Can I use an ethanol mixture higher in my subaru sport impreza, than e 10, without converting the engine, and if not, is converting the engine to use E85 a good or bad thing?
The answer is in your owner’s manual. Don’t try and use anything higher than is listed in the manual.
You better stick with E-10 to keep your engine healthy. You could convert to E-85 but the cost would be astronomical.
No and no, it’s not worth it. It’s not just a matter of changing some minor components, the fuel system has to be redesigned to properly inject the correct amount of fuel, very complicated computer revisions to mix the correct amount of E0 through E85 (since any mixture can be in the tank).
Do you work in a brewery and have access to free ethanol? If not, this is an exercise in futility…
Bingo. The more ethanol you mix into your gasoline, the worse your mileage gets.
Personally, I don’t see the benefit of converting the engine to be able to use E-85. Ethanol, as a sustainable fuel source in the USA, is a failed experiment, at least currently. It takes too much energy to make. If we were able to make our ethanol out of sugar cane, like the Brazilians, things might be different.