670s had a factory listed cc of 72, but that was probably optimistic. My wife's uncle used to race Ponitac stockers all through the 60s-70s, and said they never got set of Pontiac heads (not even RA IVs) that delived the advertised ratio. They always had to mill them.
I run 670s on my 67 428. I have a small dish in the piston with a 744 cam. Car doesn't ping on 94 octane, but I do mix toluene with my fuel. Whe I rebuild the orignal 400 HO for my '67 (with 670s), I plan on dishing the piston enough so I don't have to mix fuel anymore.
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Three times the sound peaks, falls back, peaks again. A throttling back to cruising speed, a dwindling grumble of thunder and...gone.
The frogs take up where they left off.
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