Uhhh I didn't say 175psi = 10:1 :bang: Not sure where you got that???? Just said if your saw is stock and blowing less than 175 psi it should be fine with the 87 ethanol free gas... I suppose I can add that would be in any stihl or husky saw from 50-90cc mid 80's or newer I'm sure if you look hard enough you can find some special application 2 stroke with a wacky bore and stroke that might not apply but who really cares? oke:
I was using 10:1 as an example. Generally, engines with high compression ratio (10:1 and higher) have less resistance to detonation and require high octane gas. The trapped method is preferred in coming up with a true compression ratio in a piston ported two cycle engine. I was simply trying to say what applies to a four stroke engine doesn't always apply in two strokes. IE a four stroke engine compression = volume at BDC over volume at TDC. You can't do this in a piston ported engine because the exhaust is open for a time during compression stroke. All of this is dependent on volumes, stroke, port timing and so on. The last numbers I saw from someone a lot smarter than I, showed a 50cc saw with 165psi had a compression ratio of about 7:1 because it had too much exhaust duration. Using the normal four stroke method it was like 14:1 which is false. Saw engine are not high performance engines, they are made to run on just about any pump fuel you can find. You need to get to 200psi in a saw before you start having issues with pump fuel IMHO.
Take this for what it is, I'm no expert and I still have a lot to learn.
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