Here's my experience with e85. I already had a very good tune on gas, and I was able to run 16psi and 16* timing on the twin-charged setup with zero KR. That seemed to be where I maxed out power. I could run more timing when the engine was cold, probably around 20*, but at operating temps, I had it dropped back to 16* to keep it safe.
I already had 65# injectors, a Racetronix fuel pump and rewire, adjustable FPR, and a fuel pressure sender tied into my scanner through the EGR. When my tank was just about empty, I drove to the station and filled up with e85. Before driving off, I changed my IFR table to reflect 25% more fuel. That was it. I drove around the next few days and monitored trims, and they were just about perfect. At the next fill, I adjusted the IFR a few percent richer. Trims still looked great, and I started taking a few WOT runs. First thing I noticed is that even in the 90+ degree heat I had zero KR, so I pushed up the timing and boost. Eventually, I was able to run 18-20psi with 18* of timing and zero KR consistently. I ran over 20psi a couple times, and even hit 23psi on one run, but I ran out of injector, and started going a little lean, and got a couple degrees of KR, so for now I'll keep it around 18psi until I can get bigger injectors. The difference is amazing. This car is stupid fast now.
I also raised cruise timing a lot. With e85, you can run a lot more timing across the board, and help improve the mpg's. However, I noticed that I would get a lot of blips of KR at cruise with higher timing, and the KR was random and unpredictable. I realized that the closed loop fuel oscillation was to blame, so I started running open loop full-time. Open loop made a huge improvement, and really made it easier to tune, but you really need to keep on top of the tune more because you won't have the trims to keep things in line from weather changes and ethanol content changes.
The last couple of weeks I have been running a 50/50 mix of 93 and e85. I wanted to try a mix to see how easy it would be to tune, and see how much power I could still have. Seems like it might be a good way to go for the winter also. Startups should be a lot easier on a mix. It was easy enough to tune. I adjusted the IFR and tweaked the MAF. I turned down the boost to around 16psi, but I haven't seen any KR, so it still makes good power.
As far as cost goes, I haven't really found that the increase in e85 fueling is that much more expensive. e85 is $2.39 around here, and premium (93) is usually about .50 to .60 more. So, e85 is 20-25% cheaper, and you use about 25% more fuel. Although, even if e85 was the same price as premium, I'd still buy it for the performance.
