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:


Over the years, the Society of Automotive Engineers (SAE) and similar standards-setting bodies in other countries have developed various methodologies for measuring the output of an automotive engine. These standards fall into two basic categories: gross and net output.

Gross output, which in the U.S. has typically been measured using a methodology originally codified in the SAE Gasoline Engine Test Code and later revised in SAE standards J816, J245, and J1995, is the output of a ‘bare’ engine running on a test stand with no external engine-driven accessories (e.g., alternators or water pumps), free-flowing exhaust headers with no mufflers, and optimal ignition timing. Gross ratings are also mathematically corrected to standard air pressure and temperature. In other words, gross output represents a particular engine’s maximum output under ideal conditions.

In the real world, automotive engines very rarely operate in ideal conditions. The engines of most cars are burdened with various engine-driven accessories, including the engine’s own water pump and generator/alternator and add-ons like the power steering pump and air conditioning compressor. Engines intended for on-road use typically also have restrictive air cleaners and exhaust systems, sound-deadening mufflers, and emissions-control add-ons like catalytic converters and thermal reactors. Engine tuning is further compromised in the interests of reduced noise, better drivability, improved cold-weather performance, and lower exhaust emissions. All of these factors reduce the engine’s maximum output in ways that the gross rating methodology does not reflect.

For that reason, the SAE and similar bodies have also established standards for measuring net output. Net ratings, such as the ones defined by SAE standards J1349 and J2723, are still taken with the engine on a test stand, but reflect stock ignition timing, carburetion/fuel delivery, exhaust systems, and accessories. The specific methodology varies depending on the specific standard being used, but the gist is that a net rating is a closer approximation of an engine’s output as actually installed in a car or truck.

Naturally, the net output of a given engine is somewhat lower than the gross output. For example, the 217 cu. in. (3,547 cc) “Stovebolt” six in a 1950 Chevrolet had a gross output of 92 hp (69 kW), but a net output of only 85 hp (63 kW). Chevrolet’s 1955-vintage 265 cu. in. (4,344 cc) small block V8, meanwhile, had a gross output of 162 hp (121 kW) with 8.0 compression and a two-barrel carburetor, but a net output in the same form of 137 hp (102 kW).


Net ratings have existed for many years, but before 1971, most U.S. automakers preferred to quote the more generous gross figures. Net figures might appear in shop manuals, technical papers, or other factory literature, but rarely showed up in advertising.

Until the mid-fifties, the gap between gross horsepower and as-installed output was seldom vast. However, as it became apparent that impressive horsepower numbers sold more cars, manufacturers’ advertised gross output figures began to climb. While automakers didn’t necessarily publish net ratings for their hot engines, the performance of cars so equipped suggested that the advertised figures were now substantially higher than the engines’ as-installed outputs — sometimes by 25% or more.

The late-fifties recession temporarily put the brakes on the advertised horsepower race. By the mid-sixties, the advertised outputs of many bread-and-butter engines were still unrealistically high, but the outputs of Detroit’s most powerful engines were now as likely as not to be deliberately understated. For example, in 1965, Chevrolet released the 396 cu. in. (6,488 cc) Turbo-Jet 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). At first, some Chevrolet promotional material quoted a gross output of 450 hp (336 kW) for the bigger engine, but the division then hastily restated maximum power as 425 hp (317 kW), no more than the 396. Outside observers were incredulous, particularly since the bigger engine was plainly more powerful than the smaller version.

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, officially known as a Turbo-Jet 427. Early factory literature credited the L72 with 450 hp (336 kW) at 6,400 rpm, but this was subsequently 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? One possible reason was racing. Particularly in drag racing, eligibility for different classes was sometimes based on advertised horsepower and shipping weight, so an engine that produced more power than its advertised gross rating indicated offered an obvious advantage. Racing officials were seldom fooled by such tricks for long, however, and began to handicap or “factor” advertised outputs to place a conservatively rated engine in a more appropriate class based on its actual performance.

Another reason for underrating was political decorum. Particularly at GM, the most conservative of the Detroit automakers, there was real fear of the safety lobby, which already considered the power outputs of existing engines to be irresponsibly high. In that climate, offering a 450 hp (336 kW) or 500 hp (373 kW) Corvette, for instance, might have been asking for trouble. Insurance was also becoming an issue, with a growing number of auto insurance companies levying prohibitive surcharges on very powerful cars, or simply refusing to cover such cars at all.

For those reasons, GM also had internal rules limiting all of their passenger 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 sequiturs like rating Pontiac’s 3,300 lb (1,500 kg) Firebird at 325 hp (242 kW) while claiming 360 hp (269 kW) for the mostly identical engine in a 3,600 lb (1,635 kg) GTO.

Underrating of this kind was at best an open secret. When Car Life magazine tested a 1969 Pontiac GTO Judge equipped with the $390 Ram Air IV engine, for example, Pontiac executives freely admitted that the engine’s 370 hp (276 kW) gross rating was purely a political fiction.


Between inflation and deliberate underrating, the relationship between advertised gross horsepower and actual net output was becoming increasingly nebulous by 1970. The gross ratings served a variety of useful marketing and political purposes, but as a realistic measure of engine output, they left much to be desired. In particular, the gross ratings seldom reflected the impact of add-on emissions control devices like air injection and exhaust gas recirculation, which were already beginning to have a noticeable impact on engine performance.

The final straw was the passage of a California law requiring that any reference to engine horsepower in advertisements, brochures, owner’s manuals, or sales manuals for 1972 or later gasoline-powered cars and light trucks state only the as-installed SAE net rating. Faced with that reality, manufacturers decided it was time to abandon the gross rating system entirely, at least in the United States. For 1971, many U.S. manufacturers listed both SAE gross and net ratings (providing a sometimes illuminating comparison between the two) and then switched to net ratings exclusively for 1972 and beyond, even in states other than California. (Making the change across the board was probably a prudent move from a liability standpoint, considering that in the U.S., consumers can and periodically do sue manufacturers whose products don’t perform as advertised.)

The move also provided a useful and relatively inexpensive PR gesture. By simply switching from gross to net ratings, automakers sent a message to lobbyists and lawmakers that the horsepower race was over and Detroit was no longer offering outrageously powerful engines. Beyond that, the timing of the switch helped to obfuscate the actual losses caused by added emission control hardware and lower compression ratios, which was presumably very useful for the unfortunate Cadillac salesman trying to rationalize why the 1972 Cadillac the customer is looking at seems to have 40% less power than the 1970 model he’s trading in.

The immediate result was a dramatic drop in advertised power. For example, the mammoth 500 cu. in. (8,194 cc) engine in the Cadillac Eldorado fell from 400 gross horsepower (298 kW) in 1970 to only 235 net horsepower (175 kW) for 1971. The real decline wasn’t quite as steep as it looked; the 1971 engine did have a lower compression ratio to prepare for the adoption of unleaded gasoline, but the 1971 gross rating was still 365 hp (272 kW), so the actual loss was about 10%, not more than 40%. (Cadillac did not publish net horsepower ratings for the high-compression 1970 engine, but our guess would be that it made 275–285 hp (205–213 kW).)

Rating methodology notwithstanding, the initial decline in power was relatively modest, but that wouldn’t remain the case for long. For example, the hottest version of Pontiac’s 455 cu. in. (7,481 cc) V8 still managed 310 net horsepower (231 kW) in 1973, but was down to only 200 hp (149 kW) by the time it faded out in 1976. Ford’s familiar 302 cu. in. (4,942 cc) V8, meanwhile, which in the sixties had advertised as much as 306 gross horsepower (228 kW), 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.

U.S. automakers continue to use SAE net ratings, but in 2005, the SAE issued a new standard, J2723, which clarified and amended the existing methodology, among other things requiring that a suitably qualified independent observer be present during the rating procedure. Some engines certified under the new SAE “Certified Power” guidelines ended up with lower ratings than before, while a few others actually ended up with higher ratings. In most cases, the engines were not actually altered in any significant way; the changes were in the test methodology. Most if not all manufacturers now use this methodology for their U.S.-market cars and trucks.


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 strict DIN (Deustches Institut für Normung, German Institute for Standardization) standards. DIN horsepower and torque figures are metric net ratings, and typically differ somewhat from SAE net ratings due to minor differences in test methodology (such as correction factors for standard atmospheric conditions) and the difference between metric and mechanical units (discussed further below). Some Italian automakers formerly used the Comitato Unitario dell’autotrasporto (CUNA) standards, whose metric net ratings again were similar but not identical to SAE net ratings. Depending on the time period, model, and market, other European automakers might use either DIN or SAE gross figures.

