Greg,

...selectively, so as not to annoy you more than necessary...

I believe I wrote the compression setting was not much different. The space in the combustion chamber, between the power piston and the contra-piston, is small. The contra-piston screw thread is not variable. At high RPM compression settings, a quarter turn MORE compression changes the compression ratio more than the same quarter turn toward LESS compression. Is that agreeable to you?