I've built and dyno tested several different stock Mopar iron headed pump gas 426 type motors, 434 C.I., 3.91 stroke and 4.150 stroke motors for the same customer. All of them where right at 10.5 to 10.8 to 1 compression ratio and ran fine with CA pump swill back in the early 2000s I built the same customer a D5 alumimun headed 526 C.I. motor with a stock OEM Mopar cross ram with stock OEM Holley SS carbs., it struggle to make 50 HP more on the DTS engine dyno than the 4.150 stroke motor did with the new heavy Mopar iron replacement heads and a stock 1966 inline intake manifolds and stock 1966 AFB Carbs did That motor had a little over 12.5 to 1 compression, I probally should have treid it also on CA 91 octane pump swill