88 to discourage ordering by owners who just wanted the most powerful ‘Vette available and didn’t understand the various tradeoffs the L88 posed for normal driving. I’ve seen estimates ranging from 500 to 560 hp, but since they are unofficial figures, it’s hard to say if that was with accessories and mufflers or not.

      I imagine the question you’re getting at is, “Was the L88 more powerful than the LS7?” I really don’t know. My guess would be that they are fairly close in actual developed horsepower, the L88′s radical cam and much higher compression ratio (12.5 vs. 11.0:1 for the LS7) at least partly balancing the LS7′s vastly more sophisticated engine management system and probably lower reciprocating weight. The difference, of course, is at lower speeds and in the mid-range. The LS7 will idle smoothly and pull from low engine speeds in a higher gear, which isn’t something with which the L88 was designed to cope artfully.

      1. I saw sticker prices and option lists on delivered L-88 corvettes in 1968 and the horsepower rating on the factory sticker for the L-88 option was 625 horsepower. The cars idled like a fuel dragster,shakin like cochy dancers,these cars were radical and needed open headers to run right ,carburators were set really rich from the factory. With modern tires I’m sure they’d outrun anything built since that’s naturally aspirated,at daytona they ran over 200 mph in the stock class, probably the last real race car you could buy from a new car dealer. So glad to have been alive to see it,still brings a smile to my face.

        1. The most optimistic rating I’ve ever seen for the L-88/ZL-1 was 560 hp, although again all of these are gross figures, rather than net.

  16. I read all of these posts and your comments back. Incredible knowledge base here. Kudos to you and your commitment at getting back to the questions asked. As a novice(went to a year’s worth of automotive college in the mid 80s) I can speak intelligently about the top 3 muscle cars of today. 12 Mustang GT, 12 Challenger SRT, and the 13 SS Camaro. I own all 3 – all white ones. The Mustang, with a lower HP 5.0 liter is still the fastest of all 3 because of the wt/hp ratio. Although “clunkier” to drive than the SRT-it is incredibly fast. Guys… These cars would kill the 70 LS1 Chevelle – hands down for 30k present money (on the mustang) Pony up (and bucks down) and drive real muscle cars of this era. Again – thanks so much for your willingness to answer everyone’s questions.

  17. I remember when the horsepower ratings dropped back in the early 70′s and the explanation we were given at the time was that the horsepower was rated at the back of the transmission(automatic) instead of the flywheel and that this was done to cover up the vast drop in horsepower due to the early smog/emission equipment and engine modifications made,especially in the head/cam valve area as well as the carburator jetting/secondarie actuation changes. The transmissions in those days ate up as much as 60 horsepower (turbo 400) and as little as 40 horsepower(turbo 250-350)so the larger engines would show a bigger drop in power on paper.

    1. The numbers I’ve heard for the TH400 were more like 40 hp and the TH350 as 30-35, but yeah. Of course, if that were the way net outputs were figured, otherwise identical engines would have different horsepower ratings depending on the transmission to which the engine was connected. (This is not to be confused with engines of the ’50s that did have slightly more power in automatic form to help account for the losses of early automatics.)

      As mentioned above, when the net ratings were introduced in 1971-72, there were a lot of misconceptions about what the new figures actually meant. I hadn’t heard that it was measured at the output shaft, although various magazine writers assumed the net ratings were at the drive wheels. (This of course would have the same issue as the transmission shaft: engines would be rated differently depending on the rest of the drivetrain.)

  18. I’m in the market for a 2011-2013 muscle car and I am having trouble deciding which to buy. I want a fast car, but also a car I can live with daily. I’m over 50, so ride is important. I like the camaro ss, mustang gt, and dodge challenger. Which one would you recommend of these 3? Also is there another car I’ve over looked?
    Thanks.

  19. I had a 1970-1 Firebird 400 ram air, I believe it was rated at 365 hp. Did that engine have a higher hp rating in any other Pontiac. How did that compare with the 455 in 1971?

