Sorry to ask this question, but I've searched through all the threads on Big Twin Flatties and didn't find a good answer. I probably missed something.
I have a 1937 UL on the way with iron heads, which I believe are supposed to be a 5.5:1 compression ratio, assuming they are UL heads and have not been previously modified. I realize that head gasket thickness, overbore, base gasket thickness, deck height etc. all effect CR. Even so, my question is, what volume, in CC or ML, generally speaking, equals that 5.5:1 ration, and what would it be to obtain a 6.5:1 ration.
And, how much or a cut would be milled off the head to achieve that?
Also, what is the highest compression these will run at for normal riding type running. The bike will be light, and I weigh 180. I have a '48 Dodge with a flathead 230 inch six that I have at 7.5:1, (which required a pretty big cut) and it's right "on the edge", if you know what I mean...requires premium for sure. My guess would be that 6.5-7.0:1 would work, especially as I can retard the timing with the left hand grip when lugging/pulling. I had very high compression in my knuck, and the left handgrip worked great, but I realize the flathead is a different animal.