Citroën C4X

Just the fuel switch: the ECU is adapted to run Superéthanol E85 (up to 85% bio-ethanol) with no Stage 1 chip on top. You get a small power bump from ethanol's higher octane (~104 RON) and a much cheaper fill-up. Fully reversible; the car still runs on regular petrol whenever you want.

Chip tuning re-optimised for Superéthanol E85: the ignition and boost maps are calibrated to make full use of ethanol's higher octane (~104 RON). Bigger power and torque gains than either Stage 1 on petrol or E85 adaptation alone, plus the same cheap fill-up.