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 rate the outputs of their home-market cars under the Japanese Industrial Standards (JIS), which include methodologies for both gross and net output. The difference between the two was generally around 15% — sometimes a little more, sometimes a little less. Until the mid-eighties, most Japanese automakers quoted JIS gross figures for most if not all products sold in the Japanese domestic market. The switch to JIS net ratings began around 1985, but wasn’t completed until late in the decade. Confusingly, for some model years, manufacturers would quote net ratings for some engines and gross ratings for others, even within a single model line.

Prior to 1971, most non-U.S. automakers would publish SAE gross figures for all engines exported to the U.S. However, foreign automakers seldom indulged in the kind of gamesmanship Detroit sometimes played with its gross ratings, so the differences between gross and net ratings were typically small and probably fairly realistic. 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 PS (110 kW) DIN.


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 readers. One metric horsepower (often abbreviated PS, from the German Pferdestärke) 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.) The difference is often small, but it does frustrate efforts to be consistent or precise.


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 do not reflect power losses in the drivetrain.
  2. Because of the vagaries of the old gross ratings, the widespread over- and underrating of different engines, and the considerable differences in the amount of power consumed by different intake/exhaust/accessory configurations (even for different applications of the same basic engine), there is NO reliable formula for converting gross horsepower to net horsepower or vice versa. Sometimes, the difference is as little as 5–10%; sometimes, it’s more than 25%. Unless the factory released both gross and net figures for a given engine (which some did, even in the U.S.), 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-1971 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. Some engines’ advertised gross horsepower probably exceeded their actual as-installed power by 100 hp (75 kW) or more.


Most of this information is based on decades of reading car magazines. Some of our specific references included (but were not limited to) Robert Ackerson, Chrysler 300 ‘America’s Most Powerful Car’ (Godmanstone, England: Veloce Publishing Plc., 1996); “Alfa Super Spider,” Road & Track October 1959, reprinted in Road & Track on Alfa Romeo 1949-1963, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1985), pp. 52–53, 65; the Auto Editors of Consumer Guide, Consumer Guide Car & Truck Test Monthly August 2005 and July 2006 and Encyclopedia of American Cars: Over 65 Years of Automotive History (Lincolnwood, IL: Publications International, 1996); “Auto Makers Are Changing the Way They Advertise Horsepower,” New York Times 12 December 1971, p. 82; 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., ca. 1985), pp. 8-9, and “Miscellaneous Ramblings,” Road & Track December 1961, pp. 15–17; 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; California Vehicle Code, Section 9950; “Chevrolet Camaro Z28,” Car and Driver May 1971, reprinted in Camaro Muscle Portfolio 1967-1973, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1992), pp. 118-121; Chevrolet Division of General Motors Corporation, Chevrolet 1950 Engineering Features: Passenger Cars (Detroit: General Motors Corporation, 1949); Lionel Deluy, “Technologue: SAE What?” Motor Trend, 15 October 2005, www.motortrend. com/news/ technologue-34/, last accessed 10 June 2016; Dodge Division of Chrysler Motors Corporation, “Dodge 71” [1971 Dodge full-line catalog 81-205-1041], Aug. 1970; “GM: Cadillac,” Motor Trend Buyers’ Guide 1971, reprinted in Cadillac Eldorado 1967-78 Performance Portfolio, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 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., ca. 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., ca. 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., ca. 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., ca. 2004): 20-23; Don Sherman, “Pferdestärke and other horsepower secrets revealed!” Car and Driver Vol. 30, No. 12 (June 1985), pp. 26–27; “Technical Highlights,” Car and Driver Vol. 17, No. 4 (October 1971), pp. 57, 101; “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., ca. 1998), pp. 91-95; William K. Toboldt and Larry Johnson, Goodheart-Willcox Automotive Encyclopedia (South Holland, IL: The Goodheart-Willcox Company, Inc., 1975); “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, England: Brooklands Books Ltd., ca. 1997), pp. 30-33; and Mark Wan’s AutoZine (www.autozine.org). We also verified a few details per the Wikipedia® entry on horsepower (en.wikipedia.org/wiki/Horsepower).


Add a Comment
  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.

    1. I have owned a 72 Cutlas S (180 hp) and in later years a 70 Cutlas Supreme (320 hp).
      I have always felt the 1970 Cutlas was a guttsier performer, however this is by the seat of my pants and a long time ago.
      Loved both cars but think I was closer to the 1970 model.

      1. The 1970 engine was indeed more powerful, just not 140 hp more powerful. Your Cutlass Supreme presumably had the L33 (455/320 with duals), which had gross ratings of 320 hp @ 4,200 rpm and 500 lb-ft @ 2,400 rpm on 10.25:1 compression. The regular-fuel L32 in 1971, which had the same gross horsepower rating (320 hp @ 4,400 rpm), had 250 net hp, so it’s reasonable to assume the 1970 edition was about the same, although the high-compression engine had more torque (500 lb-ft @ 2,400 rpm compared to 460 lb-ft @ 2,800 rpm for the regular-fuel version). So, the 1970 had something like 70 more net horsepower and probably more than 100 lb-ft more net torque than the ’72.

  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. 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.

  5. How much hp. does an 02 firebird with a 3.8l have?

    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.

    2. f v6 firebird had 200 net hp in those years

  6. I have a 72 cutlass supreme with a 455 7.5L. What is th HP from the factory? How can I wake this sleeping giant ?

    1. There were multiple Oldsmobile 455 engines available in 1972.

      It could be 225, 250, 270, or 300 net horsepower. The 300 net horsepower engine was the W30, if I recall.

      225 was the base 455 with single exhaust, and 250 was with dual, if I’m not mistaken.

      I don’t entirely know what the 270 horsepower option was.

  7. 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

    1. Darrow,

      Your analysis is not quite correct. Different fuels have different heat of combustion and mixture characteristics. Most gasoline now contains at least 10% ETHANOL by law. The value of the 112,000 BTUs/gal is reasonable for 87 octane gas. The ethanol has a octane value of around 129. It is added to a mixture of Heptane and other components to improve the octane value to the calculated 87 value. If one were to run aviation fuel at 100 octane the heat value would be closer to 125,000 BTUs/gal. The ethanol has a heating value of 86,000 BTUs/gal (significantly lower).

      Why do dragsters and funny cars run alcohol as a fuel. Two reasons. One, the higher octane value allows them to run high compression ratios without detonation. Second, the amount of air needed to burn alcohol is about 1/4 as much as gasoline per molecule. This allows better volumetric efficiency (better breathing) in their engines and more horsepower. So in theory, if an engine can breath efficiently then the better fuel can provide a marginal improvement in the heat and therefore power that can be extracted from the engine. As an engine “detunes” itself automatically to burn lower quality gasoline, it loses efficiency and cuts horsepower. You do get better performance from pump.

      This brings me to the next point, that is why not offer all gasoline at the higher octane ratio? It cost a little more but if most of the gasoline were produced at the higher octane value then the difference would be minimal and the fuel economy and HP would increase for cars once the automakers take advantage of the better fuel and increase compression ratio/tuning.

      1. The latter is sort of an eternal question. One day, I’d like to get some petroleum industry marketing person to explain it to me, but I assume it has more to do with merchandising than engineering!

  8. 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?)


        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.

  9. I have a 1966 427 Corvette and a 2005 Corvette. How do they compare using net horse power.

  10. 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.

  11. 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.

  12. 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!

  13. i would lie to know flywheel horse power rating on 1987 bmw 325e 2693cc and how to find gear ratio ,its a five speed

    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.

  14. Note 3rd paragraph. Pre 1972 engine was rated on a test stand, to mufflers, no accessories

  15. 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.

  16. Thankyou on your detailed explanation; Old these days but still learning…

    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.

      1. I am the original owner of a 78 trans am with a w72 400 and 150,000 miles. I want to have the engine rebuild to stock . I want to change the original cam, 274/298 .395/ 0.401 to a 301/313 .414 lift. Do you know what kind of increase net horse power that the cam change would give me? According to High Performance Pontiac Dec 2011 a few carb and timing adjustments this engine had a dyno of 225hp /313 torque at the wheels and making 270 hp at the crank. Thank You for your time.

        1. I’m afraid I’m not qualified or comfortable giving modification advice, and I’m not sure I can accurately estimate how much power that would give. (This is an area where you might be better off looking online for hot rodding magazine articles where people have made similar changes and have actual dyno or quarter mile figures to quantify the results.)

          The best I can tell you is that you might want to bear in mind that such significant increases in valve lift and duration will give you more power on top at the expense of torque and response down low. If you’re looking to race, you may know this already and not care, but if the object is just to increase power for its own sake, you might find the hotter cam makes the car feel more lethargic than before except when really wound out. A late-seventies T/A is no lightweight, so sacrificing torque at the low end will hurt around town unless you change gearing, which could be obnoxious for highway cruising.

