E85 explained

E85 is ethanol-heavy fuel — nominally 85% ethanol, 15% petrol. Tuners love it because it behaves like cheap race fuel; your fuel system hates it because you need to burn a third more of it. Both facts come from the same chemistry.

Why it makes power

• Very high effective octane (~104–108) — it resists knock, so boosted engines can run more boost and more timing. This is where the real power comes from.
• Charge cooling — ethanol's latent heat of vaporisation is much higher than petrol's. Evaporating fuel chills the intake charge, making it denser and further suppressing knock.
• Cooler combustion — exhaust gas temps drop, easing thermal load on pistons, valves and turbos.

The catch: you burn ~30% more

Ethanol carries less energy per litre than petrol, so stoichiometric AFR falls from 14.7:1 to about 9.8:1 for E85. The engine needs roughly 30–35% more fuel volume for the same air — which cascades through the whole fuel system:

• Injectors — typically one size up (see the injector guide; BSFC on E85 ≈ 0.65–0.75).
• Fuel pump — must flow ~30% more; many stock pumps run out on E85 before the injectors do.
• Lines and materials — ethanol is corrosive to some rubbers and metals and hygroscopic (absorbs water). Modern fuel-injection hardware is usually fine; older cars need checking.

Pump "E85" is rarely E85

Blender pumps dispense anything from about E51 to E83 depending on season and region (winter blends carry more petrol for cold starts). That variability is why serious setups run a flex-fuel sensor that measures actual ethanol content live and lets the ECU scale fuel and timing — and why the rest of us test the pump and blend to a known target.

Blending to a target

You don't need full E85 to get most of the benefit — E30 to E50 already unlocks meaningful knock headroom on many turbo cars. Mixing is linear by volume:

fuel A needed = tank × (target% − fuel B%) ÷ (fuel A% − fuel B%)

Example: to turn 40 L of E10 pump fuel into E50 using E85, you need 40 × (50−10) ÷ (85−10) ≈ 21.3 L of E85 (and 18.7 L of the E10). The calculator above does this in both directions and shows the blended stoichiometric AFR to aim at.

Is it worth it?

For a tuned boosted car with the fuel system to support it: usually yes — meaningful, reliable power per unit cost. For a stock NA car: the octane is wasted, fuel economy drops ~25–30%, and range shrinks. Match the fuel to the build, not the forum hype.