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Compression ratio is the big thing mechanically wize, turbo's are the make shift compromise, E85 with the higher octane rating won't dentonate nearly as easy allowing for either higher compression ratio or higher boost pressures. Saab has been making the best E85 cars for 5-6 years, the timing, boost pressure and more vary depending on fuel. They make more HP & torque with E85 than regular. One article by Saab I found a few days ago, but can't again said they expect 15% better hwy FE, same city or combined.
In 2007 someone else did some testing of Saab's FFV car, HP numbers agreed with Saab, FE didn't in standardized testing, it was the normal roughly 20% less (almost same as my wife 02 Suburban that does 12.5 on E85 vs 15 E10 on her 4 mile commute, 83% of E10's mpg). Compared to an 2010 or 2011 Malibu's 33 hwy E0 vs 23 E85's is 69% of E0).
Energy density is one thing, higher compression I think increases efficiency so a engine capable of running higher compression can be more efficent. I want to see a E85 engine that can run E0 when neccessary.
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