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Presuming that the e85 is really e85, not seasonally blended down to e70 or sometimes even lower, as is often the case here in the US, and that the pump is e10, again not seasonally blended
In a speed6, with a 16 gallon tank, I would not exceed 2 gallons of e85, and really less is better- one gallon of e is enough to feel things smooth out and to troubleshoot KR issues, 1.5 gallons, or if really needed up to 2 gallons of e85 for more additional troubleshooting but not for daily driving.
considering 16 gallons and true e85 & e10
p - e = %
16 - 0 = 10%
15 - 1 = 15%
14 - 2 = 19%
13 - 3 = 24%
Due to the extra O2 that e85 brings to the party once you reach 2+ gallons you need to tune for the specific ratio being targeted- that is, you need to tell the ECU what the new Fuel Specific Gravity (FSG) is so that Stoich can be properly maintained. Without making that adjustment to the fuel tables the car will run lean.
Since ethanol, simply as an additive, does not make power, rather you actually lose power, the only common reason for running it with an untuned ECU is for troubleshooting knock, typically to verify if really an ignition issue or an actual hardware noise issue seen as knock by the ECU- with e the symptom will clear up if ignition issue, but if hardware there will be no change to symptom.
Hope that helps.
Also- isn't this just a refresh from the same conversation two years ago?
Sold 7/2016- Mazdaspeed6 GT #3476 TiGray slicktop, fully bolted & badass
135i - E82.N54 2009 BSM - MHD S1+ / xHP S3 / PR CP & IC / RB PCV / Borla
. . . . RSFI, Koni & Eibach, Dinan CP's & M3 CA's / EBC Red's & Firehawk 500's
X1 - E84.N20 2013 MGM - JB4 - The Wife's