There is a useful compression calculator at the following link that allows one to estimate both static and dynamic compression ratios if you know the particulars for your engine. Thought you may find it useful.
On the subject of compression ratio, I have a few questions. I wonder if we might be mixing apples and oranges when we look to what KR’s ran (6.13:1 according to the Jerry Branch CycleWorld article), and extrapolate from this to what might be tolerable on the street. Rightly or wrongly I’ve always assumed that the low KR compression ratio was a necessity to keep the engine from melting down. Think about what someone would do to a poor KR on The Springfield Mile. Probably a 45+ second lap time, with WOT applied for maybe 12-15 seconds, then a short break for corner entry and right back on it for another 12-15 seconds. 12 seconds sounds like nothing, but in my mind it’s more like an eternity especially when you do it back to back 50 times in a row (25 lap race). So, in light of this example I fully appreciate how low compression ratio on a KR was mandatory for survival, but wonder if a street bike (like Jim’s?) that is used mostly for cruising under much milder conditions (dramatically lower EX temp) couldn’t employ more than 6.5:1 CR.
I haven’t seen anything from DumbClutch posted here for a while but on many occasions in past posts he seemed pretty proud of one of his rides that he claimed had ~ 8:1 CR, was very reliable, and would cruise at highway speed all day. Anybody using much more than 6.5:1, or does experience tell us you can’t go there, even for street use (where we pull it out one time, not 50 times in a row back to back)?
To shift gears to an earlier topic of this thread, I have a question for Patrick or anyone else that wants to chime in. The subject being “sparkplug location” that arose from Patricks quote -
“but don't forget that's with well cleaned up aluminum head contours, with nice straight shelf, slightly recessed but not hidden spark plug locations, piston slight pop up, etc”.
On many vintage flathead performance engines I note that the plug is in a recess. Often times this is solely to allow easy plug access, yet in other instances it is clearly by design, e.g., the head might be tapped for a 3/4” reach plug yet the factory plug will be the next reach short of that. What benefit does slightly hiding or recessing the plug serve in terms of combustion efficiency/inefficiency or heat management in flathead engines? In many hi perf OHV applications that I am aware of the plug thread is generally flush with the combustion chamber, hence my question about recessed plugs for flatheads. In some flathead applications I know of, the plug is actually designed to sit back in a hole (totally recessed), which seems absolutely backwards to me, but then again what do I know?