[1969GPSJ]

Originally Posted by Gach

Wow you’ve done your home work. Here’s a possibility the automatic cams usually are in at 108-109. Being its a 4 speed car ( sorry can’t remember what 4 speed cams are 112 ? ) that could be part of the issue. I mean you could try one of the new generation pump gas cans more suited for a four-speed. The compression part, I mean hoping those 16’s are 72 cc. So ok my 68 GTO 4 speed 400 had number 16 heads. Those motors rated at 10.75 compression.

It sounds like you’re going to have the heads off anyway so lets see what those 16’s check put ass cc wise. I may be wrong but 16’s on a 400 rated at 10.75 compression vs being on a 428 may be more true 10.75 then on a 400. Anyway I would definitely go with a cam change and definitely see what cc those heads are then decided from there what head gaskets to go with.

The difference between the 428 10.5:1 and 10.75:1 engines was a piston without a dish. The same was true for the standard GTO/Tempest 400 and RA III - all at 10.75:1 and RA IV vs the 350 HP GP engine. The GP engine had the dished piston and was 10.5:1 - almost all of the other specs matched the GTO/Tempest standard 350 HP engine.

I have considered using the 9785744 (RAIII 4 speed cam) or 9794041 cams for both more power, but lower dynamic compression. I still think that thicker head gaskets would help too. You can hear part of knock, but not necessarily detonation, and I don't want to go down that path with a stock engine and cast pistons.