Mogas Octane Loss

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Mogas Octane Loss

Postby TakeTwo » Tue Feb 13, 2018 1:18 pm

Being new to the Light Sport realm, I do not yet fully appreciate all of the nuances of operations that are possible. Poor weather and poor scheduIe has kept me on the ground for most of December and January. I currently have a mostly-full tank of mogas that has been there for about 4-6 weeks. I have read and been told that mogas will lose its octane level (starting at 93 octane) after about 30 days. I recently became aware of a fuel additive at - Fuel Fresh - that says it maintains the octane level and prevents plaqueing-out. Can anyone shed any light on this product and its efficacy in Rotax engines?

I'm guessing that one alternate course of action would be to partially drain the tanks and re-fill with fresh mogas or 100LL to boost the octane. Related questions about detonation: How much of an issue is it ? Will I even know its happening until the aftermath?

Thanks in advance.

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Re: Mogas Octane Loss

Postby 3Dreaming » Tue Feb 13, 2018 3:03 pm

Once detonation happens it is too late. I would drain some out and replenish with some fresh fuel, just to be on the safe side. I'm not sure about the additive.

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Re: Mogas Octane Loss

Postby rsteele » Tue Feb 13, 2018 3:53 pm

The additive doesn't restore octane, it keeps the volatiles from evaporating by forming a film on top of the fuel. It works well for lawn mowers. YMMV in an airplane. I'd go with mixing new fuel with old, and would use 100LL, just to be sure.

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Re: Mogas Octane Loss

Postby drseti » Tue Feb 13, 2018 8:31 pm

Everything in aviation is a tradeoff. 100LL has better shelf life, but the lead causes sludging in the gearbox and possibly sticking valves. Pick your poison.
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Re: Mogas Octane Loss

Postby WDD » Tue Feb 13, 2018 8:52 pm

1) Would not simple Sabil work just fine? Siphon most of your fuel out of the plane, put it in your car's tank. Put in fresh fuel in the plane, add the appropriate amount of Sabil. In the future when you think you'll be on the ground for a while, just treat the gas.

2) After you do that, remember that the treated fuel is in your tank, not your engine carbs. You'll have to run it up / idle for a while to bring the treated fuel into your carbs, otherwise, you risk varnish deposits.

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Re: Mogas Octane Loss

Postby drdehave » Wed Feb 14, 2018 7:35 pm

I assume you're speaking of Ethanol Mo-Gas and not Alcohol-Free Mo-Gas. In that case, definitely yes, after 4-6 weeks sitting, boost it up with at least 50% new Mo-Gas or 100LL. And drain the sump liberally, and put that gas someplace besides the airplane, before flight. I'm in CA, where's there's more "stuff" in our Ethanol than in other locations, and I could tell you numerous horror stories of phase-separated "gunk" that has come out of the sump after sitting a few weeks--that would have tried to kill me, if I let it.--Rich
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