E85 tuning — the cheap race fuel hiding at servos

E85 — roughly 85% ethanol, 15% petrol — has an effective octane rating north of 105. That means you can run more boost and more timing before knock, which is tuner-speak for 'free horsepower sitting at a bowser near you'.

Why ethanol makes power

Two things. First, the octane: more timing and boost before the engine starts fighting you. Second, ethanol cools the intake charge as it evaporates, which helps turbo cars in particular — it's a poor man's intercooler upgrade and fuel upgrade rolled into one.

On a forced-induction car with a flex-fuel setup we typically see 10–15% more power over a 98 tune, and the torque comes in earlier. On a WRX or an S15 that's 30–40kW you can't get any other way for the money.

What it costs at the pump

Here's the honest catch: E85 has less energy per litre. You'll burn 20–30% more of it, which mostly cancels the slightly cheaper pump price. You're not doing it for fuel economy — you're doing it for power, and it happens to not cost more per kilometre than 98.

Cold starts in winter can also get grumpy on high ethanol content. A good tune manages it, but it's a real thing in July.

What your car needs

• A flex-fuel sensor — reads the actual ethanol content in the tank and feeds it to the ECU
• Fuel system headroom — you're flowing ~30% more fuel, so pump and injectors need capacity. Some cars have it stock, some need upgrades
• A proper flex tune — the ECU blends timing and fueling maps based on what the sensor sees. Fill with E85, 98, or a mix — the tune handles all of it

Total for a typical flex conversion: sensor + fitting + custom tune lands between $1,400 and $2,200 depending on whether your fuel system keeps up.

Who shouldn't bother

Naturally aspirated cars gain little — ethanol's magic is boost and timing, and an NA engine can't use much more of either. If your nearest E85 bowser is a 40-minute detour, think hard about whether you'll actually fill up there. And on a diesel it's simply not a thing.

For everyone else — especially a turbo car that's already Stage 1 or 2 — it's the best-value gain left on the table. Bring the car in, we'll check your fuel system headroom on a dyno session and tell you exactly what it'd make before you commit.