[uneasyrider]

Originally Posted by pontiac jack

Hey, we're only talking numbers from a simulator here- no reason to rush to such a conclusion...

Ditto!

ALSO... What are the spec for the quarter mile on a car with this engine? There's where the hp #'s can be found for real. ALSO... anyone look at what NHRA says about this engine in a specific car? Don't they have there estimation posted under tech specs or something?

P.S. Just found a site with a 1/4 spec for a '69 GP, 428, 390hp. It says 14.1sec. So as a rough guess lets say the car is 4,000lbs and I have a formula for guessing hp by 1/4 times here...

hp = weight/(ET/5.825)^3

hp = 4,000/(14.1/5.825)^3

hp = 282hp and I think that is at the flywheel for that formula.

My guess is they rated (as I have heard many times) the engine without ANYTHING on it and put in some "correction" factor to get the #'s they wanted.

Now, if the formula I have above is for rear wheel hp that would give about 352fwhp but this is just bench racing.

P.P.S. Just found a 1/4 time for a '63 GP 421. I looked up the curb weight of the '69 GP above and it is 3,715lbs so I would guess with some fuel and a driver 4,000 isn't that far off. But, back to the 421 370hp, curb weight 3,915lbs so we will say 4,200lbs,

hp = 4,200 lbs/(15.1/5.825)^3

hp = 241hp

[mattgouras]

That's a good way of getting at the true horsepower the engine left the factory with.
The 390 HP 428 you checked out is, obviously, a lot different than the YS 428 I plugged into DD originally.
Your formula got 352 flywheel horspower. Plugging a 428 with 068 cam, 10.5 compression, factory performance manifolds and stock d-port heads ... I get 370 horsepower. (I attached screen shot)
Both figures are at least in the ballpark of the factory claimed horsepower.
The 360 horsepower claim for the small-valve 428 ... not so close it would seem.
Correcting the head flow I had for small valve heads to match an actual test, I get a best of 272 horsepower for this engine.
As an aside, My BONE STOCK 428/360 horsepower (with 86K miles) ran a 15.4 (92) quarter. Using a 4,200 pound car weight, that calculates to 260 actual FWHP according to one online calculator.
Now, this engine has probably lost a bit over 40 years. But still, it would seem Pontiac really over-rated this engine's horsepower leaving the factory!
Enough fooling around for now. It's Saturday morning, and I have to get out to the garage and start tinkering ...

[428 78T/A]

I about had a coniption when my builder gave me the dyno sheet of my .030 over 428, he said 344 horse and 452 torque. I keep wondering how in the heck is it only getting 344 horse when the lowest rated 428 of that year is 360, with #46 heads. Guess I got my answer, they were severly overated.