Quote Originally Posted by FlyingRon View Post
The problem isn't so much the energy density as the fact that ethanol has some nasty reactive propeties part of which is because it's hydroscopic and part is that fuel systems weren't designed to encounter it. This has been a big concern in the boating industry where WATER is a more invasive problem. The sad truth is aviation fuel tends to sit in tanks a lot (avgas is specifically stabilized) which car gas is not and it gives the ethanol time to attack the tanks, etc...
This is not entirely accurate. Water is a problem for boats and aircraft due to the open-vented fuel systems used, not because they are operated near water. Modern cars, by contrast, used closed, pressurized fuel systems so there is less water absorbed from the atmosphere. Statistically, private boats and airplanes are operated about the same annually, around 50 hours, the rest of the time they sit in a hangar, shed, etc. If fuel remains in the system, and it contains ethanol, it will eventually reach saturation and phase separation occurs. The resulting ethanol/water mixture sinks to the bottom of fuel tanks, lines, pumps, carbs, etc. and starts eating into the surrounding surfaces.

Quality gasoline without ethanol (aka Mogas) has a stable shelf-life of about one year. Avgas shelf life is 1-2 years. Put ethanol in gasoline, or in 100LL for that matter, and the shelf life drops to a few weeks.