I would run the 91 tune and just make sure you purchase fuel from top tier stations. E85 does not have a true octane rating of 105. Generally it is 93 or lower. What it does have is a high latent heat of vaporization value which is where it derives its antiknock properties. Making the swap to E85 isnt always as easy as filling up the tank and getting a new tune. Depending on the current duty cycle of the fuel pump and injectors, you may need additional hardware. Also, I think it is safe to assume since you are in Colorado, E85 out there is subject to seasonal blending which each station may do differently. In the fall/winter months more gasoline is mixed to allow for easier starts and quicker warm up. Minor changes in the mix will have a big impact on your tune causing it to run rich or potentially very lean. This means constant testing before fill ups.
Having already done the fuel mixing thing for a couple years and been at a similar cross road I decided to stick with race fuel.
Great response, man, thanks! Since I've driven on strictly E85 in my daily driver in all 4 seasons the last year I agree with a lot of what you are saying. I not only tuned my car, but logged like a mad man, had it on the dyno a bunch of times, drag raced it, etc.
First, I believe E85 is >>93octane. Closer to 100. Either way, though, we agree that it does have fantastic anti-knock properties derived from more than just octane. I don't care to argue this point though since it doesn't really affect my decision here.
Next, I agree I'll need to swap out some hardware. Injectors for sure and I think we'll wait to see what the fuel pump duty cycles look like. Currently I've got 39# and the tuner I visited today said 42# would likely suffice.
As far as the seasonal blends go, I agree that it happens but at least here it doesn't happen to the degree that is commonly believed. That said, I've never once tested E85 with a fuel tester. But I watched my fuel trims closely in my previous car and they changed very little with the changing seasons. I expected big negative trims in the winter but they only went slightly negative. In theory E85 can vary between 85% and 70% ethanol, but I believe that most of it falls between 80% +/- 3%. The car is already built to handle a 10% deviation in ethanol (E10 gas in much of the U.S., sometimes changing with season), so even if E85 did vary the full 15%, the pcm has enough fuel trim authority to deal with it.
My main reasons are safety and low to mid range torque. I sincerely believe I would have blown my previous OEM engine several times if not for E85.
What I'm considering doing is converting it to E85, but leaving it de-tuned above 4500rpms or so. That way, at high rpms I'm not making really any more power, but I have a great degree more safety. At low and mid-rpms, add extra spark to get a lot more grunt.