          1. Hi Aaron, you seem incredibly knowledgeable, so I’m hopeful you’ll be able to help…

            I’ve been thinking that, for a given displacement (and narrowing the inquiry to the flywheel for simplicity’s sake, as I’m just asking this as a matter of principles) …

            Would the change in temperature of a given CFM be proportionate to the amount of power created? If at TDC the fuel-air mixture’s 200 C and then becomes 1400 C, the pressure-change would be cognizable. And there’d be an argument to say that the more air that undergoes a greater temperature-contrast (in and out), the more “power” that’s made.

            I’d assume the fuel-air ratio will dictate the stoichiometric limit …
            A NA-engine will be some percent of atm. (14.7 PSI) x displacement.

            Or, at exactly 1-bar, we’d have a known value of displacement (fluid) changing temp, at each RPM.

            I get that it’s way easier to measure than idealize a maxim and conjecture it, but my only point / curiosity is to ask, is the change in temperature of a given volume of air indicative of the torque / power..?

            Or more importantly, however YOU would prefer to word that.

            Thank you, very much.

          2. I’m flattered by the faith in my knowledge, but this represents a level of engineering knowledge I can’t claim.

            The only thing I can say is that generated torque (and thus power) is most directly related to compression ratio and expansion ratio. While those are obviously related to temperature, the relationship isn’t quite so fixed as you’re suggesting, particularly in modified intake valve closing cycles such as the Atkinson cycle or Miller cycle. In a Miller-cycle engine, for instance, delaying intake valve closure reduces charge temperature (which among other things reduces NOx formation), but increases charge density, which produces more power despite the temperature reduction.

  17. 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.

        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.

  18. 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.

  19. 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?

    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.

  20. 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!

  21. 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…

  22. 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.

  23. 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.

  24. 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.)

  25. 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?

  26. 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.

  27. 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.

  28. 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.

  29. 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.

  30. 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.

  31. 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.

  32. 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!

  33. 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.

  34. 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.

  35. 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.

  36. how many horses does a 1976 Chevy Vega have?

    1. The base engine had 70 net horsepower, the optional engine had 84, and the Cosworth had 111.

  37. 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.

  38. Hello, I have a 73 Ford Mustang with a 351 C 2v, dual exhausts. Not the original motor. According to the number cast into the block behind the starter this motor was manufactured in 1972, app. 13 months before my 73 was assembled. Believe it was rated at 177 BHP. Does that sound accurate and can I assume that this HP figure was derived with the motor being tested with accesories connected and an exhaust system ? Of more interest I have located the letters/numbers stamped in driver’s side of the block on the smooth surface below the head. I believe this number is supposed to show that a motor came with the car off the assembly line, (matching numbers). The letters CAR appear in this position … any idea what they mean ? Thanks

    1. I assume the “CAR” lettering indicates that the whatever vehicle the engine was originally built to power was a passenger car and not a truck. If the engine was indeed from a 1972 non-California 351-2V Mustang, its SAE net rating was 177 hp, although it’s interesting to note that the same engine in a Torino was rated at 161 hp.

      1972 was a confusing year as far as power ratings went: A lot of contemporary consumer and car magazines were very confused by the switch to net ratings, which to compound matters appear to have been released later than usual at the start of the model year. So, if you look at different catalogs or buyers’ guides, there are all kinds of estimated figures that aren’t necessarily accurate.

      To answer your other question, net ratings, which the 177 hp figure was, were taken with accessories and exhaust system installed. Even so, there was (and remains) some variation between different engines produced on the same line, so not every engine is going to produce precisely 177 hp — some are going to be a little weaker or a little stronger just due to normal production variation (different tolerances and so forth).

  39. I have a 66 Chevelle 327 275hp what would the net hp in today’s standards.

    1. Hmm. At a guess, probably something between 200 and 220 hp. That’s not a factory rating — I’ve never seen net ratings for the 327 — but my very rough estimate based on the net ratings for comparably tuned Chevrolet 350s.

      1. If I’m following you correctly the 327 275hp Gross is equal
        To 220 net and with a 4 speed 1:1 is probably around
        190HP at the rear wheels? I have a 66 Nova 327
        4 Speed that does 230 HP at 4,000 RPM at the rear wheels on a chassis Dyno with Headers and a 3:55 rear end. . What do you think the gross HP at the flywheel would be? Thanks

        1. Net horsepower is measured at the flywheel, not the rear wheels. How much power you get at the rear wheels is net horsepower minus whatever power is consumed by the transmission, driveshaft, differential, and drive axles, with the transmission being responsible for most of that. I think 220 hp at the flywheel is likely to give you a good deal less than 190 hp at the wheels, even with a light-duty four-speed manual transmission; that’s a loss of only 13.6%, which is unrealistically low. (My guess is that you’d lose at least 20–25%, which would be more like 165–175 hp at the wheels.)

          As I keep saying, there is no gross-to-net conversion formula. Having 230 hp at the wheels suggests that you probably have at least 285 hp at the flywheel, but, short of pulling the engine and setting up dyno tests to replicate the methodology of gross horsepower testing — which seems like a lot of work — the best you can say about the engine’s likely gross rating in its present state of tune is “more than that.” Probably at least as powerful as a stock L79 327, which claimed 350 gross hp, at a guess.

  40. Hello, do you know how the hp was reduced between 71-70 olds? were the pistons used to lower the compression or the cyl heads? thanks Ellis

  41. All I am going to say was that back in the Muscle Car Days of the late 60’s and very early 70’s I was there. The Boss 429 Mustang was more a Nascar engine and not a Drag Strip engine. The 340 Mopar Cuda was faster than the 429 Boss Mustang on the street. The LS6 Chevelle SS’s ran away from most of them.

    1. The Boss 429 was pretty explicitly a NASCAR engine — it was only offered in the Mustang because Ford needed to sell the requisite number of engines (in anything) to meet NASCAR homologation requirements.

      I should also emphasize that the gross and net output ratings discussed here are nominal factory ratings for stock engines, and as such don’t reflect production variations — which in those days could amount to 15–20 hp — or after-sale tuning and modification. For racers or tuners who see factory equipment mainly as a blank canvas and think in terms of “potential” rather than off-the-shelf performance, the factory ratings (and the accuracy thereof) are largely irrelevant except maybe as a way of identifying which engine options come with which desirable components. It’s pretty clear, for instance, that the Chevrolet LS6 and L78 engines (454/450 and 396/375) could be made to produce a good deal more power than they necessarily made when they first rolled off the dealer lot.

      Also, accelerative performance of course depends on a lot more than just developed horsepower (whatever that may be). Power-to-weight ratios, weight distribution, gearing, tires, and even suspension design can play a major role. It was not terribly difficult, particularly with bias-ply tires, to end up with more power than you could effectively use.

      1. Thank you Aaron for a well-researched article. I was told several years back that the difference between gross horsepower and net horsepower was 37%. You say it is 25% which seems more credible.

        1. As the article explains, there is NO set formula for converting gross to net horsepower. There are some engines where the difference was indeed 35% or more — for example, the 1971 Cadillac 500 cu. in. engine, based on Cadillac’s published gross and net outputs (235 vs. 365). In other cases (e.g., the Triumph TR250), the difference was less than 10%.

          Unless the manufacturer published both gross and net ratings for the same version of the same engine (which some did, although for U.S. automakers, that was more common for trucks than cars), the best you can usually do is make a rough estimate based on quarter mile trap speeds or something like that. (I suppose technically you could pull the engine, make sure it’s in healthy stock condition, and run it through the appropriate test regimen on a dyno, but that’s not exactly an easy weekend project!)

  42. Hi, there. You are doing the great work.

    I am currently in process of restoring a 1968 Dodge Charger, 440 CID and a four barrel carb. I was wondering what is the engine Net rating? Car is in a sad shape, and the engine too. I am also considering putting the 3×2 (six pack) 3 2-barrel carbs on, since I need to rebuild the engine and transmission anyway, new intake mainfold and carb(s) are a must have anyway. What is the 3×2 engine Net rating, too? I wonder how much ACTUAL performance increase it would make – go 6-pack, or stay stock?

    Surprisingly, the car still pulls incredibly strong, i keep getting around 14.5-15 sec 1/4 mile…depending on the state of tires currently on (always used ones, never bought new set). But it burns a ton of oil (leaks all over my yard, too), about 2-3 liters every tank of petrol, and the gearbox is so leaky I have to top it up almost every week, engine also leaks water and starts very bad (sometimes hard to stop, too, lol)…never going around without 2 l of gearbox fluid, 5 l of water and 7 liter of engine oil…and never go far :(, being towed 2x yearly on average.

