Fwiw, I'm running unported 16's on an .030 400 with a 744 cam (224/236 @.050" 115.5 lsa) with no detonation on pump gas. Right around 10:1 compression.
There's no magic number as if you put the wrong cam in, it can detonate at 9.5:1. If you have mildly ported heads and a larger cam, it can run fine at 11:1.
What are your constraints? So far we have unported d-ports on a 400. What's the intake, transmission, rear gearing, etc? Or have you yet to select them? All of those factors will determine the rpm range of the engine, which will then influence camshaft selection, which will then determine your safe CR.
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