There has been a lot of talk about compression ratios and how much is too much. I've done extensive research and built numerous motors, and would like to offer the following as a guide to the mysterious compression ratio.
Andy's Comprehensive Compendium on Compression
Compression ratios can be understood in two different ways: static compression and functional
compression. Static compression is based ONLY on cylinder volume versus crankshaft movement. Static compression is fairly easy to determine. Functional compression accounts for cylinder volume,
crankshaft movement, AND valve opening. This is how you figure "real world" compression numbers. Because air flows differently into
a cylinder at different RPM's, it is very difficult to know your
functional compression ratio at every point in the RPM range.
