BMW S55 · Flex Fuel
BMW S55 Flex Fuel Tune
M3 · M4 · M2 Competition · M2 CS
An S55 flex fuel tune uses a fuel-composition sensor to tell the DME how much ethanol is actually in the tank, then interpolates between a conservative pump-fuel calibration and an aggressive ethanol one. Instead of committing to a single blend, the car adapts: fill with 93 and it runs the pump map, fill with E60 and it moves toward the ethanol targets. The trade is hardware and setup complexity for not having to mix precisely.
- Platform
- S55
- Sold as
- Custom S55 flex-fuel calibration
- Base calibration
- Custom quote
- Revisions
- Log-based, included
S55 Flex Fuel, quoted per car.
There is no catalogue price for S55 flex fuel, so it is quoted per car from your hardware, fuel and logs. What the car needs first is listed further down.
Send the car, its hardware and the fuel you run, and a tuner quotes the flex fuel calibration.
Talk to a tunerWhich cars it applies to.
F80 M3, F82/F83 M4 and F87 M2 Competition cars fitted with a flex-fuel content sensor and the wiring to read it. Without the sensor, a fixed E30 map is the correct path.
- S552015-2021M3, M4, M2 Competition, M2 CSChassis and years
What the calibration changes.
- 01
Two calibrated endpoints, then interpolation between them
The map is built as a conservative pump-fuel end and an optimised high-ethanol end, with boost, ignition and fuel targets interpolated against measured content. Both ends are calibrated on real logs — an interpolated map is only as good as the two endpoints it is drawn between.
- 02
Ignition scheduled against measured content
Advance follows the sensor rather than an assumption. As content rises the map moves toward its ethanol timing; as it falls it retreats. Knock correction on your car remains the check on both ends.
- 03
Fuel demand scaled across the range
Fuel mass demand climbs with ethanol content, so rail-pressure targets scale with it. Boost targets at the high end are written under what your fuel system sustains there, not under what it manages on pump fuel.
- 04
Sensor sanity and failure behaviour
A content sensor can read wrong or drop out. The calibration is set up so an implausible or missing reading falls back toward the safe pump-fuel end rather than leaving the car running ethanol timing on gasoline.
What your car needs first.
- A flex-fuel content sensor correctly plumbed and wired, reading in your logs before we start
- Healthy high-pressure fuel delivery on both banks across the whole blend range
- Fresh plugs suited to the highest-ethanol targets, not the pump-fuel ones
- Consistent access to E85 if the upper end of the range is the point
- Logs at both ends of the blend range, not just on the fuel you like
What to log.
- RPM and vehicle speed, so every other channel can be read against load
- Boost target vs actual boost (or absolute manifold pressure), full pull
- Wastegate duty cycle, and actual wastegate position where the platform exposes it
- Ignition timing per cylinder and any knock / ignition-correction channel
- Intake air temperature and coolant temperature
- Ethanol content as reported by the sensor, on every log
- Fuel rail pressure, target vs actual, on both banks — particularly at the high-ethanol end
- Knock or ignition correction per cylinder
- Boost and wastegate channels for both banks
Reading what comes back: the BMW data logging guide goes channel by channel, and the wastegate duty cycle guide covers boost control.
Flash, log, review, revise.
The first file is conservative. Your logs decide every revision after it, and log-based revisions are included in the calibration price.
- 01
Submit your build
Chassis, engine, full mod list and target fuel. This defines a safe calibration starting point before any file is touched.
- 02
Receive a base map
A conservative calibration matched to your hardware and fuel, flashed with your own software at home.
- 03
Log and send back
Full-throttle pulls and part-throttle data captured on our channel list, then sent over for review.
- 04
Review and revise
We read the logs, adjust the calibration and repeat until the car is running clean and on target.
S55 Flex Fuel questions.
More answers- What hardware does an S55 flex fuel tune need?
- A fuel-composition sensor plumbed into the fuel supply and wired so the DME or your tuning platform can read it. That reading has to appear in your logs before any calibration work starts — a flex-fuel map without a trustworthy content signal is just an ethanol map with extra steps.
- Is flex fuel better than a fixed E30 map?
- It is more convenient and more tolerant, not automatically more powerful. At any given blend a dedicated fixed map can be optimised slightly harder because it does not have to remain safe across a range. Flex fuel wins when your fuel actually varies — different stations, travel, seasonal content swings.
- What happens if I fill up with 93 octane on a flex fuel tune?
- That is the point of it. The sensor reads the lower content and the calibration moves back toward the pump-fuel targets automatically. Give it a few minutes of running for the new fuel to reach the sensor before you go looking for full power.
- Can you set up flex fuel remotely?
- Yes. The install is yours to do or have done locally; the calibration is ours. We need logs that show the sensor reading correctly, and then logs at both a low and a high blend so both endpoints are calibrated on your car rather than assumed.
Built around your S55.
Chassis, model year, hardware and the fuel you actually run is all we need. If you already have logs, include them: it usually saves a revision round.