Did Detroit 'Muscle Cars' Really Have Higher-Than-Rated Horsepower?

I’ve heard that many of the older ‘muscle cars’ of the 1960’s and 1970’s actually had much higher horsepower than they were originally rated for, presumably because the ‘real’ horsepower figures, if published, would result in higher insurance costs for their owners.

However, since these cars obviously weren’t marketed for their safety, reliability, fuel economy, drivability, comfort, handling characteristics, spacious interiors, or for not requiring a lot of maintenance, I have to assume that the factory horsepower ratings were the major consideration for muscle car purchasers.

Therefore, if a manufacturer specified lower horsepower than their car actually produced, this would clearly lead to reduced sales and profits for the company.

Are we to believe that the Big 3 were so magnanimous that they were willing to tolerate reduced revenues, simply so their consumers could have slightly reduced insurance costs?


You have it backwards. The published numbers are inflated versus today’s methods of testing and reporting. Today is more legit in terms of what the engine will produce in production mode, with real fuel, timing, etc.

What they DID do was hold back the numbers relative to their era.

So, if they had a 400hp motor (which might be 285 today), the might report it as 365, and this might be for insurance reasons, or just because the internal directive from the corporate decision makers was to cut back the HP (for image reasons, safety, etc).

Oh, btw, some famous muscle cars were produced as ‘sleepers’ (family sedans with monster motors and few accessories) because the engineers literally snuck a performance car out AGAINST the directive of the company execs.

John Delorean is famous for rolling out muscle cars while simutaneously taking directives from headquarters that said “stop the hot rodding”.

Under rating the power was the exception rather than the norm, but it did happen.

Some examples:

Late 60’s Corvette with the 427 big-block was initially rated as 450hp, and then lowered to 425 in early production. Engine was unchanged. Reason was insurance co balked at the high power number.

Another example would be the L88 Corvettes (again late 60’s). These were really meant for road racing but had to be sold to the public due to sanctioning body rules. They had no fan shrouds and would overheat in traffic. To discourage people from ordering them they were rated at 425hp, while another engine which WAS intended for street use was rated at 435hp. This made consumers prefer the 427/435 over the L88. Later dyno tests have showed the L88 producing over 500 hp.

This question has always bothered me, and I don’t think I’ve seen a definitive, proven answer to it.

I reject the hypothesis that the numbers were lowered for insurance reasons, because AFAIK all of the “true” power values were well-known by enthusiasts, hot rodders, and racers. It’s not like the insurance companies operate in a vacuum - if they see chat in hot rod magazines and racing journals of the time, my guess is at worst they would only be 6 months from raising the rates to reflect the “true” horsepower.

Another reason is, according to my State Farm rep who I called just now, he doubts very highly (but cannot offer a definitive proof) that cars were ever rated at different premiums based purely on horsepower anyways. The way he put it, “A Corvette with a 454 would rate the same as a Corvette with a 350, unless the one with the 454 was labeled by the manufacturer as a ‘special edition’ - which it often was - or was a very limited release. In that case, we’re insuring it at a higher rate because it’s collectable and hard to replace. Most actuarial tables back in the 1970’s didn’t break anything down as detailed as engine power, or even engine size. But they did differentiate between V6, V8…”

I also doubt that any group of engineers was able to “sneak” an engine or engine configuration/tuning out. There are so many people involved in the production and manufacture and outfitting of an engine and the parts that make it up - everything from initial design to testing to contracting OEMs to delivery scheduling to emissions and safety certifications…I really, really doubt that anything got out of the Big Three with statistically significantly more (or less) power than intended.

I don’t have cites to offer right now other than the 3rd-part account I quoted above, nor do I intend to offend any poster’s experience here. I’m just offering why I have doubts over a couple of the premises I’ve heard a few times before. And I could very easily be wrong on both counts.

"In 1963 Pontiac’s racing fling was ended when General Motors placed a corporate ban on factory participation in stock car racing. Wanting to keep Pontiac’s new-found performance image alive, Estes, now general manager, and De Lorean, now chief engineer, started experimenting with their intermediate Tempest model that had been introduced in 1961, but hadn’t enjoyed great sales success.

First they tried one of the division’s 5.3 litre V-8 engines in the Tempest and were gratified at the snap it gave the lightweight Pontiac. They then tried their 6.4 litre V-8 in it, and of course, it was even faster.

Satisfied that they had a winner they decided to offer a GTO (named after a Ferrari model, and standing for Gran Turismo Omologato) option in the redesigned Tempest for 1964. Besides the big V-8, it got heavy duty springs, shocks and brakes to handle the higher power. A 4-speed manual transmission was also offered, along with the regular 3-speed manual or 2-speed automatic.

**This activity had been concealed from senior GM management by Estes and De Lorean, and by the time the 14th floor found out, it was too late to abort the GTO option. They weren’t pleased, however, feeling that it undermined the spirit of the responsible corporate image that GM was trying to project by pulling out of racing. Pontiac’s pessimistic general sales manager Frank Bridges refused to include more than 5000 GTOs in the 1964 production schedule. " **


Also, often the engine would be underpowered by adding a simple throttle-stop to the carburetor…which could be removed by the owner in 30 seconds with a screwdriver.

Another trick was to artificially adjust the car’s power to weight ratio by sneaking sheets of lead weight under the upholstery…also easily removed by the owner.