  20. I bought a new 1968 Vette L 88. As you say, the invoice, sticker, and air cleaner all said 430 HP. It also came without smog pump, radio, or heater. But had headers and off road exhaust, and only F-60′s tires on 7″ wide rims.

    Perfectly tuned it was wickedly quick, most estimated 500+ hp. But in the real world, the 435 hp Tri-Power would often run with it, as would the 425 hp 426 dual 4 barrel 426 Hemi in the light Cuda and Challenger, which mostly just needer new plugs.

    Yenko later put the 454 in some Cameros that were as quick. None of the Shelby GT 500′s I saw were in the same class.

    With todays better tires the above cars would run with the new cars, except for some of the European million dollar rides.

    Thank you very much for a very informative article.

    1. In terms of real-world performance, it’s important to remember that whatever the measuring system, the rated output — over- or underrating notwithstanding — is a peak value and doesn’t necessarily tell you a lot about the engine’s actual torque curve. Especially in the era before variable valve timing and computer-controlled dual-runner intake manifolds, getting massive horsepower generally involved tuning for high-end breathing at the expense of low-speed and mid-range response (and vice versa). That’s part of the reason that, for example, a 426 Hemi could potentially be clipped by a 440 on the street: Sometimes you don’t have enough room, traction or time to get the highly tuned engine into its power band, but if you do, you leave everyone else behind.

  21. What about the 5 liter Mustang of the late 80′s, rated at 220 hp, based on the performance it is hard to believe it was less than half of the car of today?

    You are good.

    1. The 5.0′s rated output, which was 225 hp and about 300 lb-ft of torque from 1987 to 1992, was a net rating, although in 1993 Ford reduced it to 205 hp in a way that suggested the 225 hp figure had perhaps been a little optimistic. Nonetheless, the 302 still had a lot of low-end torque for that era and the LX 5.0 was not a particularly heavy car — around 3,000 lb for a basic LX 5.0 coupe — so it had a lot of grunt off the line.

      Today’s engines are two generations ahead in technology, although a lot of cars are considerably heavier than they were 20 years ago, which makes a difference.

  22. I have 66 Corvette with a 327/300HP, 600cfm Holley, close ratio 4 speed and 3:08 posi. I’m thinking about putting in a crate motor rather than building the original, numbers matching engine. Chevy has a 290HP crate motor. Would that be stronger than the old engine.Thanks!

    1. I don’t know for sure, but I would suspect the crate motor would be at least a little stronger. I haven’t seen any net output numbers for the 327, which was replaced by the 350 before GM switched to net ratings, but my guess for the 300 gross horsepower engine would be something in the 240 to 250 net hp range. Also, from a collectibility standpoint, not molesting the numbers-matching engine seems like a wise move.

  23. I have a 2008 Mustang GT that test on a dyno 269 horsepower but they advertised it as having 300 horsepower. what is with that.

    1. @Mike: Both gross and net horsepower ratings are measured at the flywheel. A chassis dynamometer measures output at the drive wheels, which means that the dyno readings reflect the engine’s power at the flywheel minus the power lost to friction in the transmission and drivetrain, which can amount to 10% or more. I haven’t studied a lot of chassis dyno figures for late Mustangs, but if the 269 hp figure is indeed at the rear wheels, it sounds consistent with the engine producing at least its advertised horsepower at the flywheel.

