Does anybody know off hand what Pontiac did to raise the compression from 10.5 to 10.75 in the 428 engines. My service manual identifies the pistons and the cylinder heads as being the same with both engines using no 62 heads. I know the camshafts are different upon application and the exhaust manifolds flow much better on the 390hp. Did they raise the compression with a thinner head gasket ?
The difference could be the way the factory calculated static compression and the 1/4 point can also be done with the camshaft. A hotter cam can raise the cylinder pressure slightly and that may be the difference.
I think you can vary the CR that much by choosing different gaskets. Been awhile since I did the calculations, but I used to know how much the CR on a SBC changed by decking the block, shaving the heads or using a different gasket. I have my calculations somewhere - it's pretty easy to calculate if you know the bore.
-- Edited by 66 Beau on Sunday 5th of September 2010 03:56:40 PM
I won't guarantee it but I think there is two different chamber sizes on 62 heads. Seems odd, I know. I will look in a book that I have for those engines. It seems to me the answer to your question is in there.
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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles
1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)
I have some info on 62 heads saying they came in both 72 and 75cc chambers. That would makes sense, as a it also says that a difference of 5 cc's in that size of chamber makes a difference of approx .5 on the ratio.
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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles
1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)
I have some info on 62 heads saying they came in both 72 and 75cc chambers. That would makes sense, as a it also says that a difference of 5 cc's in that size of chamber makes a difference of approx .5 on the ratio.
Great info Carl I found some information that the 428/390 used #62 or #16 heads. The #16 heads are 72CC. It looks like the heads are the difference.
Thanks Carl. The 3 cc difference would be about 1/4 point in compression. I could make that up with a thinner head gasket. I think the major hp difference in the two engines is the camshaft and the exhaust manifolds. A couple of my friends are encouraging me to use the 428 in one of my 55's. They all figure it would be really different and a one of one. I am not totally convinced yet. I am going to put the 69 GP back in storage for another year as I just don;t know what to do with it yet, part it out or try and find a rust free rolling body and chassis. I had the car on my hoist and spent a day hammering the frame to knock the scale off. The body needs a trunk floor lower rear quarters, a drivers side floor panel, both front fenders and inner fender panels, hood and trunk lid.
