Figure out exactly how much E85 and pump gas to add at the pump to hit your target ethanol blend. Works in gallons or liters.
Ethanol's effective octane rating (~113 AKI) lets turbocharged engines run more boost and timing without knock. But full E85 demands roughly 30% more fuel volume than gasoline, which maxes out stock pumps and injectors quickly. Blending captures most of the benefit at a fraction of the strain - which is why E30-E50 "blend tunes" are so popular on direct-injected platforms.
Fuel sold as "E85" is legally allowed to be anywhere from 51% to 83% ethanol, and stations blend seasonally - winter E85 often tests closer to E70 for cold-start performance. If you're mixing to a tight target without a flex fuel sensor, test the pump fuel with a cheap ethanol tester first, and enter the real number in the "Pump E85 Actual Content" field above.
Add the calculated volumes in either order - the tank mixes within a few miles of driving. The blend changes your air-fuel requirements: only run blends your tune supports, and if you're datalogging, watch fuel trims and HPFP pressure the first few pulls after a blend change. The AFR/Lambda calculator shows how stoichiometry shifts with ethanol content.
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