That shot from the Pawnee ethanol STC again:
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So straight ethanol is OK. Straight 100LL is OK. But mixtures of ethanol and Avgas are limited to a max of 25% Avgas. Why would that be? Obviously, there must be a reason. Do our resident chemists have any idea?
If we go to the other STC, the one for the O-540 engine, we see the same thing:
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If we bought the STC from the owner, we would get Instructions for incorporating it. Those instructions are not posted in the STC database. I am certain that changes to hoses and seals would be required. Ethanol is just too destructive to the older polymers.
NBR (Acrylonitrile-butadiene rubber) is used, in similar compoundings, in the MS28775 and MS29513 O-rings. 28775 is commonly used for hydraulic fluids, 29513 for gasolines. If we go to a handy chart like this one, we can see what effects gasoline and ethanol have on NBR:
https://www.allorings.com/o-ring-compatibility
Let's see if I can chop the chart up and show the relevant sections:
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We see that NBR has poor chemical resistance to ethanol, good resistance to gasoline, and for gasohol (the mix of gasoline and ethanol) it is not recommended at all.
There are a half-dozen compounds shown there that would work for straight ethanol. They would have to be narrowed down to those with good temperature stability, resistance to other chemicals they might encounter (gasoline, of course, and maybe Varsol) and so on.