Fuel quality is one of the most important factors in the performance and safety of a tuned engine. A Stage 1 calibration is typically developed for a specific fuel grade — and running lower-quality fuel can trigger knock, reduce performance and potentially cause engine damage.
For petrol BMWs: Stage 1 calibrations are typically developed for 98 RON (Super Plus / V-Power). Some calibrations are available for 95 RON but with reduced gains. Never run 91 RON on a tuned European-market BMW.
For diesel BMWs: standard diesel fuel is sufficient, but premium diesel (higher cetane rating) can improve combustion quality and reduce particulate generation. This is particularly relevant for DPF-equipped vehicles.
E85 and ethanol blends: some high-performance calibrations (particularly N54 and S55) are developed for E85 or E30 blends. These require specific fuelling hardware and should only be used when the calibration explicitly supports them.