    Oil burning is sometimes a problem with the road police and registration, though…

    I am trying to wake this sleeping beauty (or Pipe smoking granny as my friends call it).
    I am also very disappointed with the car top speed, it hovers around 200-220 km/h, I expected more from all that power and the Autobahn…Fuel consumption is also in the range of 30l/100 km, which is maybe too much. I am considering putting a LPG installation on, and raising the compression ratio a bit.

    Thanks a lot in advance, fellow gearhead.

    1. I am assuming your car has the 440 Magnum, which in 1968–70 claimed 375 gross horsepower. For 1971, the compression ratio was reduced a bit, bringing the gross rating to 370 hp. 1971 was the first year Dodge published net ratings for passenger cars (at least that I’ve seen), so they also published net ratings of 305 hp and 400 lb-ft. The higher-compression version was probably a tad higher; let’s say perhaps 310 hp net.

      The 440 Six Pack previously claimed 390 gross hp and dropped to 385 hp for 1971 on slightly lower compression. Interestingly, the 1971 Six Pack’s net rating is 330 hp, 25 hp higher than the four-barrel engine. If you were drag racing, that would be worthwhile, although whether it’s worth the bother on the street is debatable. (The concern with the Six Pack is that the linkage is vacuum-controlled rather than mechanical, so you sometimes get all three carburetors whether you want them or not.)

      Regarding top speed, keep in mind that most American engines of this vintage, even the performance-oriented ones, do not relish high revs and are geared for speeds under 160 km/h. I don’t know what axle ratio you may have, although 3.23:1 was typical for 1968, which would be roughly 37.5 to 40 km/h per 1,000 rpm depending on what tires you have. A top speed of 220 km/h would therefore mean an engine speed of 5,600 to 5,800 rpm, which is fairly ambitious for most big U.S. engines of that time (particularly if yours has cooling and oiling ailments!). So, going much faster would likely require either a taller axle ratio (say, 3.08:1) and/or doing some work on the valvegear to allow you to pull and sustain more than 6,000 rpm. (I have no idea how much of that would be legal under your current licensing laws — I had been under the impression that German generally required the hardware to remain stock.) Taller gearing would of course hurt performance at lower speeds.

      The truth is that the speeds you’re already observing are a lot more than these cars were ever intended to achieve except perhaps on a NASCAR oval track with a lot of advance preparation. In the U.S., going that fast on public roads is a good way to end up in jail unless you’re a policeman or are white and well-connected in local politics. The cars generally didn’t have the chassis, the tires, or the brakes for that sort of thing and it starts to raise some questions about engine cooling and lubrication as well. (American cars of the ’60s didn’t always have an abundance of cooling capacity even for 110 km/h highway driving in hot weather, much less high sustained speeds.)

  43. Thanks a lot for the reply.

    25 hp difference is maybe worthwhile, since it would be part of an engine overhaul – car restoration anyway.

    I am thinking about stroking the engine reasonably (block is good, just the oil rings are bad, etc), along with boring it just a little bit (.030) over, the new pistons, stronger cooling and oil pump, ported heads…

    I can not tell if the head is leaking oil or the headers or the manifold…if it is good, maybe i will keep the original and just bolt on a bigger carb – more cfm than stock for sure.

    I tested the top speed on the Autobahn because, why not – but it gets kind of scary past 180 km/h, the whole car shakes, noise is horrible, and I have a feeling at about 200 km/h that the steering wheel is gonna fall off, lol.
    Guess that the aerodynamics is a bitch after 160 km/h. Not driving even close to that regularly.
    RPM is about 5000 at 200 km/h (max power curve anyway?), since my tires are 275/60 R15, and yes 3.23:1 ratio is right.

    I plan on putting an overdrive on for sure, disc brakes etc…

    I use it for an occasional drag race, yeah, but not for the long distance hauls (fuel costs, not that much cargo space compared to the Voldo – although spacious by standards of most people)
    and it gets tiresome to drive after more than an hour anyway), or Autobahn cruising – have my Volvo 850 R for that, so I am not overtaken by diesels…it can cruise at 220 km/h without effort, and there is power up to about 270 km/h for overtaking…

    I like to drive to work in Charger (not in winter though, do not wanna crash it in a tree since no ABS+rwd+1960s brakes), only 30 km every day there and back, and of course when I go out with girls – they all detest my Volvo, black boxy estate with tinted windows, so they think it is resembles the undertaker car, but always fall for the Charger…and the spacious rear bench.

    Regarding the cooling, had no trouble at higher speeds, but in summertime during the traffic jams sometimes it runs a bit warmer than it should – i avoid going out with Charger in a rush hour… Climate is not that hot here, so it gets cooled by airflow pretty well…

    I have put together about 10 k $ that i am ready to invest in the car at this point, so some work has to be done – engine first, and tranny second. bodywork is mostly OK, just a few rusty spots in the trunk.

    1. The ’68 Charger looks great, but its aerodynamics were none too good, as evidenced by the work that had to be done to make it competitive in NASCAR: first smoothing out the recessed grille and moving the backlight to eliminate the roof buttresses, then the nosecone and the wing on the Daytona. So, you have quite a bit of drag and more lift than you would probably like to think about. Also, while the Charger’s suspension was on the firm side by contemporary American standards, the stock suspension bushings (which may also be old and tired) were intended to take the edge off of bumps at 40 km/h, not at Autobahn speeds. That could conceivably compound your aerodynamic woes, resulting in momentary deflection steer from the rubber bits in the suspension as the car batters its way through the air. This is what you get from powerful cars designed to look good and go quick (but not fast) in the land of the free and the home of the speed limits!

  44. Thanks mate for everything. I am considering several options for an engine build, Autogas is going in for sure…but not just as a bean counter solution, octane rating of Autogas is in the range of 110-112, so i am going to up the compression and play with the cam profile once i rebuild the engine with high performance parts.

    Currently I run at pump gas, 95 octane, which is about the same as USA 91 if I am not mistaken, with blei-ersatz-additive to keep the valves healthy.

    I also need more range with the car, it is annoying to stop at gas station after 250 km, especially when I drive fast it is after a little more than an hour… 70 l fuel tank is ridiculous, my land Rover has 80 l, but it consumes only 10l/100 km…so I will have another fuel tank.

    My stock compression was what? 10.1:1 (i am not getting more than 8:1 on at least 2 cylinders, lol). I am thinking about conservative 12.5.1. I can always fill with 100 octane as spare fuel in the gas tank.

    Sportier big block engines in the 60s had some crazy compression ratios…I think about adding the turbocharging, too. I like the low-end torque of the stock big block, and supercharging is the most common – bt in europe turbochargers are extremely cheap…and under the hood is enough place for 2 turbos, one for each bank of cylinders…I just have to custom fab my own intakes.

    Turbos are more fun IMHO, especially when you have big displacement to start with – so turbo lag is minimal. And it is more comfy for high speed cruising and overtaking on Landstraßen.

    330ish Net hp is kinda low for 7.2 liters…it is not even a 50 hp/litre!

    Also, I can get my hands on E85, or even some very cheap toluene – illegally, but at half price from pump gas, since no road tax. my friend runs his milled BMW 6 (E24) at 13.5:1 with
    1/3 toluene/pump gas mixture (i am a doctor, he is a chemist – he can get under the counter chemicals, I know because he always ask me to help him – 200 l steel drums can not fit in a BMW, while my Landy takes 5 in trunk without a problem).

    I always have crazy builds- my land Rover runs on used vegetable oil most of the time, sometimes even used engine oil – double heated tanks and all, just 10% gasoline in mixture in winter to start easier.

    1. U.S. pump octane numbers are the average of two other anti-knock indices (research and motor octane), but I think 95 RON is generally similar to 91 pump octane, yes.

      Generally, only the hottest ’60s American engines (that is, the ones that were intended for racing homologation and were only notionally street engines) had nominal compression ratios upwards of 11.0:1. Engines of that time tended to have progressively increasing octane requirements with age due to deposit buildup in the combustion chambers. If the engine demanded super premium fuel when it was brand new and had pristine combustion chambers, that was a problem, as you can see.

      Engines of that era generally were not very ambitious in terms of specific output. In the U.S., 1 gross horsepower per cubic inch (equivalent to 61.2 gross horsepower per liter) was considered hot stuff. Without a supercharger, which in the mid-sixties had a rather low reputation, getting much beyond that tended to make an engine harder to live with: too much valve overlap and far too much carburetion to be happy in normal driving. Americans didn’t go in much for specific output because it was easier to just get a bigger engine instead — this is a market, after all, that considered sixes of 3.3 to 4.1 liters “puny little engines” not strong enough for any kind of heavy work.

      I can’t advise you on modifying or turbocharging your engine — just try not to blow it up!

  45. We used to have a 1967 Chevy Biscayne with a two barrel 283 rated at 195 gross horsepower. It seemed to have decent passing power for a stock, plane Jane sedan. No power steering and no air. Plane Jane. What would that 283’s power have been in net horsepower?

