E85 has a bad reputation among those who never tried it, and a deserved one when it’s poorly tuned. Here is what actually matters before you decide.
What E85 actually is
E85 is a fuel blend of 85% bioethanol in summer and around 65% in winter, the rest being SP95. In France, this ethanol mostly comes from locally grown beet and cereal crops, typically priced between €0.90 and €1 per litre at the pump, against €1.55 to €1.65 for SP98.
Two physical properties explain everything that follows: ethanol has a lower calorific value than petrol, and a lower flash point.
Why it takes more fuel
E85’s stoichiometric ratio requires roughly 30 to 33% more fuel by mass than SP98 to burn the same amount of air correctly. Converted to volume (ethanol being denser), the real injection increase needed is around 30%. That is the figure — 30% — that circulates everywhere, and that scares people off.
It only applies to the injection rate required, though — not to your actual day-to-day fuel consumption.
What you actually burn
At steady cruising speed, the air/fuel mixture runs lean: the observed extra consumption is then only about 6%, far from the theoretical 30%. The two moments where the gap really widens are cold starts (richer mixture needed, more so on E85) and full throttle (enrichment for engine protection). Over mixed use, the real extra fuel burnt is largely offset by the price difference at the pump.
The real challenge: cold starts
E85’s flash point shifts with its seasonal blend: around -7.25°C for the winter mix, +3.75°C for the summer mix. Below that temperature, the mixture ignites less easily — hence the sluggish cold starts in cold weather if the ECU isn’t tuned to compensate. It’s the point our clients raise most often, and the only one that genuinely needs careful tuning.
What actually gets retuned in the ECU
Switching to E85 without retuning means running permanently too lean — bad for performance, bad for the engine. A proper dual-map reworks:
- Injector sizing and response latency
- Part-load and full-load fuelling tables
- Transition tables between the two fuels
- Cold-start enrichment
- Ignition timing and knock tables
- Boost-pressure tables
- Coolant-temperature-based enrichment
Dual-map or Flex Fuel: two methods, not the same result
A generic « Flex Fuel » box works on an average: a sensor estimates the ethanol content and the ECU applies a setting interpolated between two extremes. It’s simple to fit, but it’s a compromise — the engine never runs exactly on the right setting, only on an average, with possible flat spots and hesitation.
The dual-map OPTI-MAP installs works differently: two complete, distinct maps, no averaging, with an event-triggered switch rather than a continuous estimate. Each fuel gets its own setting, built for it.
Our honest recommendation: for a remapped car with a power increase, a dedicated dual-map is the only setup we install ourselves. A generic Flex Fuel box remains a reasonable choice on a stock-power car.
Pricing for the Flex Fuel option and the dual-map is on the Pricing page. For practical questions (reliability, timing, legality), see the FAQ.