what is the loss of hp to the rear wheels compared to net horsepower

There are a lot of misunderstandings among car enthusiasts and historians about vintage horsepower ratings. It'southward piece of cake to assume from a coincidental glance at ads or spec sheets that fifty-fifty 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 almost emissions controls and anemic, low-octane unleaded gasoline — which is true, only 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 aforementioned mode as modern engines. "A horsepower is a horsepower, correct?" yous say. While a horsepower, pre-smog or post, remains 746 watts (or 736, for metric horsepower), the fashion that output was measured has changed quite a bit. Permit's explain:

GROSS HORSEPOWER RATINGS

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

Gross output, which in the U.S. is typically measured using the methodology laid out in SAE standards J245 and J1996, is the output of a 'blank' engine running on a test stand with no external engine-driven accessories (e.one thousand., alternators or water pumps), gratis-flowing exhaust headers with no mufflers, and optimal ignition timing. Gross ratings are besides mathematically corrected for standard atmospheric weather. In other words, gross output represents a detail engine's maximum output under ideal conditions.

In the existent globe, automotive engines very rarely operate in platonic conditions. The engines of most cars are burdened with various engine-driven accessories, including the engine'southward own water pump and generator/alternator and add-ons like the power steering pump and air conditioning compressor. Engines intended for on-route apply typically likewise have restrictive air cleaners and frazzle systems, sound-deadening mufflers, and emissions-command add together-ons similar catalytic converters and thermal reactors. Engine tuning is farther compromised in the interests of reduced dissonance, better drivability, improved cold-weather functioning, and lower exhaust emissions. All of these factors reduce the engine'south maximum output in means that the gross rating methodology does not reflect.

For that reason, the SAE and similar bodies have also established standards for measuring cyberspace output. Cyberspace ratings, such as the ones divers by SAE standards J1349 and J2723, are even so taken with the engine on a exam stand, simply reflect stock ignition timing, carburetion/fuel delivery, frazzle systems, and accessories. The specific methodology varies depending on the specific standard existence used, just the gist is that a net rating is a closer approximation of an engine's output equally actually installed in a car.

Naturally, the net output of a given engine is somewhat lower than the gross output. For case, the 217 cu. in. (3,547 cc) "Stovebolt" 6 in a 1950 Chevrolet had a gross output of 92 hp (69 kW), but a cyberspace output of but 85 hp (63 kW). Chevrolet's 1955-vintage 265 cu. in. (4,344 cc) modest block V8, meanwhile, had a gross output of 162 hp (121 kW) with viii.0 pinch and a two-barrel carburetor, simply a net output in the same form of 137 hp (102 kW).

ADVERTISING, OVERRATING, AND UNDERRATING

Net ratings have existed for many years, simply before 1971, about 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 upwards 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 operation of cars so equipped suggested that the advertised figures were now substantially higher than the engines' every bit-installed outputs — sometimes by 25% or more than.

The tardily-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, just the outputs of Detroit's almost powerful engines were now as probable equally non to be deliberately understated. For example, in 1965, Chevrolet released the 396 cu. in. (6,488 cc) Turbo-Jet V8 as an selection for Corvettes, rated at 425 gross horsepower (317 kW). The following year, the engine was bored to 427 cubic inches (6,996 cc). At start, some Chevrolet promotional cloth quoted a gross output of 450 hp (336 kW) for the bigger engine, simply the division then hastily restated maximum ability every bit 425 hp (317 kW), no more than the 396. Outside observers were incredulous, particularly since the bigger engine was evidently more than powerful than the smaller version.

This is a 1966 Chevrolet Corvette Sting Ray's 427 cu. in. (vi,996 cc) L72 engine, officially known equally a Turbo-Jet 427. Early on mill literature credited the L72 with 450 hp (336 kW) at six,400 rpm, but this was afterwards amended to 425 hp (317 kW) at five,600 rpm — the aforementioned horsepower every bit the previous yr's 396 cu. in. (six,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 than ability than its advertised gross rating indicated offered an obvious advantage. Racing officials were seldom fooled by such tricks for long, nonetheless, and began to handicap or "factor" advertised outputs to place a conservatively rated engine in a more appropriate class based on its bodily operation.

Another reason for underrating was political decorum. Peculiarly 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 exist irresponsibly high. In that climate, offering a 450 hp (336 kW) or 500 hp (373 kW) Corvette, for instance, might accept been asking for problem. Insurance was also becoming an upshot, with a growing number of insurance companies levying prohibitive surcharges on very powerful cars or just refusing to comprehend such cars at all.

