6.5 to 1 is pretty much the limit because above that everything is too crowded to let the valves and heads breath effectively - you can't run a piston right up to the valves like an OHV can do, when they are off to the side.
Harley Davidson put huge amounts of effort into developing the flathead long after it was obsolete, well into the '60s, due to a loophole in the US racing rules that let HD run 45 c.i. sidevalves against 30 c.i. OHVs. While stock sidevalve Harleys ran compression ratios similar to the 8ns, the fastest racers usually had even LOWER compression ratios, 5.5 or so, since there was no way to port them without lowering the ratio, and good breathing was found to be more important than compression ratio.
It's likely some 8ns are now nibbling at the upper limits of compression without benefit of trick heads due to the heads and/or blocks having been resurfaced.
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