Quote:
Originally Posted by red-beard
Lower octane is allowing the higher compression engine to compression ignite.
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This. Your engine needs the higher ignition temps of the the 93 octane to prevent pre-ignition and dieseling. The dieseling is only the symptom you hear, too-low octane fuel increases engine temps and wear and tear at all running speeds. It's ironic I know, but fuel that ignites at a lower temperature (89 octane) makes the engine run hotter than fuel that ignites at a higher temp (93 octane). If gasohol is the only 93 octane fuel available, then use it. It's better for an engine that requires 93 octane than using 89 octane pure gas.
BTW my Stihl dealer recommends the the highest octane available at the pump for the 2 cycle equipment he has sold me (in the past 5 years). They are built for high octane gasohol and run best on it. That doesn't speak for older 2 cycles with soft fuel system parts that might be damaged by ethanol.