  24. Good article
    Starting in 1970 i owned a service station,till 1978.
    I have all my old shop manuals.Starting in 1973 all american manufacturers had this disclosure at the end of their engine specifications."Horsepower and torque are SAE net figures.They are measured at the rear of the transmission with all accessories installed and operating.Since the figures vary when a given engine is installed in differend models.Some are representative rather than exact."
    The figures are for automatic transmissions.So a torque converter and the hydraulic unit,which all automatic transmissions have,you would loose about 30% of your power vs a manual transmission,which is still true today.
    Recently i acquired a 1977 corvette manual trans.Here are the specs for the L48 and L82.
    L48:180hp at 4000rpms,and 270ft lbs of torque at 2400rpms.
    L82:210hp at 5200rpms,and 255ft lbs of torque at 3600rpms.
    So if you want to argue the L48 is more powerful.Thats the way it always was,and still is.Took of all the garbage of the engine L48,A.I.R. pump,air conditioning compressor,replaced the fan with light weight aluminum fan,put weaker springs in the distributor,high performance module,high voltage coil.And i have all the power i’ll ever need.Simple modifications.
    The A.I.R.pump takes about 40hp.air conditioning about the same,the original fan 10 to 15hp.What the distributor and coil give i don’t know,all i know its a hell of a difference.

    1. It’s very peculiar that it claimed the figures were at the transmission output shaft. Net rating systems are measured at the flywheel/flexplate and don’t reflect transmission losses.

      While some engines have different horsepower ratings with manual or automatic transmission, that’s because the engines are tuned differently. In some cases, the automatic version is tuned for a bit more power to account for the reduced efficiency of the automatic (that was common in the early ’50s); in some cases, engines slated for automatic are de-tuned (or retuned in a way that sacrifices peak power for mid-range torque) either to mate better with the automatic or to not strain the automatic’s torque capacity. However, there are a great many engines that have exactly the same rated net output (whether SAE, DIN, JIS, or ISO) regardless of transmission choice.

      Automatic transmission does not consume 30% percent more power than manual transmission. Total drivetrain losses can be 30% or more, but that’s not just the converter — that reflects frictional losses through the transmission itself, plus the prop shaft (if any), differential, etc. Manufacturers are typically reluctant to release power consumption figures for their transmissions, but the figures I generally hear for older American cars are about 20 hp for Powerglide, 40 hp for a TH400.

      Manual transmission also consume some power, just not quite as much. Assuming the clutch is fully engaged, there aren’t the slippage losses of an unlocked torque converter or fluid coupling, but there are still frictional losses, seal losses, and so forth. And of course manual transmission cars will still have the same losses through the rest of the drivetrain as a car with automatic.

      With the L48 vs. L82, the torque figures are significant. At lower rpm, the L48 does indeed have more power than the L82. Horsepower is a function of torque and rpm; based on those figures, the L48 would have about 123.4 hp at 2,400 rpm. Without having a complete torque curve for the L82, I don’t know how much torque it had at 2,400 rpm, but we know it was not more than 255 lb-ft. Even if the L82 had 90% of peak torque at 2,400 rpm (i.e., about 230 lb-ft), it would be making only about 104.9 hp at that speed, which is 17.7% less than the L48. The L82 ultimately has more power, but based on these figures, its advantage is almost entirely at engine speeds over 4,000 rpm, where the L48′s torque curve is dropping off rapidly. From that, I would expect that if you were to drag race an L48-equipped car against an L82 Corvette of the same weight and gearing, the L48 car would be faster off the line and maintain its lead until a fair ways down the strip, but that the L82 would eventually catch its less powerful cousin, pass it, and finish first with a 4 or 5 mph higher trap speed.

      1. True,but only if they had exactly the same gear ratio in the rear end.
        I once tried an open 2.50 gear in the rear end,on a 1969 GTO,and there did not seem to be an end to the top speed,i gave up when my palms started sweating,at 180mph.

        1. That’s why I said "of the same weight and gearing." An engine like the L82 would benefit from a shorter (higher numerical) axle ratio because it would not only provide greater torque multiplication off the line, it would also allow the engine to get into the meatier parts of its power curve more quickly. If the L82 had the optional 3.70 gears (or a 4.11) instead of the standard 3.36, it would be a different story.

          1. Sorry about that,i totally missed "of the same weight and gearing".You are absolutely correct.Most people miss the importance of gear ratios.
            I worked in a helicopter power transmission plant,and we used to test them on a simulated engine.The idea was to take 10,000rpms,and reduce them to 300rpms at the rotor,that was for the torque,i believe.

