Re: Chamber vs compression
"but pistons were in the hole a ways and I used a .039" gasket"
Well, that is one way to create detonation and pinging from too much quench distance. GM set these engines up to have between .020 and .025 negative deck (piston down in the bore), and an .020 steel shim head gasket for a reason, it gives proper quench distance. Back in the olkden "I know it all" days of design engineers that did know it all, and some that didn't, some one that didn't thought pinging required a bigger chamber volume, and told everybody. Others of his kind we nt bonkers with idiotic "fixes", stuff like head gaskets that were out of control thicker than the .020. There were ones that went to as large as .080. Then, another one at Silvolite pistons, near here in Carson City, came up with the penultimate moronic modification, raise the pin center on the piston .020, "Destroking" the piston by lowering the piston deck down even further down into the bore.
This worked excellent, it made the 4engine ping far worse than just a thicker gasket. Math: stock deck: .020 to .025 down in the bore, add Destroked height: .020, and the .080 head gasket, result, a completely impossible to save quench of .125 inch, detonation city, NO ping control. All because of two things, lower fuel quality, and a few scatter brained wrong design engineers that had no business in the auto industry.
Now, stock GM flat top pistons also had 4 valve reliefs, adding to the compression volume, so, even lower c/r. All in all, I remember a flat top, 4 valve relief piston, down the stock engineered .020 in the bore, and .020 steel shim head gasket on a 4 inch bore in a 327 gave right at 10.00:1 c/r.
Time, and sensible people revised settings, came up with "zero deck" dome heights, .039 to .045 thick head gaskets, correct quench, and a gasket that was less sensitive to leakage, and NO pinging, win, win, win, win situation. It still works today, ZZ4 crate engine has 10.25:1 or so c/r, .042 head gasket, zero deck, and when the distributor is fixed past what GMPP forced on them, the engine works, doesn't ping, nor over heat, all the way win situation continues.