    1. A power/torque graph issued by Chevrolet in 1963 for the 283/195 engine indicates an output of about 150 net horsepower at 4,100–4,200 rpm with net torque of about 245 lb-ft at around 2,400 rpm. These are approximations from the power and torque curves, but they jibe pretty well with the figures published for the Chevrolet truck version of the 283, which was rated at 175 hp/275 lb-ft gross and 145 hp/245 lb-ft net, with net power and torque peaks of 4,200 rpm and 2,000 rpm respectively.

      This version of the 283 was obviously very mildly tuned, sacrificing a lot of maximum output potential (obviously, even in 1967, 32.3 net hp/liter was taking things easy) to put the fattest part of the torque curve right in the normal cruising speed range. A B-body Chevrolet with the 283 was respectably quick for the time (0-60 mph in 11 seconds or so even with Powerglide, which was faster than a fair number of contemporary sports cars), but the point was to make it feel like something was happening as soon as you put your foot down, even without a downshift.

  46. The 1973 Pontiac SD-455 was rated at 290 net hp. The oft-quoted 310 was a pre-production figure, that while widely publicised, never actually made it to the consumer.
    Perhaps an SD-455 T/A and Formula story is in order?

    1. A fair point. The SD455 is a complicated story (and ultimately sort of a weird historical footnote) that’s obviously beyond the scope of an article like this one.

  47. Hello, I am French and I own a 1975 continental Mark IV (460 ci Ford, 4 bbl Motorcraft). A difficult year to know the net HP !… According to my shop manual, in 1975 some continental had only one exhaust line (but most 2!) Some cars with a catalytic converter on an exhaust line (and not catalytic on the other exhaust line…!). And some other car with a catalytic converter by exhaust… “Ultimate American V8 engine data Book” gives an HP power of 194 for 1975 but elsewhere I find powers of 220, 223 and 224 HP (off version 460 ci “police”). V8 engine data book seems to hesitate too since in 1976 the power of the 460 ci is given with a simple exhaust line 194 HP @ 3800 RPM and dual exhaust line to 224 HP @ 4000 RPM … (compression Ratio 8.0). I do not understand anything ! Thank you if you have an answer to give me … :-)

    1. Franck,

      It is very confusing indeed. All of these numbers are net hp ratings, but there is a lot of confusion about which versions of the engine were available in different cars. (A civilian buyer could not normally order a police engine, and not at all on a Lincoln.) Catalytic converters were not yet fitted on all U.S. engines in 1975, in part because they were still in fairly short supply. The catalyzed engines were mainly for California, which had tougher emissions standards than the national requirements. Surprisingly, several sources indicate that in 1975, California-bound Lincolns had the catalytic converter and dual exhausts, with more power rather than less. The website Automotive Mileposts gives the output in that form as 220 hp, but two contemporary sources list net output with dual exhaust and catalytic converter at 223 hp @ 4,000 rpm and 366 lb-ft @ 2,600 rpm. I’m afraid I can’t give you a more precise answer than that; Lincoln’s 1975 ads and brochures don’t list power output at all and I’m not sure how technically precise these other sources are. (Contemporary American magazines generally hated cars like the Mark IV because such cars were aimed at an older audience not concerned with performance or handling, so cars like this got much less attention than smaller sporty cars.)

      Of course, you can easily determine whether your car has dual exhausts and a catalytic converter or not!

      1. My Continental Mark 4 has a dual exhaust line with a catalytic on the left but not on the right of the line, which is incoherent, no ? When I repaired double exhaust, I removed the catalytic, disconnected EGR system and if I relocated the air pump to place, I did not put the belt for this pump. It seems to me that the engine has gained power (power gain). I am anxious to respect a “matching number aspect” that is why I visibly kept all the 1975 period accessories (but not functional). What do you think ?

        1. The exhaust layout you describe sounds very strange — you’d have to look at the shop manual to determine if it’s supposed to be that way, as I’m not familiar enough with the mechanics of the Mark IV to say. It’s possible it was a strange compromise to minimize the additional back pressure the catalyst creates, but I’m not sure. Early emissions controls tended to reduce power, either by forcing compromises in engine tuning or requiring engine power to run. For instance, the pump for an air injection system consumes some horsepower, much like an air conditioning compressor.

          I’m afraid I can’t advise you on modifying your car. I don’t know what French law specifies, but in the U.S., disconnecting the emissions controls (except during repair or replacement) is generally illegal, and a technician who did it for you could get in a great deal of trouble.

          1. In my country, legislation is more flexible for the “classic cars”. EGR system, catalytic, etc. is a legislative obligation that from 1994 …! Therefore, 1975 = no problem ! I think : the state of California was very strict (from 1972) that there was a requirement of a catalytic system by exhaust line. The other states being less severe for pollution and the manufacturers put only one catalytic even if there were 2 lines on the cars ! Sometimes none (40% Lincoln had no catalytic in 1975…). To go back to the subject, they are “HP net” in 1975 but totally impossible to find accurate and reliable data … I believe there is a historical archives service at Ford … I would have to find The address … maybe they will be able to inform me ? Congratulations on your article. I already knew the difference between HP gross and HP net but it is important to remember! (excuse me if I make errors of spells) like many French people, I’m no good in foreign languages ​​and especially with the Sakespeare language… ! :-)

  48. I have bought a 68 Firebird 400 that has a 73 185 HP engine (with a 2 barrel) that now has a 4 barrel. Just wondering how much less actual HP this has than the original 330 SAE gross rating that this engine would have (stock VS stock).

    1. GM unfortunately hasn’t yet digitized its vehicle information guides for Pontiac, so I don’t have an official word for you in that regard. However, Pontiac did publish both gross and net ratings for the Firebird’s low-compression 400/2V engine: 185 hp SAE net, as you know, or 265 hp SAE gross.

      The 400/4V is a more complicated question because of Pontiac’s policy-driven habit of tailoring its published gross power ratings to a GM corporate rule mandating no more than 1 advertised gross horsepower per 10 lb of curb weight. This means the 400/4V engine that claimed 330 hp @ 4,800 rpm in the Firebird was rated at 350 hp @ 5,000 rpm in the 1968 GTO despite identical cams, valves, carburetion, and ignition timing (at least initial timing — I don’t know if the advance curves are the same, although I assume they are). Bizarrely, in 1971, when Detroit quoted both gross and net ratings for the last time, this difference still appeared in the net ratings, though not the gross ones. So, the 400/2V was listed at 265 gross horsepower in both the F-body and the A-body, but 185 hp net in the Firebird and 205 hp net in the A-body cars. Similarly, the 400/4V is listed at 300 gross hp either way, but claims 250 net hp in the F-body and 255 net hp in the GTO and GT-37. (The latter might be attributable to different exhaust packaging, but the 20 hp difference on the 2V engine is very curious indeed.)

      That said, the only difference in specs between the 1971 and 1968 400/4V engine is compression ratio, which fell from 10.75:1 to 8.2:1. I would therefore guess that the high-compression engine would probably have a net rating of something in the realm of 265–275 hp. So, the 1968 four-barrel engine probably has an edge of around 80 to 90 real horsepower over a 1973 400/2V. If anyone has the vehicle information guide, it may have net power curves for the ’68, which would provide a more precise estimation. I don’t know, however, if those curves would also maintain the fiction that there was a 20-hp difference between the Firebird 400 and GTO versions of the 400/4V engine, which as far as I can see was purely for the benefit of corporate management.

  49. Presently restoring a 1968 Ford Country Sedan. It left the factory with a 390 V8 which was taken from the car before I bought it. I have a ’68 Country Squire with the 390 2V and I’m always a little disappointed in the available power. I have a 302 2V V8 from a ’68 Galaxie I was going to swap in to the Country Sedan, but wonder if It’d be smarter to find a wrecked early 90’s Mustang and donate its 302EFI (5.0L) to the cause instead. I saw a spec that rated the 1968 302 at 235hp (gross) and I know the 1992 5.0 was rated at 225 net (although I read and respect your notation that Ford may have exaggerated by 20hp or so) . My question is: What would the ballpark difference be between the available hp between the ’68 302 and an early 90’s 5.0? Thanks for you help with this!

  50. Looks like I erred in the 235hp rating for the ’68 302. That would be for the J code motor, probably a 4 barrel carb in a Mustang. The 302 2V in a ’68 Country Sedan is rated at only 210hp gross. So the question is, how much of a difference in “apples to apples” hp would there be between the 210 gross in 1968 and 225 net in a ’92 5.0L ?

