Compression ratio is about 7.5:1 on this engine according to a Honda shop near me. Best I have gotten out of mine is about 6400 or so but that was with the stock manifold. I have made a new one to rotate the carb and matched it to the carb bore so I should see a gain. I also have a 10.5mm carb to try once I get the plane finished. Simple compression ratio formula is:

Volume at BDC divided by volume at TDC.

However this formula does not take into account dome or dish pistons, gasket thickness, bore, stroke, volume of the combustion chamber, deck clearance, etc.

Nitro may be the ticket to increase the power. I know the boat guys do different cams or grind the stock ones etc. Who knows what happens to reliability though. I won't be messing with mine much as I am in the air verses the guy with a boat and all the mods who is in the water. We have way more to loose.