Thread: Octane for Gas Engines

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  1. Octane for Gas Engines 
    I've read your column a few times and find it interesting to read. I'm interested in moving up into gas engines and I'm curious about the octane rating of the gas for these model engines.

    For a typical gas model engine: like DA, Fuji, or Zenoah, etc... what is the recommended octane rating of gas used for these engines ?

    What happens if you use higher octane gas like Sunoco Ultra-94 (94 Octane) in these small engines ?

    Can you damage the engine using higher octane gas ?

    Do you have to change the oil mixture or high/low needle / carb adjustments if you use different octane gas ?


    Dear Greg
    There is a mistaken belief that the higher the octane rating of the fuel, the more power an engine will develop. This is definitely not the case. Higher octane fuels are less prone to detonate when higher compression ratios are used. Detonation is caused by the expanding gases compressing the fuel mixture ahead of the flame front causing a secondary explosion. The piston diameter (bore) in our model engines is too small, and the flame front too short a distance to travel, to cause this to happen. There is no need to use higher octane fuel than 87. Additives used to increase the octane rating can actually form more unwanted deposits in an engine.

    Clarence Lee
    (to write to Clarence, email

    Join Date
    adrian, mi. usa
    high octane can more problems than any increase of performance neadles become more fussey.

    Join Date
    Santa Catarina, brazil
    There is another aspect to this. Regular gas-station auto fuel, at least in my region, is not only extremely variable in quality even when buying from the same gas station, but also has a very high ethanol content, in the order of some 20-25% (which incidentally keeps varying, too). All attempts to maintain constant adjustment to a Zenoah engine with gas pump fuel failed miserably. I gave up and started using 100LL AvGas. In other forums a number of people would keep telling me that all sorts of bad things would happen to the engine but I have been running AvGas on it for some 7 years now, with no ill effects whatsoever. All I get is easy starts, consistent power, no more fiddling with needles, no significant increase in build-up in cylinder or spark plug. As long as I can keep purchasing AvGas I don't think I'll ever go back to auto fuel again.

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