Go to rotaxowners.com and you should be able to pull up the specs on your engine. There are two types of compression listings for a 2 stroke. I can't remember the specific connotations but one is for when you crank it over by hand and the other is when it is running. I had an old Ducati 1965-66 maybe, but it had a 12.5 to 1 ratio and ran on ethyl pump gas ( mid to upper 90's octane) it was finicky on tuning but it ran like stink. 4-stroke engine.