    1. I was hoping Ford might have done what GM and Chrysler did in 1971 and list both gross and net outputs, but they don’t appear to have gone in for that. Nonetheless, the 1972 302-2V, which was not terribly different, was rated at 140–143 hp depending on the application. (I assume the slight variation in rated net outputs was due to different exhaust packaging.) Since the 302-2V was a mildly tuned, regular-fuel base engine, I don’t think the ’72 was terribly different from the ’68 in output, the vagaries of early emissions control not withstanding. So, one could probably reasonably assume a net output of about 145 hp for the ’68. Given Ford’s later de-rating of the late 5.0L engine, the ’92 may be closer to 205–215 hp than 225, but that’d still be an advantage of at least 60 net horsepower over its carbureted ancestor. The advantage holds in torque, as well: The carbureted engine’s net torque peak was 230–245 lb-ft, depending on application, whereas the ’92 was rated at 300 lb-ft and the more conservative ’95 rating was still 285 lb-ft. Wonders of electronic engine control.

  51. Aaron, thanks “a ton” for your help on this. I appreciate it!

  52. As a Mopar guy,I noticed at least with Mopars when they went to net h in 1972 all engines had lower compression,weaker cams,weaker crank and heads that didnt flow as good..

    They also got the hp/torque at lower numbers thus actual net hp would have been higher if tested the same rpm as the older engines..As the engine let off before the greater power band even weaker low compression engines..

    I did have a 71 454 Chevelle SS and a 72 454 Chevelle SS and it was more of just switching to net as some say,they cars were both the same 454 or what people said to me lol,just hp was in net..Well,the 2 cars both identical,same axle ratio,same factory exhaust,same trans etc the 71 would run 6 car lengths ahead of the tuned,compression checked 1 owner previous 9 year old Chevelle SS that was never abused and was maint..The 71 had higher mileage but was in good condition and ate the babied 72 !

    I still own the 72 as it was a true 40,000 mile car,the 71 had 130,000 in 1978 when I bought it..The Chevelle SS 454 is my only GM in my Mopar collection…and will never be sold,lke my 4 68-69 Charger RT…But I had th 72 so long I just couldnt sell it !

    1. A point I want to address here, because it’s a common misconception, is about the power curves of gross and net horsepower. Gross and net power and torque peaks sometimes occur at the same rpm, but that isn’t always the case.

      Developed horsepower is a mathematical function of engine torque and engine speed. Once the engine reaches its torque peak, torque begins to decrease, although power may continue to increase so long as engine rpm climbs more rapidly than torque declines. In as-installed net condition, it’s not uncommon for an engine’s power and torque peaks to be reduced somewhat, due to greater back pressure restriction or other breathing limitations. This is true even if the engine’s heads, valves, and camshaft profile are unchanged. For instance, for non-performance V-8s of the sixties, the gross power peak might be at least 400–600 rpm higher than the engine would willingly pull when installed in a car, and if you tried it, it would be pretty clear that you were getting more complaints than power at that speed. So, you shouldn’t assume that if the net power peak was at a lower rpm than the gross peak, the engine would produce more net power if you just revved it higher; that was generally not the case.

      Regarding the Chevelle, the Chevrolet specs indicate that the ’72 454 gave away 15 net horsepower to the ’71 due to additional emissions controls. So, the ’72 was legitimately less powerful than the ’71. I don’t have torque output specs close at hand, but I would assume the torque curve of the ’72 is less generous even if the peak is not far off.

  53. Been trying to figure out the Net ratings for Jaguar XK6 engined cars which was only rated in SAE, does any straightforward conversion formula exist to convert SAE to Net?

    1. I’ve seen estimates that put the 265 hp (gross) 3.8- and 4.2-liter XK engine at about 180–185 net horsepower in stock form, which seems plausible judging by the DIN ratings of ’70s and early ’80s Jags with the 4.2-liter XK (which ultimately got I believe 170 hp DIN with carburetors and 205 hp DIN with Lucas/Bosch injection). Since late dual-carburetor 3.4 XKs got 162 hp DIN in relatively unrestricted British form, I would assume the ’60s versions were in the same realm, although I haven’t seen a breakdown for the various combinations of carburetors and state of tune.

      There is no formula for converting gross to net or vice versa. The best you can do is either hope to find a factory source that quotes both for the same engine or else do some conservative estimates based on similar cars and engines. It’s possible to estimate actual power and torque based on quarter mile elapsed times and trap speeds, but those are still estimates and introduce an additional area of uncertainty in estimating how much power is being consumed between the crankshaft and the drive wheels.

      As a minor point, “SAE” is not synonymous with “gross.” Since 1971, the SAE has had standards for both gross and net output, although their methodology for the latter is not identical to DIN or JIS net ratings. (There’s also the pesky issue of metric and Imperial units, which British publications have a frustrating habit of getting very wrong.)

      1. Understand

        Can see why Jaguar sought to replace the XK6 with the V12 and 60-degree V8 project even if they were better off making the latter a 90-degree V8.

        Do any figures exist for the abandoned 3-litre XK6 engine project? Some say it lacked power though have yet to see any figures, while others say the 3-litre XK6’s issue was the lack of torque.

        1. Do you mean the 3-liter racing iteration from the E2A prototype? I’ve seen its output variously quoted as 293 hp, 294 hp, “nearly 300 hp,” or “300 hp” with Lucas Mk1 petrol injection, which for a racing engine is about as precise as one can probably expect. The torque figure was 230 lb-ft @ 6,000 rpm. As you can see, power was not its problem, but it would have been high-strung for non-racing use even discounting its mechanical problems (failed injector, burned pistons, snapped connecting rods, et al). I don’t think the 3-liter engines were ever intended for street use except perhaps in a limited homologation sense. (There were several different approaches to the 3-liter formula, none of which seemed to have worked out terribly well.)

          I suppose if Jaguar had really wanted to have a V-8, they might have been better off starting from a 90-degree V-8 and making a 90-degree V-12 from it rather than the reverse. It was a reminder that engine commonality isn’t always a benefit!

          1. Not sure, it is said a 3-litre / 2997cc version of the XK6 was considered for the Jaguar XJ alongside the V12 as well as possibly the XJ21 E-Type replacement project, yet despite the top end power lacked low speed torque.

            It is not mentioned whether it was a road-going detuned version of E2A or a 3-litre version of what became the 2.8-litre (albeit potentially putting out 120-145 hp NET).

            Indeed though would have thought Jaguar could have benefited from both a 90-degree V8 and 60-degree V12 with common components had they partnered up early on as alluded to in William Lyon’s bio.

          2. The idea of using a street version of the 3-liter engine from the E2A as a base engine for the XJ sedan doesn’t make a lot of sense to me. Aside from its state of tune and narrow power band, and lacking the torque one would want for the heavier saloon, the racing engine was full of expensive exotic bits like titanium connecting rods and the alloy block. The 2.8-liter XK didn’t do terribly well in the XJ6 either, but it at least wasn’t quite so dear.

          3. Apparently the 2.8-litre XK6 was originally envisaged as a 2.6-litre in combination with the 3.0-litre XK6 for the XJ, prior to the 3-litre being dropped and the 2.6-litre being enlarged to 2.8-litres.

            The 3-litre was tested by Normal Dewis in 1965, who found the engine in the E-Type gave good low-speed torque, economy and quiet running though torque was not sufficient compared to the 4.2-litre XK6.

            (Above via Jaguar – the Complete Story by Heiner Stertkamp)

            However that did not stop Jaguar from considering using the 3-litre XK6 in other proposed models, yet it is possible a production 3-litre would have been enlarged to 3.2-litres inline with how the 2.6-litre was enlarged to 2.8-litres.

          4. Interestingly the follow notes the planned 3-litre XK6 was putting out around 185 hp and that it likely featured an alloy block or was basically a short-stroke version of the existing XK6 engine, with little to no indication of being related to the E2A prototype engine.


          5. Very interesting. I know that link says the 3-liter engine’s output was 185 net horsepower, but I really have to think that was an SAE gross estimate, as was the 2.8-liter production engine’s 180 bhp rating. (Which would make sense, to compare the ratings with Jaguar’s then-extant lineup.) It seems like the drop from 3.0 to 2.8 liters was primarily to keep it under the punitive taxation threshold in markets like France rather than a radical rethinking of the engine design, so the loss of a few horsepower and a modicum of torque would seem a likely outcome.

            Confusing gross and net ratings is an extremely common issue, and one of the reasons I originally wrote this article. A lot of times, the source material is vague, and when it comes to engines that didn’t see actual production, there may not be enough context to know for sure. In this case, I look at the actual production 2.8 XK6 and think, “Well, 185 net horsepower is a lot more power than 180 gross hp — that’s probably a difference in net output of at least 25 hp. If there was a major change in specification (such as a valve timing change to sacrifice power for torque or vice versa), that might make sense, but if the change was just a reduction of about 200 cc displacement, that doesn’t really track.”

            I may be wrong, but that’s my logic.

