Muscle Car Horsepower – How Exaggerated Was It?

Forty years after the end of the “classic” muscle car era, there is still some confusion over horsepower ratings, especially how they relate to today’s cars. Let’s try to clear it up.

Prior to 1972, American carmakers used the SAE (Society of Automotive Engineers) “gross” measurement of horsepower. Gross meant the figure was taken from an engine running on a test stand, with no air cleaner assembly, accessories or exhaust system connected.

By 1971, carmakers began reducing compression in many engines in order to meet upcoming emissions requirements and to use unleaded fuel. General Motors and Chrysler began advertising both gross and SAE net figures in 1971, derived from an engine tested with air cleaner assembly, accessories or exhaust system connected.

The net ratings, which were applied across the board for 1972, must have been a shock to some customers. Suddenly, muscle cars appeared to lose 100 hp or more.

For example, the Corvette’s optional LT-1 350 cu. in. small block V-8 had 370 gross hp in 1970 (with 11:1 compression), then a 330 hp gross rating (with 9:1 compression) for 1971-1972 with a 255 hp net rating. The mighty Chrysler 426 cu. in. Hemi kept its high compression and 425 hp gross rating for 1971 and showed 350 net hp.

Jim Campisano, editorial director of Muscle Mustangs & Fast Fords and Super Chevy magazines, has revisited the classic muscle car horsepower topic numerous times over the years. The magazines have compared old and new muscle cars and have also put classic models on a chassis dynamometer to record rear-wheel horsepower.

“Rear wheel horsepower was at least 30 percent lower than the reported gross figure, in some cases even more,” Campisano said.

Some Super Chevy readers must have been stunned to see that an LS6 Chevelle SS, with 450-hp rating, put down 288 rear wheel hp in the dyno test. That would have put a net hp rating at around 350 hp for that legendary big block.

You don’t need a dynamometer to estimate net horsepower for classic muscle cars, or to check claims of current models. Roger Huntington, the renowned technical writer who penned articles for many car magazines into the 1980s, developed a formula to show the relationship between quarter-mile performance and power output. Others have refined those formulas and developed calculators, in which you can use performance figures and vehicle weight to get estimated hp. (To check hp figures for this article, we used calculators at

Keep in mind that some muscle cars came specially prepped from press fleets, sometimes with non-factory supertunes. That’s one reason that making comparisons using vintage road tests can be sketchy. Different axle ratios, testing methods, drivers, test conditions and other variables also affect results.

But we can try anyway.

In 1970, Car & Driver tested a Pontiac Firebird Formula 400, which had a 330 gross hp rating and was equipped with a fairly tall 3.07 axle ratio. They recorded a 14.78-second ET at 98.9 mph. A 1970 Trans Am with the same engine, but with a 4-speed and a 3.55 axle ratio, was tested by Muscle Car Review magazine in 1995. That car burned the quarter-mile in 14.68 sec. at 97.17 mph, quite close to the C&D test 25 years before. Pontiac gave that engine a 255 net hp rating for 1971.

Now, let’s add a later model into the mix. When C&D tested a 1979 4-speed Trans Am with the emissions-controlled W72 400 engine, the one with a 220-hp net rating and the “T/A 6.6” decal on the shaker hood scoop, it ran a 15.3 second ET at 96.6 mph. That car had a 3.23 axle ratio. As a drag racer will tell you, the mph figure is the better indicator of horsepower than ET. So, the 35 net hp deficit from the 1971 engine seems accurate, and not nearly as bad as some might have thought three decades ago.

Some myths still persist, though, one being that the 1969-1970 Ford Mustang BOSS 302, which had a 290 hp gross rating, really had “around 400 hp.” Vintage road tests show mid-to-high 14-second ETs at 94-97 mph for a car weighing about 3500 pounds with a driver and test gear. That’s about 100-150 pounds less than the 400-powered Firebirds cited above. Given those figures, the BOSS 302’s 290 gross hp rating seems accurate, pegging net hp closer to 240.

Contrast that with the 2012-2013 Mustang BOSS 302.

Ford rates the modern BOSS with its DOHC 5.0-liter V-8 at 444 hp. Car & Driver, driving one the way most drivers would (not powershifting), recorded a 12.8-sec. ET at 113 mph. Muscle Mustangs & Fast Fords, with hot shoe Evan Smith banging off full-throttle powershifts and the car running on drag radials, scorched the quarter-mile in just 12.07 sec. at 114 mph.

Those similar mph figures easily substantiate the car’s 444 net hp rating and leave no doubt that the 1969-1970 BOSS 302 was at least 200 net hp below that.

To be clear, debunking myths does nothing to tarnish the place that classic muscle cars hold in our hearts and garages.

“We still love the old ones,” said Campisano. “They’re cool looking, fun to drive and fun to look at. It’s just a different performance world today.”

