Al Downs Posted April 4, 2014 Report Posted April 4, 2014 I am checking along my routed for Mogas and finding some have a blend anywhere from 89 to 91 octane. Is this useable or will it cause problems?
Anticept Posted April 4, 2014 Report Posted April 4, 2014 90 is the lowest you can go in a pinch, 91 is what the manual calls for. I use 93 and only get from big town fuel pumps because they seem to be the least variable.
Al Downs Posted April 4, 2014 Author Report Posted April 4, 2014 Can I had a few gallons of the 89 to the 93 left in the tank to get me to a place I can get higher octane or should I add a little 100ll?
Anticept Posted April 4, 2014 Report Posted April 4, 2014 You can mix. The problem is that hi-test (high octane) tends to sit for a while, and if it has ethanol in it, ethanol can precipitate separate out with moisture. Ethanol acts to stabilize the fuel at a higher performance number, so as you loose ethanol, you lose octane. Another reason why I pay the few extra cents for 93. The person who previously owned my CT once flew in to a gas station a ways north of OSU. They filled with hi-test, and they were having problems with the fuel. Never went back.
Ian Posted April 4, 2014 Report Posted April 4, 2014 Ethanol does not "precipitate" out of fuel! Ethanol does mix fully with water so fuels containing ethanol can absorb moisture without it being obvious to you - ie simply draining some fuel off will not show a water layer as it would if water had got into avgas!
Anticept Posted April 4, 2014 Report Posted April 4, 2014 Had to look up the definition of precipitate. Seems in chemistry, it's specifically the act of forming a solid from a liquid. I meant to refer to ethanol's ability to separate from the gasoline mixture when water is introduced (phase separation). Water suspended in the fuel isn't the part that is the issue; it's when it separates and you get layers of water/ethanol, and old fuel. It doesn't take much water to start separating an E10 blend. Around .05%! Yeah, that period and percent sign are correct.
Doug G. Posted April 4, 2014 Report Posted April 4, 2014 So 34 gal. usable will take .017 gal. of water before phase separation. Trying to imagine what two hundredths of a gallon is, two tablespoons? In 34 gallons, doesn't seem right.
Anticept Posted April 4, 2014 Report Posted April 4, 2014 .05% is when it starts becoming detectable at low temperatures. It becomes more and more cloudy. Get it cold enough and that will be it's saturation limit. Here's an EPA document worth reading. http://www.epa.gov/otaq/regs/fuels/rfg/waterphs.pdf At .5ish%, it will separate into layers. Here's another document that warns how water at the bottom of the tank can pull ethanol right out of the fuel, even with percentages less than .5%. http://nationalpetroleum.net/Ethanol-Water-Phase-Separation-facts.pdf
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