E85 Flex Fuel for Performance Cars Explained
E85 Flex Fuel Explained: What It Is, Why Performance Cars Run It
If you follow the performance car world, you’ve heard people talk about E85 like it’s magic fuel. The truth is more nuanced — E85 is genuinely powerful when used correctly, and genuinely useless when it’s not.
What Is E85?
E85 is a fuel blend of approximately 85% ethanol and 15% gasoline. Ethanol has a significantly higher octane rating than gasoline — premium pump gas is 93 octane in the US; E85 effectively rates around 105 octane. That’s not a small difference.
Why High-Octane Fuel Matters
Octane rating measures a fuel’s resistance to knock — premature detonation before the spark plug fires. Higher octane lets the tuner advance ignition timing further and run more boost. E85 also cools the intake charge as it evaporates, meaning denser air and more power.
What Kind of Power Gains Are We Talking?
Going from a 93 octane tune to E85 on a turbocharged engine typically adds 15–30% more power. A BMW S55 making 550 whp on 93 octane often crosses 650 whp on E85 — same car, same hardware. That’s the fuel doing that.
The Flex Fuel Option: Best of Both Worlds
A flex fuel kit adds an ethanol content sensor. The ECU reads whatever blend is in the tank and adjusts in real time. Fill up with E85 for full power; use 93 octane when that’s all that’s available. No retuning required.
What the Tune Needs to Change for E85
E85 requires 30–40% more fuel by volume. The tune must increase injector duty cycle, raise boost safely, advance timing significantly, and adjust fuel trims for ethanol’s different stoichiometric ratio (9:1 vs 14.7:1 for gasoline). Running E85 on a 93-octane tune will cause a lean condition. Don’t do this.
Want to run E85 on your performance car? ECMTuner in Moonachie, NJ builds flex fuel tunes for BMW, Mercedes, Porsche, and Aston Martin.