I am just finishing the completely stock 396/360hp engine in my 1966 Chevelle. I have heard that back then GM overrated the horse power in engines, and I have also heard that they underrated the horsepower in engines. Does anybody know what the actual horsepower of engines were back then as compared to their stated horsepower?
Thanks,
Bob

Don't know the answer, but you could pay $50-60 for a couple of pulls on a chassis dyno, and you'd know exactly how much power it was making. if it's actually making 360HP at the crank, you should see 300-310 at the wheels I'd expect.

there was a member who just did a 350 hp or 360 hp engine and had it dynoed and it was pretty close to the rating. do a search on it. it was about a month ago I think.

I believe that rebuild was a 360hp engine, and it dynoed at 357hp. Not to shabby.

In 1966 they used gross horsepower rating. Thats with no accessories. Today they use Net (with accessories). Gross obviously gives a higher rated horsepower. Realistically a bone stock engine back in the day is going to more often than not, make less horsepower than its rated at. Exhaust manifolds, dinky factory exhaust, and all accessories on the engine, really choke it up. You've heard the whole "a 426 HEMI really makes 500 hp". Its not true. A stock Hemi Coronet 4 speed was chassis dynoed, and it made a whopping 315 hp to the wheels. Assuming 18% drivetrain loss, thats about 384 bhp. I also seen a vintage dyno sheet from chrysler, that had a stock 426 Hemi at 433 hp, and 465 ft-lbs of torque. I think thats with headers too. Another example is the ZL1 dyno results i posted a while back. 375 hp with full stock exhaust, and all accessories, but 523 hp with open headers, and no accessories. To get true horsepower to what the factory rated at, you generally will need headers, and then larger full exhaust will often take above the rating. But like someone said, take it to a chassis dyno. I wouldnt be too shocked if made 265-275 rwhp with an automatic.

Your pretty close SS454, my 455" Stage 1 G.S. was rated at 270 net and 340 gross if my shakey memory is correct. I would think the net number for a 360 H.P. - 396 would be about 290 or there abouts at the flywheel, not counting driveline losses. A stick drivetrain looses about 13% and an automatic looses about 20%.

My car dynoed out at 357hp. We were expecting more, but it had 454 torque which was good news. I have heard different info on what Chevy was saying back then and wondered if anybody recalled. I believe the reports were based on environmental concerns.
Thanks,
Bob

"Net" horsepower is still rated at the crank, and does not include drivetrain losses. That's one of the biggest myths going on out there, especially among people who weren't there back when the change was made, is that somehow, the change had something to do with accounting for the drivetrain. It doesn't.

The typical "gross" HP "ratings" from days gone by, were artificially inflated by pressure from marketing; and artificially deflated by pressure from insurance concerns. Consequently they didn't accurately reflect what a car really could do. But, since they rarely involved anything resembling a dyno, they were just a bunch of hype anyway.

Some motors perform close to their "ratings". Others.... are a joke. The "375 HP 327" is one of the best examples of such a joke that I can think of.

As said, the modern system rates them in the form that they are actually sold and used: with all accessories (that means emissions devices, not things like power windows and A/C) installed and operating; with the exhaust system and air cleaner that match the chassis (for a good example of the air cleaner, look at the difference between a 88 Z28 and 88 Trans Am.. identical motors except for the air filter tract); fuel and timing curves set according to actual "specs", not tweaked out for max; etc. etc. etc.

There is no magic "formula" for converting from engine HP to RW HP. Everything in the drivetrain is a variable; rear end lube, pinion angle, auto/stick trans, and so on. Even if you have hard measured numbers from an engine dyno'ed on a stand, the conditions that it was run under, may be completely different from what it will give in a car. It's amazing how not having to power a water pump, PS pump, alternator, and so forth, and having a stack air intake instead of some stupid drop-base restriction, and long-tube large-diameter equal-length headers and no mufflers, will add up.

A typical "360 HP" engine from back then, would make probably 320-330 as installed in a typical chassis; maybe 270-275 of that would make it to the wheels. So, I'd expect to see about that much from a "360 HP" 396, on a piece of real test equipment, in the real world. Personally, I have my doubts that it will make that much; but, until you dyno it, all else is speculation and drivel and mouth racing. Go to the dyno and get a dose of reality when it's running. Be prepared for a humbling experience.

Just curious wonder what a stock 68 Z28 rated at 290hp would dyno?

Don't know off-hand; they were mid-14 sec cars when new.

Thanks for the great info, guys. We used a DTS dyno machine on my car which I understand gives a pretty true reading.
Bob