For those reasons, GM likewise 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.ane kg) of adjourn weight, leading to curious non sequiturs similar rating Pontiac's 3,300 lb (i,500 kg) Firebird at 325 hp (242 kW) while claiming 360 hp (269 kW) for the more often than not identical engine in a 3,600 lb (1,635 kg) GTO.

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

NET HORSEPOWER RATINGS

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 diversity of useful marketing and political purposes, but equally a realistic measure of engine output, they left much to be desired. In particular, the gross ratings seldom reflected the bear on of add-on emissions control devices like air injection and exhaust gas recirculation, which were already beginning to have a noticeable touch on engine operation.

The final harbinger was the passage of a California police requiring that new cars sold in that land advertise only the more than conservative SAE internet figures kickoff with the 1972 model yr. Faced with that reality, manufacturers decided information technology was time to carelessness the gross rating system entirely. For 1971, many U.S. manufacturers listed both SAE gross and cyberspace ratings (providing a sometimes illuminating comparing between the 2) and then switched to internet ratings exclusively for 1972 and across, even in states other than California. (Making the alter across the board was probably a prudent movement from a liability standpoint, considering that in the U.S., consumers can and periodically practise 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 bodily losses acquired 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 effect was a dramatic drib 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 cyberspace horsepower (175 kW) for 1971. The real pass up wasn't quite as steep equally it looked; the 1971 engine did have a lower compression ratio to prepare for the adoption of unleaded gasoline, just the 1971 gross rating was all the same 365 hp (272 kW), so the bodily loss was about 10%, non more than 40%. (Cadillac did not publish cyberspace horsepower ratings for the high-pinch 1970 engine, only our approximate would exist that information technology made 275–285 hp (205–213 kW).)

Rating methodology nonetheless, 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. (vii,481 cc) V8 still managed 310 internet horsepower (231 kW) in 1973, but was downwardly to simply 200 hp (149 kW) by the time it faded out in 1976. Ford'due south 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). Information technology was not until the widespread proliferation of electronic fuel injection in the 1980s that net power outputs again began to climb.

U.Due south. automakers go on to use SAE cyberspace ratings, just in 2005, the SAE issued a new standard, J2723, which clarified and amended the existing methodology, among other things requiring that a suitably qualified contained observer be present during the rating procedure. Some engines measured nether 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 mode; the changes were in the test methodology. Most if not all manufacturers now apply this methodology for the U.S.-market cars and trucks.

Non-U.South. HORSEPOWER RATINGS

What about cars not built by U.Due south. automakers? As with many things, the respond is, "It depends."

German automakers take long rated their engines under the DIN (Deustches Institut für Normung, High german Institute for Standardization) standards. DIN horsepower and torque figures are metric net ratings, like simply not identical to SAE net ratings due to modest differences in test methodology (such as correction factors for standard atmospheric conditions) and the deviation between metric and mechanical units (discussed further below). Some Italian automakers formerly used the Comitato unitario dell'autotrasporto (CUNA) standards, whose metric net ratings once again were similar but non identical to SAE net ratings. Depending on the fourth dimension menstruation, model, and market, other European automakers typically used either DIN or SAE gross figures.

Until the early on 1970s, some British automakers quoted SAE gross figures for horsepower and torque while others listed cyberspace figures; some manufacturers listed both. After the U.Yard. joined the European Economic Community in 1972, the British motorcar industry switched to DIN ratings.

Japanese automakers rate the outputs of their domicile-market cars under the Japanese Industrial Standards (JIS), which include methodologies for both gross and net output. The deviation between the two was generally around 15%, sometimes a footling more than, sometimes a little less. Until the mid-eighties, near Japanese automakers quoted JIS gross figures for most if not all products sold in the domestic market. The switch to JIS net ratings began effectually 1985, simply 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 fifty-fifty 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.South. Nevertheless, foreign automakers seldom indulged in the kind of gamesmanship Detroit sometimes played with its gross ratings, then the differences between gross and internet 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.

MECHANICAL VS. METRIC

There is an additional complication when because non-U.South. ability ratings: the question of units. DIN, JIS, and CUNA standards are typically — merely not always — quoted in terms of metric horsepower rather than the mechanical horsepower more familiar to our American and British readers. 1 metric horsepower (often abbreviated PS, from the High german Pferdestärke) is about 736 watts while i mechanical horsepower is nearly 746 watts, so 1 PS equals 0.986 hp. For example, 300 metric horsepower would exist nigh 296 mechanical horsepower while 150 PS is about 148 hp.

Unfortunately, many sources are maddeningly inconsistent in their awarding and even agreement 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 ane 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 attempt to avoid information technology.) Since the difference is less than 2%, information technology'southward not a big deal, merely it does frustrate a lot of efforts to be consistent or precise.

