Underrating muscle car horsepower?

I’ve heard this spouted off as common knowledge since as far back as I can remember, and it just popped up again in another thread here. The idea is that Ford/GM/Chrysler would underrate the horsepower on some of their top muscle cars to keep insurance rates down, or not to piss off the Feds, or whatever reason they come up with.

The other thread pointed out that old horsepower ratings used to be measured on a carefully blueprinted engine with no accessories and only a nominal exhaust, which makes sense to me – get the highest number you possible can out of the engine, then advertise the hell out of it to get people in the showrooms. This means that most engines would be overrated.

But it doesn’t make sense to me that they would ever underrate them. Insurance companies only care about actual crash statistics and costs of repairs. They may make an estimate for new cars based on things like target demographics and horsepower, but if people started wrecking a certain muscle car frequently because it had a ton of power, rates would quickly rise to reflect that. So underrating wouldn’t affect the rates for longer than a few months, but it would definitely hurt sales, unless you were really counting on underground rumors to sell your top models.

And the Feds never had a say in it, unless the automakers were worried that they would try to change that, and this was a pre-emptive strike. Still, that seems dubious, and I’ve never heard of any proposed bills from the 60s/70s that would limit hp output on cars. They did that pretty effectively with emissions controls, anyway.

Is there any actually evidence that this was common practice? Or, like I suspect, is this just an example of men hearing what they want to hear, which somehow persisted for decades?

I can’t speak directly to that reason, but I know that Chevy has tried to avoid ever copping to the fact that sometimes the Corvette wasn’t faster than the Camaro.

Assuming a case where the two cars had the exact, same engine, what year(s) was this the case?

I don’t know about the Camaro, but the Buick GNX is generally accepted to be quite a bit faster 0-60 than GM said it would be, for that reason.

As it happens, I was reading about the GNX this last week and it is possible that there were some very limited versions of it that made it the fastest GM car for that model year, you are correct. I was just wondering if there were any cases where the same engine was in the Camaro versus the Corvette and the Camaro was still faster, just from an academic interest only.

You have:

HP (or BHP): The power of the engine all by its lonesome.
RWHP (Rear Wheel HP) or FWHP (for front wheel drive cars): The power of the CAR sitting on the dyno. This is a more accurate picture of what the car can do (aside from weight and aerodynamics). That Lincoln with catalytic converters, icy-cold AC, and a slush-box of a transmission might have a great HP, but is RWHP is gonna drop.
SAE net HP: similar in use to RWHP - the HP of the engine with all of the goodies attached.
PS: Some damned German thing, because German horses are weaker than our American horses.

Yes, back in the late 60’s to early 70’s, the car companies occasionally fudged on the HP numbers to keep insurance rates down. This practive didn’t last long because everyone got wise to it, but it happened. Some insurance companies based their premiums on the power of the cars engine, so for a while, it was a game the car people played.

It was not unusual for a certain engine to be under-rated. Sometimes an engine would be available that came with higher performance camshafts or carburetors, yet be rated at LESS than the standard engine it was derived from. Sometimes an engine with several significant performance options would only be rated at 5hp more than the unmodified version. An engine like a Ford 428 Cobra Jet would be rated at 335hp, which was probably about a hundred short or its real potential when properly tuned.

In most cases, engines before 1972 were rated an a bare engine on a test stand with no transmission or parasitic power losses - just pure power. In 1972, most car companies lowered the compression ratio of their engines to comply with emission standards. They also started rating the engines loaded with full accessories and drivetrain losses. This is more like actual rear whell horsepower, and is more like engines are rated today. It made a big difference in the power ratings, usually a 25%-30% decrease in listed horsepower. A modern engine that makes 200hp is a lot more powerful than a 200hp engine from the sixties.