L78 vs COPO in 69 ('real' engine horsepower ratings)
 

I recently saw a 2016 motortrend L78 evaluation of how much GM was cheating on what the true horsepower is of a stock 69 L78 engine. They found it was under-rated by 50 HP with stock manifolds for a chevelle application. Add headers and you gain 30 HP.

So 375HP was likely more like 425 (which oddly enough actually matches the standard advertised HP on a 65L78 corvette I have, which made me wonder they are both L78s (65 vs 69 so how did they get away with the HP derating for the same engine model?). I even now remember Bill Thomas taking a 65 L78 apart and blueprinting it and it produced more than the GM stated rating of 425. I have the book that showed how he did it somewhere, but I think it got to 450HP, but it was north of the 425 rating obviously.

So now I wonder, and I know the COPO 427 is more, but 'how' much more is it? stock manifolds (from factory) and the headers numbers. My guess it's likely no more than 7.5% better if you look only at the displacement increase and equate that to horsepower gain. But I have no clue really on what the true HP rating are for a COPO 427 vs a L78 in 69, all speculation on my part.

Back in the day, I imagine if you had stock manifolds and the other guy had headers, the race may not go your way looking at the horsepower benefits.

L78 vs. L72
 

We recently 'freshened' the engine for our 68 L78 Nova...made 425hp at 6,000 rpm....no trick parts.

I would imagine a blue-printed short block, performance valve job, distributor & carb tuned correctly, would get you close to 450hp.

As for the L72...they were briefly rated 450hp before the drop to 425hp. Same things would apply and likely get you well north of 450hp.

-wilma

I would suspect the L72 would have very similar gains from headers as an L78, around 25-30hp. Same 840 heads, cam, intake, carb etc. Very similar engines, more cubes in the L72. Regardless, both engines come alive higher in the rev range with the 143 cam.

I would suspect an L78 with open headers would pull on a stock L72 if both cars had same gearing, weight etc. Both engines need a recurved distributor and bigger jetting to live up to their potential. I have found one medium weight and one light weight spring from the Moroso advance kit bring full centrifugal advance in around 2500rpm. I run my L78 with 37* total timing, 50/50 mixture of sunoco 110 leaded with 91 octane pump. Very nostalgic experience to say the least. Once it gets to about 3500rpm it really starts rippin good. Puts a smile on my face every time!

Remember, HUGE flow difference between streamlined Corvette exhaust manifolds and Chevelle/Camino ones!

Real world comparison, all other things being equal, I believe you can safely add 31 HP for the extra 31 cubic inches.

One of the old tricks the manufacturers used was to take the HP reading (to be reported to the world) at a LOWER rpm and use that, eventthough the power band was still advancing up. One of the tools to keep the insurance companies happy...

Correct…..the L88 was rated at only 430hp at 5500rpms. But we all know that wasn’t even close to the rpms the L88 could twist. It’s my understanding that they didn’t want to outdo the hyped up 435hp tri-power motor.

Kurt

Also keep the lethal L88 out of the hands of novices. It was as close to an all out race motor GM had built at the time.

HI back in the day i owned a 1969 Camaro gold copo and my friend had a 1969 Camaro 396-375 hp we raced the copo would always be 2 car lengths ahead ... both cars m21 trans and 410 rear.

This is a great data point. Did the 396 have cowl induction like the 427? Most would not have, which only adds to the beating the copo would have dealt out. I am assuming both cars had the same setup between them (headers or stock, same 'racing' fuel, same size tires). Very cool info.