  25. OK, you explained some of the differences, however, if we go back to the early 50′s, at least in the truck market there was another system. I owned a 1950 1 ton GMC with 270 CID straight 6. It was rated at about 30 HP, yet later models with identical engine specs had HP ratings of around 100 HP. Were the earlier trucks rated as "usable" HP similar to tractors that were rated at the PTO? Was this also applied to automobiles of that era because again there was a significant jump in horsepower between early 50′s and late 50′s cars?

    1. My guess is that the figure you saw was 34 hp, which would be the engine’s rating on the old RAC taxable horsepower scale. The RAC formula, which the U.K. and some U.S. states used to use to calculate road tax, is the square of the cylinder bore in inches times the number of cylinders divided by 2.5. Using that formula, the GMC 270, with a bore of 3 25/32, would have a rating of 34.3, which would round to 34 hp. When the formula was first created early in the last century, it did produce a reasonable approximation of actual developed horsepower, although that was no longer true even by the early 1930s. The RAC formula, incidentally, was the reason a lot of older British engines tended to be extremely undersquare (with stroke exceeding bore) — a small-bore engine with a longer stroke had a lower taxable horsepower rating than a big-bore engine of the exact same total displacement.

      I think a few states still use the RAC formula to calculate annual road tax and that practice was more common when your truck was new, so it wasn’t uncommon for the spec sheet to list the taxable horsepower figure. Again, it has no connection to developed output.

  26. when president NIXON started pulling troops back in 1973 I was sent to ft. bragg nc to the 82nd airborne. I special ordered a 73 plain 2 door nova with sport suspension, power disc brake, and 3.42 rear diffentral. I also ordered in it 3 speed manual with the L48 engine. I was informed by the dealer two weeks later that I could not get the L48 engine with the manual trans and I had to either go 4 speed or automatic.I went auto for insurance reasons but the car came in with the L65 2 bbl instead. the first day I took delivery I punched it and watched the needle go out of sight. it did not make since. after adding headers, intake and spreadbore holley, plus a total accel ignition it was hard to beat. I had owned a 66 GTO and a 67 396 chevell. they had much more torque but could not an any way run with this car on top end. beleive it or not this is a true story.

    1. A 350 Nova, even an L65, did have a weight advantage over the earlier Chevelle SS396 or GTO — on the order of 200 pounds, depending on exact options choice. Adding a bigger carb, headers, and a new intake manifold would obviously also give the L65 more than the rated 145 hp!

  27. This write-up is, for the most part, quite good.

    However, the alluded to 340 MOPAR’s "325 HP" NRA figure is a "factored" number. It’s an average of what a particular engine running in given NHRA classes make for power with the MODIFICATIONS permitted within that class. These figures are therefore largely impacted by the NHRA tech specs, which permit milled cylinder heads (sometimes resulting in nearly 3 full points of added compression for some engines). Doing this results in minimum allowable chamber size, and is also known as "cc’ing the heads."

    NHRA tech specs also permit 3 (and now 5) angle valve jobs in the "stock" classes, and well as engine over-bores. Obviously long tube headers, intake tweaks, and some aftermarket internals are also permitted.

    Additionally, the same engine can be "factored" at several different HP figures – depending the various weight classes in which is runs.

    So by no means is an NHRA "factored" HP number representative of a truly stock engine.

    1. Thanks for the information — that’s helpful.

      Based on the 340′s performance in stock (not drag-prepped) condition, I think the 275 hp rating is a probably a reasonable approximation of the stock engine’s *net* horsepower. Even the ’72, which was detuned quite a bit, was rated at 240 net horsepower, which leads me to conclude that the earlier engines were more in the 265-275 net range.

  28. I also have a 1972 Olds 455, and I’d really like to find out how much power it really has. Do you have an idea what the gross HP or the ACTUAL net HP was for these engines in ’72?