            (Thanks for that link, BTW — very interesting.)

          6. The impression one gets is that the originally planned 2.6-litre and 3.0-litre XK engines were intended to be much different compared to the production 2.8-litre unit, which appears to be an ill-regarded lash up that was approved on the grounds of cost.

            The 185 hp net rating of the 3-litre XK engine does made it fairly competitive compared to similarly displaced 6-cylinder engines from other rivals, as opposed to what the underdeveloped 2.8-litre unit up out.

          7. That the 2.8-liter ended up being ill-considered is a matter of record, of course, but I’m still doubtful. I suppose the DOHC Maserati/Citroën C114 V-6 made 170 PS from 2.7 liters, but 185 net hp was probably about what the three-carburetor Jaguar XK6 managed, so I’d have to know a lot more about how the mooted 3-liter differed to find that credible. (Obtaining the same power at a sacrifice of 800 to 1,200 cc couldn’t be done without cost, presumably in the area of torque output.)

          8. Would have to agree regarding torque, that said it is to be expected for larger displacement engines in general to put out more torque and especially since the 3-litre (along with the planned 2.6-litre) was apparently a short-stroke version of the XK6.

            Had the BMW M30 in mind despite being a SOHC Straight-6 though as with the Citroen/Maserati V6 there are others as well.

          9. The thing that strikes me about the XK6 is that by even ’60s standards, it didn’t get a lot out of its fairly exotic and expensive configuration. Its specific output, in net terms, wasn’t any better than a fair number of contemporary SOHC sixes. A short-stroke version would potentially be sweeter and more willing to rev (and obviously the exotic racing version had lots of power), but it was still showing its age, which is why I’m skeptical about the 185 hp figure being a net rather than gross output.

  54. I’ve come into possession of seven engines/blocks that are a mix of the Buick 215 and the Rover 3.5 liter. It is my understanding there may have been low and high compression versions. Any ideas on the gross and net output of these mills? Thanks.

    1. There are a bunch of different versions of that engine, in different states of tune, so it really depends on what pieces they have and how they’re set up — compression ratio, carburetion/injection, cam, manifolding, and so forth. The most common British versions, for instance, had either 135 or 155 PS DIN (133 or 153 net hp), but the early U.S. low-compression 2V engines had only 150–155 GROSS horsepower, which was probably in the realm of 110 hp net. It’s also possible to get significantly more power out of the engine, especially if the displacement has been punched out some.

  55. You are a brain for sure! I purchased an open chamber L88 427 Chevy short block in 1972 for $508 new. Purchased OC big port iron replacement heads new, with 2.19-1.88 valves, springs and retainers for $250 pair. (These replacement heads have the standard-shaped square exhaust port outlets, but are slightly larger than a 425 hp 427 outlet.) Swapped L88 cam for ZL1 cam. Stock dual plane intake with divider cut down. #3418 Holley 850 vac sec carb. Engine made 545 HP with open M/T super scavenger headers (2″ tubes). By the way, ZL-1 engines were factory dyno’d at 525 hp with open headers. Most tests that suggest more than that had ported heads.

  56. Speaking of US emission standards have read the US in the mid/late-60s gave exemptions to cars with engines less than 50 cubic inches or 819cc, which affected the likes of the Fiat 850 and Saab 96 though unsure what other cars were impacted nor how long such rules remained in effect for.

    Was there a rationale behind the 50 cubic inch threshold? Since it would have appeared to have made slightly more sense placing the threshold at from 52 cubic inches up to a maximum of 60-61 cubic inches.

    1. I would have to dig through the statutes to see when that changed, as I don’t know offhand how long it remained on the books. I’m not certain of the rationale, although I can see several possibilities. One is that the earliest U.S. emissions standards were based on percentages of exhaust volume rather than total emissions output in g/km or g/mi, and in that respect, smaller engines were sometimes at a disadvantage in ways that did not reflect their actual emissions output. Another is that small engines (and I’m confident the 50 cubic inch figure was chosen because it was a round number that at the time seemed very small rather than that it had any relationship to imported vehicles with metric engines) already had quite limited power, generally speaking, and the kind of performance penalties involved in running air injection pumps and other early emissions control equipment may have been deemed impractical. A third possibility, since legislators respond to lobbying, is that there was a push from manufacturers of small engines and off-highway vehicles — golf carts, for example, a subject near and dear to many Congressional hearts — fearful that immediately applying the rules to them would effectively put them out of business. It may have been some combination of the three, although again my assumption is that it was aimed at certain American manufacturers rather than imports. (During this period, engines smaller than 200 cubic inches/3.3 liters were regarded as dinky economy-car stuff in the U.S.!)

    2. I don’t know what the situation was with the Fiat 850. In the United States Saab offered the sub-50-inch^3 two stroke only for model year 1968. I don’t know what the rationale for a special one-year-only engine might have been, unless they wanted to be able to advertise a low list price. The exemption may have ended with MY 1969, but the two-stroke was on the way out anyway. I believe 1968 was the last year for the 96 two-stroke in all markets, but I would have to confirm this.

      Ca. 1978 my Saab mechanic was overhauling a ’68 two-stroke for another customer, using stock-bore pistons for an 841cc engine. I joked that he was restoring the engine to its rightful displacement.

      1. It’s conceivable there was some other regulatory issue other than emissions — drive-by noise rules, for instance. (I really don’t know, but there are a number of possibilities.)

  57. You are realy amasing in Your knowlige!!
    Fredrik from Norway

  58. This has really been a fun read…So we are now in the 3rd decade of the millennium, is there a standard answer across makes & countries for todays autos? PARENTHETICALLY, I REALLY MISS EDMUND’S OLD “FUN TO DRIVE” RANKING, WHERE YOU HAD TO BE ABLE TO DO 0 TO 60 IN EIGHT SECONDS OR LESS TO MAKE THE LIST. THAT’S STILL A PRETTY USEFUL RULE OF THUMB, IMHO, BUT WITHIN MY BUDGET, SEVEN’S ABOUT THE BEST I CAN HOPE FOR.

    1. There’s really not a single standard across markets. Some markets have laws explicitly requiring that advertising and brochures for new vehicles use specific standards and/or units, and even where it’s not that specific, manufacturers are usually going to want to follow the general market trend. (If you quote outputs in kW while your competitors only specific mechanical horsepower, you’re just going to confuse people!)

      Something I miss is Car and Driver‘s old “street start” test, which was a 5–60 mph rolling start acceleration time. This was a helpful metric because it was more reflective of real-world performance, and it helped to illustrate the shape of the power curve. For instance, turbocharged cars in those days would often lose a second or more compared to their 0–60 times, making clear that the latter were obtained by very aggressive clutch-dropping launches that you probably wouldn’t want to do often with a car you were relying on to get you to work on time.

  59. Thank you so much for writing this informed and literate article. Thank you especially for clearing up the idea that net horsepower is measured at the rear wheels. I have heard and made some pretty dumb comments in this context over the years. My first clutch-dropper was a ’53 Ford convertible. I also abused a ’67 Pontiac Beaumont (a what? a what?) sport coupe. Current ride is a 1996 Buick Roadmaster wagon with the “260 hp” LT-1 motor, which seems to have lost its mechanical fan and standard air cleaner somewhere…

    Would you like to say something about the hilarious horsepower ratings of the 1967-68 Z28 Camaro?

    Anyway, thank you again, and thank you for the astoundingly competent privacy statement.

    Will you ever reveal your own car history?

    1. I’ve talked about the Z/28 at some lengths in the article on the first-gen Camaro (https://ateupwithmotor.com/model-histories/chevrolet-camaro-z28/). As with the Mopar 340, my back-of-envelope estimate is that while the Z/28 302 engine was certainly underrated in a gross horsepower sense, its official ratings were probably fairly close to its actual net output. Given the complex internal politics surrounding GM performance engines of the time, that was likely more coincidental than by design.

      My own history, which I’ve mentioned here and there, is hardly a secret, but less interesting than you might think. The most interesting car I’ve ever owned, from an enthusiast standpoint, was a third-generation Honda Prelude Si 4WS. I have at points considered writing about the Prelude, either in its own right or as part of an expanded version of an article I originally did for Autoweek on four-wheel steering, although, perhaps embarrassingly, I have NO photos of that car. (I think the only ones I ever took were with a terrible disposable camera and would not have been usable even if I still had them, which I don’t.)

  60. Sir,I tip my hat to you for your overall knowledge of such a vast array of engines and information…SO, I need to put you to the test.

