Originally Posted by hotrod
Yes with a 50/50 mix you would need to add about 10% -12% more fuel to get the same effective mixture as gasoline only.
The higher octane of the E85 might allow you to get away with the same tune but you would be leaner than your tune on gas. Only way to know would be to work up in small increments and see how the car likes it.
Even a 20%-30% mix of E85 makes a noticable difference.
I would expect a Subaru ECU to handle a 50/50 mix quite well and to automatically add the necessary extra fuel. The ECU would quickly recognize that the new fuel has a different stoichiometric ratio and would adjust the long term fuel trim (A/F Learning) to compensate. The entries in the ECU's open loop fuel table are target equivalence ratios, so I don't see where the problem is, unless the fuel injectors are already nearly max'd.