Very interesting, Philster. Perhaps it was mainly due to the times/era that they could get away with that for so long. I would have a hard time imagining that happening post 1980, but in the context of the thread topic here and now, I see your point.

Enola - throttle stops I believe entirely; I removed one myself on a car. But lead sheets? Can you give an example? :confused:

A boss I had in Philly said thats what AMC did with the AMX and Javelin.

Its kind of the same with high performance German cars too, isn’t it? They have a 155mph limiter that can be legally removed by any mechanic. It was there to stop the government bowing to green groups who wanted lower speeds on the Autobahns. Bit more complicated than the throttle stop I’m sure, but same principle.

IIRC, two things were going on here. First, Detroit seriously overrated the HP on smaller engines. Essentially they looked at the horsepower produced at the crankshaft on a test stand without accessories like generator, power steering, even the water pump. They also were looking at engine horsepower and not horsepower at the wheels. Compare a stock early Mustang with a 289 / 271 HP engine with a late Mustand (much heavier) with a 275 HP 302 engine and you will see how optimistic the early ratings were.

For the performance engines, a different situation arose. Basically it was agreed, to keep the do-gooders at bay, that engines with more than (I think) 425 HP would not be produced. Anyone who thinks that a late model Dodge Hemi with two four barrel carbs produced only 425 HP had to be smoking something.

do not believe that is what the OP is asking about. Back in-da-day the ratings were net HP. Today they test the engines with all the accessories and exhaust in place so the numbers are less than if tested with nothing extra on the engine and using dyno headers.

But on to the OP. YES, and it is still done today. That is one reason my Bonneville that weighs more and has less rated power than my buddies Honda still blows away his Honda. My Buick was rated at 235 HP------at 4,000 RPM’s. At 5,300 RMP’s it is more like 300+.

Of course the area under the torque curve is what really tells the story.

I thought it was pretty well established that the ‘racing’ versions of some muscle car engines were rated low intentionally, either for insurance reasons or just to inhibit people from buying them (the company had to offer them to meet the rules of various race bodies). The engines I’m thinking of in particular were the GM L-88, LS-7, and the Dodge 426 Hemi.

Some cars are definitely under-rated, even today. For example, the Subaru Forester XT is advertised to have 210 HP and 235 lb-ft of torque. But its performance numbers (0-60 in 5.3 seconds, and 1/4 miles times under 14 seconds, weight 3200lbs) don’t add up with those numbers. Cobb tuning then put one on a dyno, and discovered that it’s making more like 240HP, and about 260-265 ft-lbs of torque.

Why would Subaru under-rate the engine so much? Could be lots of reasons - marketing may have wanted to make sure that the HP figure was below the WRXs 227, to prevent confusion in the marketplace. Perhaps the marketing material was done before the final tweaks were done to the design, and the engineers who gave marketing the numbers were conservative about what they could do with the exhaust and timing. Or perhaps they were afraid of being caught OVER-rating the horsepower like Mazda did with the RX-8.

I believe the Dodge SRT-4 is also under-rated by 20-30HP.

One of the tricks I recall reading in a Mopar book was an even simpler bit than a throttle stop or anything else.

The 2X4bbl Hemi, for example, was rated at the brochure at something like 425 hp at 4,300 RPM.

Which was entirely correct and accurate. Except that wasn’t the HP peak. They’d simply taken the HP measurement at some arbitrary point on the HP curve, whereas the actual HP peak was closer to 510.

When did the change occur? Was it a response to the oil embargo, burgeoning environmentalist political sway, shifts in the insurance industry and/or something else?

Would my 1999 282 HP car be a 400 HP bruiser or a 185 HP weenie by 1968 standards?

Ringo, you need to find a graph of the torque output. 282HP would rate even higher using the net HP method of the past. If it is an import, it would rat LOWER than how domestic engines are rated.

Sam, those numbers actually do make sense. 14.0@93mph in a 3,300 LB car = 200 HP on my ATI Power Speed Calculator. Also, of the dozens of WRX’s I have seen at the track. Only modified WRX’s make it to the 14.0 range, and lots of mods get them into the 13’s. Even in the cold air of November.

Una rises a good point directly relevant to the OP that remains unanswered. How much of this horsepower low balling, to the extent it occurred, was really because of insurance companies actuarial concerns re risk management. The notion that actuaries would be finely parsing insurance rates, to the extent there would be significant cost differences based on 20-40 horsepower differentials between similar models seems a stretch.

Looking on the net this notion is widely taken as gospel. Are there any dopers that know someone who would have been selling auto insurance during the 60’s that could answer this definitively?

I can tell you guys that when the insurance company looked at the VIN for my Buick Regal in 1987, my rates went from about $600/yr to over $1,200/yr. VIN 7 means there is a higher HP engine under the hood. So HP does make a difference sometimes.

Ringo, would your car be a BMW 540i?

My WAG: About 320-345hp in 1968 terms (net hp).

I can tell you guys that when the insurance company looked at the VIN for my Buick Regal in 1987, my rates went from about $600/yr to over $1,200/yr. VIN 7 means there is a higher HP engine under the hood. So HP does make a difference sometimes.

Blown & injected, when the engine code reveals a special order VIN 7 (turbocharged) engine, that implies both “high performance” and “expensive to replace” ; To put it in simply, there is quite a bit of difference between a grocery getter Regal and a Grand National. Your insurance co. is not dumb and their rates reflect this.