    I purchased a 1977 corvette, with what I was told is a 1972 BBC engine in it. I checked the casting number, which was 3999290, which indeed can be a 1972 Big Block or earlier. But on some sites, that shows up as either a 396ci engine, or a 402ci engine. Based on the engine serial number which is T0225CTB, the engine was built in Tonawanda, Feb 25, with 240 horsepower, TH400 and was put in either an “A”,”B”, OR “F” body. The vin portion of the engine serial number is 12K588833, which decodes as a (1) Chevrolet, (2)1972 engine, (K) built in Missouri, (588833) index number. So, with that information on the block, I am extremely confident it is truly a 1972 engine block, but I am still uncertain if it is a true 402 or 396 based on some sites showing both size blocks available in 1972.So, my question is, based on reliable information stating that a 1971 402 had 300 horsepower, which was gross, and the 1972 402 showing 240 horsepower which was net, is there any difference in the internal components of a 1971 engine versus a 1972 engine…or for that matter even a 1970 402, other than a much less compression ratio? They did put a mild aftermarket cam in it, with an Edelbrock manifold and Edelbrock Quadrjet (795cfm) carb(yes, Edlebrock did make “Quadrjunk” carbs for a few years on it, and Hooker headers. Other than that, it’s stock. So, my concern is, what would be maximum rpm on this motor? Could I crank out the same rpm rating for a 1970 or 1971 402 with my thought process being that the critical internal components are equally as strong as a 1970 or 1971 402?

    Again sir, thank you for your time and knowledge!!!

    1. The 396/402 distinction is a confusing one. In MY1970, Chevrolet offered both a 396 (called “Turbo-Jet 396”) and a 402 (called “Turbo-Jet 400”), which had different displacements and different specifications. The Turbo-Jet 400 had a milder cam and a cast rather than forged crank, giving a quoted 330 hp @ 4,800 rpm and 410 lb-ft @ 3,200 rpm (SAE gross) compared to 350 hp @ 5,200 rpm and 415 lb-ft @ 3,400 rpm (also SAE gross) for the 1970 Turbo-Jet 396.

      In MY1971, Chevrolet retained the separate “Turbo-Jet 396” and “Turbo-Jet 400” nomenclature, but it appears that both names now described the same engine: In the January 1971 Chevrolet Sales Album, intended for dealer training, both options share the same column in the engine specifications table, which list identical bore and stroke dimensions (4.126 x 3.76 inches), compression ratio (8.5:1), and output (300 hp @ 4,800 rpm/400 lb-ft @ 3,200 rpm gross, 260 hp @ 4,400 rpm/345 lb-ft @ 3,200 rpm net). My assumption is that Chevrolet was simply reluctant to surrender the familiar “SS 396” nomenclature, even though the 396 cubic inch version was dropped or phased out. (Why they didn’t take the opportunity to call the bigger engine the “Turbo-Jet 402” I have no idea, since that would have helped to distinguish it from the SBC “Turbo-Fire 400” engine offered at the same time.)

      The “396” nomenclature continued into 1972, albeit only on the Camaro, whose ’72 brochures describes it as the “Turbo-Jet 396 402-cu.-in. V8” engine. Most other Chevrolet sales literature I’ve seen for MY1972 describes the engine as “Turbo-Jet 400” (although the trailering brochure helpfully describes it as “402 CID”). Dual exhausts were now a separate option, so there were two variations: The single-exhaust version was listed at 210 hp @ 4,000 rpm/320 lb-ft @ 2,400 rpm; the dual-exhaust engine was listed at 240 hp @ 4,400 rpm/345 lb-ft @ 3,200 rpm, which appears to be what you’ve got. (Both are SAE net ratings; Chevrolet no longer quoted gross output figures for 1972, at least in the U.S.)

      You’ll notice that the dual-exhaust version has the same net torque output as the ’71, but lost 20 net horsepower. I don’t have camshaft, carburetion, or valve size specs, but obviously the ’72 gave up something compared to the ’71. (Without looking up more detailed specs, my guess is that the cam remained the same — since the net power and torque peaks were unchanged — but there were other changes that sacrificed some top-end power.) Either way, the 402 cubic inch engine was pretty clearly already a milder engine than its high-performance 396 cubic inch cousin, the use of a regular cast iron crank rather than the SS 396 engine’s forged steel crank being a particular giveaway in that regard. Chevrolet didn’t publish net output figures for 1970, but at a conservative guess, the “Turbo-Jet 400” lost 40–50 net horsepower between MY1970 and MY1972. (Its gross rating dropped 30 hp from 1970 to 1971, due mainly to the lower compression ratio, so one might reasonably extrapolate a 1970 net output of 290 to 300 hp.)

      As for maximum rpm, that’s not something I feel competent to advise you on (not least for liability reasons — I’m not a mechanic or an engineer, so I’m not qualified to give technical advice!). It’s a question better posed to engine builders experienced with Chevrolet Turbo-Jet (BBC) engine builds, who could probably also better advise you on the cost and difficulty of any engine modifications you might contemplate. The best I can tell you in that regard is the factory did de-fang the engine from the SS 396 performance days, so I personally would be wary about expecting the milder ’72 to perform like its high-compression ancestors without some work.

  61. Aaron, I think you have some misinformation in your reply. All Turbo-Jet 396 and Turbo-Jet 400 engines in 1970-1972 actually displace 402CI. This can be verified from the specs published in the Vehicle Information Kits which are readily available for download from the GM Heritage Center. I suspect that for a proper hierarchy, Chevy couldn’t have a small block (Turbo-Fire 400, actually 401CI) that was larger than the big block 396. But the SS396 was an iconic brand for Chevelle and Camaro from ’67 to ’69 so marketing likely didn’t want to give up and that – somehow SS396 just sounds right and SS402 doesn’t.

    Oddly, in other Chevelles from ’70 to ’72 the milder LS3 version (330HP in ’70, 300g/260nHP ’71 and 240HP ’72) were all marketed as Turbo-Jet 400, while in ’70 the SS396 got the more powerful 350HP L34 version as standard and it was advertised as a 396. In Camaros, all 3 years advertised as a 396 and in ’70 was the 350HP L34 version, but in ’71 and ’72 it was the LS3 lower performance version as the L34 was discontinued.

    1. They were certainly all 402 cu. in. for 1971 and 1972, but at least one version of the 1970 Chevrolet Sales Album (which you can see on the Old Car Brochures website.) indicates the concurrent availability of an actual 396 cu. in. engine (bore 4.094 inches, stroke 3.76 inches) AND the 402 cu. in. version (bore 4.126 inches, stroke 3.76 inches). Now, it’s entirely possible that this information was wrong, or that it changed during the 1970 model year, for example after Dec. 31, 1969. (That version of the Sales Album has a 1969 copyright date, so it was printed in the first part of the 1970 model year, although it already lists assorted internal revisions, so there were clearly a variety of changes and/or corrections throughout the model run.) I would not be at all surprised if subsequent versions of that same 1970 Sales Album noted that that was no longer the case or that the original listing had been in error — there’s a reason car brochures and advertisements usually have disclaimers saying the specifications and options may change without notice! — but that’s what this one said.

      One of the challenges with tracking this kind of stuff retroactively is that cars of that vintage frequently underwent a multitude of running changes — some trivial, some surprisingly involved — that are not always reflected in the official specifications or factory literature. This can confound restorers, who may start taking things apart to discover some component that, while obviously OEM, is significantly different from what’s reflected in the shop manual (or the factory wiring diagrams, which is especially fun). Even if the printed specifications were accurate at the time they were printed (which is not always the case), they may not have remained constant throughout a given model year, and it can be hard to know for sure, or to know why certain things changed, if they did.

      Anyway, I don’t doubt that you’re correct about what the GM Heritage Center specs say, but for at least part of MY1970, Chevrolet was telling its salespeople that the SS 396 had an actual 396 cu. in. engine (as you say, the RPO L34 version), distinct from the 402 cu. in. version. I don’t claim to know WHY the factory specifications differ on this point — I have several hypotheses, but they’re just speculation, and I don’t have any way to verify them — but they DO differ, and I feel I would be remiss not to point that out.

      Regarding nomenclature, before 1970, the decision to round displacement of both the Turbo-Fire and Turbo-Jet engine to “400” would have made some sense in view of the corporate policy limiting the A-bodies to engines no larger than 400 cu. in.; Buick did the same thing in calling the 401 cu. in. Nailhead the “400” when installed in the GS 400. However, that rule was obviously rescinded for MY1970, as evidenced by Chevrolet’s SS 454 models, so that doesn’t seem altogether plausible. (I’m not talking about the desire to hang onto the “SS 396” badges, which is easy to understand, but rather their decision not to dub the Turbo-Fire 400 the “Turbo-Fire 401” and the Turbo-Jet 400 the “Turbo-Jet 402.”)

  62. What about lawn mower engines. There was a horsepower war, at least on the labels, going on and now the do not advertise hp. What’s the deal with that.

    1. I’m really not familiar with trends in small gasoline engines, so I don’t have any insights to offer on that front.

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