compression ratio


Jun 4, 2008
Somehwere i read the HT engines have a compression ratio of 6:1 it would not cost much to have a machine shop shave the cylinder head. how high of a ratio can we go up to in a 70cc engine? I read that 8.5:1 is the limit for regular unleaded gas. is 10:1 possible here?
Jun 3, 2008
10:1 "may" be a little high even on premium fuel and "may" cause detenation, Yes the compresion is low on these, the piston sits about 4mm or 0.160 inch below the deck and the combustion chamber is huge (I haven't measured CC's). and the head gasket is around 4mm thick.
Even raising to around 8 - 8.5:1 would need a fair amount of material to be removed from the head and or the top of the barrel, you can't go back once you've done it.
Another "easier" way is make a thiner head gasket from copper and remove the base gasket, but, and there's a but, these are piston port timed motors so removing the base gasket will lift the comp a fair amount but will retard the exhaust and transfer port timing, and advance the inlet port timing.
I have removed the base gasket on mine and lifted the exhaust and transfer roof's and just "cleaned up" the inlet port, faced the top of the barrel and head face, ( 0.005 inch at most, Just to make sure they were flat) and a 0.010 head gasket.
I have since gone back to standard head gasket as was a little hard to start, actually had to bounce on the seat a little or it would try and lock the wheel.
But over all, way smoother runing, better milage and quieter running.