Seeing effective compression ratio has been mentioned I'd better say that any figures I've given have been geometric compression ratios...obviously, seeing I used the engine's stroke as part of the formula. Virtually all RC sport engines have an exhaust timing within a degree or so of opening 75 degrees BBDC so using geometric will give the same results for nearly all engines.

Effective compression (defined as starting when the exhaust port closes, which seems logical ) actually varies with revs reaching a peak at the max torque rev. Either method would work but if you used effective compression then the max would likely be more like 9:1 instead of the 13.5 I used.

I thought I'd better point out the difference in case someone tried to modify an engine to get a 13.5:1 effective ratio. Then you could really run it on diesel fuel .

The only manufacturer I know of who's always (or at least, used to) publish compressions is Enya.