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    This is completely disingenuous. The main reason cars from the late 60s and early 70s had high et’s was because of their lack of traction, not their lack of horsepower. Had you put wider wheels and slicks on those cars back then, their ETS would have dropped two or three seconds.
    I work for a speed shop back in the day that was one of the rare chassis dinos available in those years, and the horsepower numbers for most muscle cars were devalued by at least 30% in 1970. If properly tuned, a 426 hemi was closer to 500 horsepower than 400, and the 70 ram Air 4 from Pontiac was around 450 horsepower. A DZ 302 was about 340, as was a boss 302. The TA and AAR were around 360

    That’s false. Change from gross to net power figures for the 1972 model year is the main issue. Many vintage road tests were performed on slicks, yet most of them were still slower than a new 2.0 liter Honda Accord. There are also vintage chassis dyno results proving how bogus the old gross power figures were. For these power levels, add 15% or so to get flywheel HP. 1970 LS6 (very well documented production line stock but well tuned example) produced 285 peak HP on an inertia chassis dyno, as witnessed by “Super Chevy” magazine staff. That’s 335 SAE net at the crank, give or take a little. Also, unlike ET, trap speed will remain similar – regardless of tire choice for cars in this general weight to HP ratio class.

    Chrysler in 1972 all engines were changed..Cam,crank,heads they were less powerful not because of the NET rating,because engines were weaker…1971 and older engines were low balled in power,basically the Gross hp is the actual Net hp…The 71 426 Hemi net numbers were again low balled and you dont lose 75 hp from gross to net,if you know you know.I dyno’d over 300 of my personal engines,I owned 3 different dyno’s.I rebuld engines and dyno tune them.

    The Honda you talk about has a low gear in the transmission and axle ratio are pretty low thus for that car it make sit get up and go the best it can,change the gears to old car gear sets (remember not only axle ratio but transmission plays a huge part)and its a 16.5 second 1/4 mile car and a 8.4 second 0-60 car..Old 727 Chrysler trans has a 2.45 1st gear ratio vs the new 8 speed with a 4.71 1st gear ratio thus is quicker than a 2.45 1st gear.Your Honda has a low trans and axle gear set.

    Example in 1984 I had a low mile 1972 Charger Rallye 440 4bbl,3.23 gear,car ran perfect no issues and 60,000 original documented miles…The car ran 2 lengths ahead of my brothers stock original 1968 Coronet 2 door (b body like my Charger)with a 383 2 barrel non high performance..The 383 2bbl had lower compression and different cam than the same year 383 4bbl version that was 270 gross hp..Also he had single exhaust I had duel exhaust! I ran low 14’s in the 1/4 mile,he ran 15 flat..He probably has 220 -230 the 383 2bbl was a lower compression engine vs the same year higher compression 4bbl.

    I could not beat my buddies stock 4 door 1969 Chrysler Newport Custom and he had a 383 4barrel all bone stock and 140,000 miles! That car was 2 cars ahead of me! Again I ran low-mid 14’s in a smog 440,low compression,weaker cam engine.My engine was in perfect condition,compression checked etc..Later that buddy bought a 74 Road Runner GTX thats the 440 and it ran door to door with my Charger…

    I finally bought the car I wanted a 68 Charger RT original,stock from a elderly couple(gold/black) well the car was what I wanted,color not so much…But that Charger blew the doors off the 72 440 like it was standing still…Low 13’s in the 1/4 mile 100% stock besides the then new ultra wide at the time 275/60/15 tires!!

    That was NOT with the stock 185/75/14 inch series white Wall bias ply tires I ran 14’s-15’s with those stock bias ply tires going sideways and spinning,then another run took off like I was on ice until 35 then floored it it missed all of 1st gear… I tuned up the 68 and ran high 12’s in the 1/4 bone stock except new points,850 carburetor replaced the 750 cfm carb..I put 275/60 series 1985 radial tires at the back.High 12’s it what they ran pre 1971 440’s vs 14’s for the smog 72-74 440’s it wasn’t only changing to net..The cam,crank,heads,carb were changed,plus lower compression..It neutered the engines!

    The so called tests you talk about with slicks lol,those old slicks were equal to a minivans 225 wide tire,they never hooked properly..Tracks were not sticky like today most tracks were not sticky until the late 1990’s..Old road tests were done wither car starting in 2nd losing all of 1st gears power or walk it out to 30 mph then flooring it,losing the power the cars make in 1st gear in an automatic or standard trans…Also some tests they just floored them and they had tire smoke and thats that..

    I have old magazines from the and each car ran different times by each magazine/same car..Then you could have the same year of car with a bigger engine run slower times than the same car with a smaller engine..Not only magazines but I owned hundreds of cars from that era..From bone stock to wild.

    Basically most old car tests were up to 2-3 seconds off the 1/4 mile time…unless they had a straight 6 those hooked!

    2 or 3 seconds quicker with only slicks? That’s laughable, and you must never have been racing at the drag strip to make such a statement. Quarter mile trap speed is a great indicator of HP, I can have a horrible launch (2.24 60 ft) with my 1968 400 5-speed 3.55 rear geared LeMans and run a 13.2 @ 107.4. then the next run a decent 1.99 60 ft a 12.7 @ 107.8. Hardly a 2 or 3 second difference, and I may get another few tenths quicker with slicks, but my trap speed may not move much. Almost all muscle cars back in the day were very lucky to run 13s stock. Now with day 2 or 3 mods, yes they were much faster, but all modded cars are.

    This article is very good, but I take issue with it. The first version was a complete rip off of rock solid data I posted on various auto related sites, right down to specific examples and commentary. I wrote the author about that, never received a reply and later noted that the author heavily revised it. The author should have noted his source of this information for the original version in particular, given that I essentially wrote it. I have many usernames on various auto related website, with the “1LE” and “Koki” characters generally appearing in them.

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