E85: E70, E74, E80...changing pump mixture a problem for tuning?

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How the hell do the flexfuel chevy's do it? I imagine they have a little module in the gas tank or fuel line that measures ethanol content and then adjusts a/f's for it...would it be possible to hook up something like this to AEM EMS and then just use something like the nitrous controls (or something more progressive, not just on/off) to add or remove fuel? Obviously that would be a lot more tuning time, but would be really cool if it would work!