Have worked with MON and RON and the average of the two over the years, when considering high performance competition 2 strokes..easy to get the fuel/compression wrong and you will have an expensive seizure..

Broad-brush 'conventional' wisdom says compression needs to be no more than 10% of the octane rating (being non-specific about which version of octane rating)...hence my earlier post about a max of 9:1 on 92 and probably less (especially given the old-style combustion chamber design).

It is also my experience that some/many octane improvers don't achieve much, especially where they don't contain lead. As they don't, if they want to be acceptable in street engines in most of the western world these days.

Fuel quality, as a function of time and handling, is also a factor: that tin of gas that goes 'whoosh' when you take the lid off is less of a fuel than you might think..the whooshing bit is all the stuff that makes it good. We've all heard how the fuel goes off over time, with the light ends (the whoosh) evaporating and reducing the fuel quality. It's true - read up on Reed Vapour Pressure. Had this experience driven home to me in recent times, where the fuel had lost all it's whoosh and was consequently harder to vaporize in the engine (effectively coming out of the needle jet in bigger 'globs' than normal, which are consequently harder to vaporise). The end result is that the spark plug (and presumably any air-fuel sampling device) sees rich, though the combustion is actually lean...so the tuner goes smaller in the jetting..worsening the problem.

Kerry W

PS: sometimes simply backing the spark off is no more than a band-aid..the compression needs to be right for the fuel

Last edited by Kerry W; 06/12/19 6:54 am.

No generalisation is wholly true, not even this one.
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