SOME COMMON MISCONCEPTIONS

Let'southward articulate upwardly a couple of common misconceptions almost gross and internet horsepower ratings:

1. Contrary to some assumptions, internet horsepower ratings do Non measure horsepower at the bulldoze wheels. Both gross and internet ratings are at the flywheel and do not reflect power losses in the drivetrain.
2. Because of the vagaries of the former gross ratings, the widespread over- and underrating of unlike engines, and the considerable differences in the amount of power consumed by dissimilar intake/exhaust/accessory configurations (fifty-fifty for different applications of the same basic engine), at that place is NO reliable formula for converting gross horsepower to net horsepower or vice versa. Sometimes, the difference is as petty as five–10%; sometimes it'southward more than 25%. Unless the factory released both gross and net figures for a given engine (which some did, fifty-fifty in the U.S.), the best you can do is brand 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 ambivalence of gross horsepower ratings means that many pre-1971 American cars were actually a lot less powerful than the advertised figures would advise. While the late sixties were a golden age of horsepower compared to the late seventies or early on eighties, the differences weren't quite every bit vast every bit they appear at starting time blush. Some engines' advertised gross horsepower probably exceeded their actual as-installed ability by 100 hp (75 kW) or more.

FIN

NOTES ON SOURCES

Near 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," Route & Track October 1959, reprinted in Route & Track on Alfa Romeo 1949-1963, ed. R.One thousand. 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); John R. Bond, "Automobile of the Year: The 1949 Cadillac," Motor Trend November 1949, reprinted in Cadillac Automobiles 1949-1959, ed. R.G. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1985), pp. eight-9; Bernard Cahier, "Route Test/10-63: Mercedes-Benz 230 SL," Sports Motorcar Graphic May 1963, reprinted in Mercedes 230SL – 250SL – 280SL Ultimate Portfolio 1963-1971, pp. 31-34; California Vehicle Lawmaking, Section 9950; "Chevrolet Camaro Z28," Auto and Driver May 1971, reprinted in Camaro Muscle Portfolio 1967-1973, ed. R.K. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1992), pp. 118-121; Chevrolet Segmentation of General Motors Corporation, Chevrolet 1950 Engineering Features: Passenger Cars (Detroit: General Motors Corporation, 1949); Lionel Deluy, "Technologue: SAE What?" Motor Tendency, 15 October 2005, www.motortrend. com/news/ technologue-34/, last accessed 10 June 2016; Contrivance Partition of Chrysler Motors Corporation, "Dodge 71" [1971 Contrivance full-line itemize 81-205-1041], Aug. 1970; "GM: Cadillac," Motor Tendency Buyers' Guide 1971, reprinted in Cadillac Eldorado 1967-78 Performance Portfolio, ed. R.Yard. Clarke (Cobham, England: Brooklands Books Ltd., ca. 2000), pp. fourscore-83; "GM: Oldsmobile," Motor Trend Buyers' Guide 1971, reprinted Oldsmobile Muscle Portfolio 1964-1971, ed. R.1000. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1999), pp. 138-140; Tony Grey, "Olds 4-4-2," Road Exam 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.Yard. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1988), pp. 16-17, 31; Jikayousha [Private Car] Buyer's Guide Jump '87 Edition, Feb 1987; Steve Kelly, "Beware the Quiet Fish," Hot Rod Jan 1971, reprinted in Plymouth 1964-1971: Muscle Portfolio, ed. R.Yard. Clarke (Cobham, England: Brooklands Books Ltd., ca. 2003), pp. 120-122; John Lamm, "King of the Colina: Cadillac Eldorado vs. Lincoln Continental Marking 4," Motor Tendency July 1972, reprinted in Cadillac Eldorado 1967-78 Functioning 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 Nov 1953, pp. 58-62, reprinted in Chrysler Majestic 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 Commuter Vol. xxx, No. 12 (June 1985), pp. 26–27; "Technical Highlights," Auto 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.Chiliad. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1998), pp. 91-95; William Thousand. Toboldt and Larry Johnson, Goodheart-Willcox Automotive Encyclopedia (South Holland, IL: The Goodheart-Willcox Company, Inc., 1975); "Triumph TR4 (Route Exam No. 26/62), The Motor 11 July 1962, reprinted in Triumph TR4 – TR5 – TR250 1961-1968 (Brooklands Road Examination Series), ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1997), pp. thirty-33; and Mark Wan's AutoZine (world wide web.autozine.org). Nosotros also verified a few details per the Wikipedia® entry on horsepower (en.wikipedia.org/wiki/Horsepower).

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