    I’ve read that the advertised BHP was as low as 225 HP (360 ft-lbs) on some models, and only as high as 300 HP (410 ft-lbs) for the W-30′s with Air Induction. I don’t think the engine I have was originally a W-30, but it has the W-30 "Ga" heads on it.

    I’d really like to know what kind of HP I could expect out of this engine, and whether it’s worth a full rebuild as-is or if the ’72s are actually doomed to be low-HP engines without changing the heads, pistons, etc. Even an estimate would be much appreciated! Thanks.

  29. I have an 1966 olds F85 2dr post coup with a 1976 190 net hp olds 455. That net rating was for an air conditioned, power steering, power brake car with Exhaust Gas Recirculation intake manifold ,small block exhaust manifolds with single exhaust using a catalytic converter.As installed in the f85 with standard steering and brakes,no air of course,edlbrock non EGR intake,performance cam, big block duel exhaust manifolds with 2.5 in mandrel bend with Xpipe system no catalytic converter the car feels like a 400 gross horse power car,even before the mods it felt like 350 gross hp. There really isn’t much difference from the late 60s engines, the 2 point drop in compression can be felt but can be more than made up for in cam profile and free flowing exhaust ect: It seemed easier to guess at gross horse power ratings after modifications but I will take a wild one and say I jumped from 190 net to 290, any other guesses?

    1. I don’t think factory net ratings generally reflect options like power steering or air (unless they’re standard), although obviously the additional accessory pumps do consume some power. In any case, 290 net horsepower from an uncorked 455 seems entirely plausible, although the surest way to find out would be to put it on a chassis dyno.

  30. yes, in the late 60s GM did have a corporate ‘rule’ that no vehicle other than the corvette could have a horsepower rating in excess of 10% of the vehicle weight. that did mean that the ‘same’ engine that went into the heavier gto was rated at 360 hp vs the 325 hp for the ‘same’ engine that went into the firebird. except that the engines were not identical. or i should say the carburetor linkages were not identical. there was a restrictive tab installed on the 4bbl quadrajet carburetor that went on the firebird. it restricted the back 2bbl from opening completely. once that tab was removed you had a fully functional 360hp gto engine in a firebird. that car flew.

  31. The earliest figures for the 360 (1970-1971) are SAE gross numbers, so those are harder to compare directly to the post-1972 figures. However, all the later numbers are SAE net, so there aren’t any issues of conversion or rating systems (there have been some changes to the SAE net measurements in the past decade, but I don’t think there were any between 1972 and 1984).

    The later figures are reasonably typical for early emissions-controlled engines. For comparison, a 1976 Chevrolet 350 was rated at 165 net horsepower, while the Mopar 360 was rated at 170 or 175 horsepower. Even the big blocks were way down on power — the 500 cu. in. Cadillac engine was down to 190 hp by 1975, and late Mopar 440s rated only 195 hp.

    The main issue was trying to meet federal (and California) emissions standards with carbureted, non-computer-controlled engines that in most cases hadn’t originally been designed for the new standards. Moreover, it wasn’t just a matter of meeting the standard with one engine in one car at a time — every engine had to comply, despite normal production variations, and you had to prove that they would continue to do so for 50,000 miles. Eventually, automakers figured out how to meet the standards without compromising power so severely (thanks in part to the widespread adoption of fuel injection and computer controls), but in the short term, many opted for a very mild state of tune to make sure they met the requirements.

    In terms of specific output (horsepower per cubic inch or liter), bigger engines tended to suffer more than smaller ones. The earliest federal emissions standards were based on percentages of exhaust volume (parts per million), but by the late seventies, the standards were for total output (grams per mile or kilometer), which gave smaller-displacement engines an advantage. That’s one of the reasons the big block engines nearly died out, except in trucks — the smog-controlled big engines still had more torque, but they didn’t necessarily make much more power than smaller ones, and they were potentially a lot thirstier.

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