E85 conversion kit or ECU remap: which solution is right for your car?
Are you considering switching to E85 superethanol and hesitating between two solutions: the conversion kit (Flexfuel, eFlexFuel, Biomotors) or engine remapping? This is one of the most common questions we receive. Both approaches allow you to run on E85 and significantly reduce your fuel budget, but they don't work in the same way and aren't suited to the same profiles.
At RS-TRONIC, our network of over 80 specialised dealers offers both solutions. This article gives you an objective comparison, with no commercial bias, to help you make the choice best suited to your vehicle and your expectations.
How does each solution work?
The E85 ethanol conversion kit
A conversion kit (also known as an ethanol kit) is an electronic module that sits between the original ECU and the engine's injectors. Its role: to correct the amount of injected fuel in real time to compensate for the difference in energy density between petrol and E85.
In practical terms, the original ECU continues to manage the engine as if the fuel being used was standard petrol. The kit intercepts the signal and adjusts it so that the engine actually receives the right amount of ethanol. It can be fitted with an ethanol content sensor to adjust the dosage dynamically.
E85 conversion kit: pros and cons
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Engine remapping (chip tuning) for E85 ethanol
Remapping your car for ethanol consists of extracting the original mapping from the engine control unit (ECU), then modifying it to adapt the operating parameters for E85 ethanol use: injection curve, ignition timing, air-fuel ratio, and lambda sensor management.
This is an intervention performed directly on the original ECU, with no external component added. At RS-TRONIC, this mapping is developed and certified by our engineers, then installed during your appointment at the dealer.
E85 ethanol remapping: pros and cons
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Quick comparison: conversion kit vs remapping
| Criterion | Conversion kit | Engine remapping |
|---|---|---|
| Principle | Electronic kit added between ECU and injectors | Direct modification of the original ECU mapping |
| Injection compatibility | Indirect injection only (for most models) | Direct and indirect injection — broader scope |
| Customisation | Standardised settings per model | Custom mapping tailored to the vehicle and its specifics |
| Power gain | Neutral (objective: E85 compatibility) | Possible by combining E85 and Stage 1 optimisation |
| Reversibility | Physical removal of the kit possible | Return to original configuration included at RS-TRONIC (5 years) |
| Vehicle registration | Mandatory (change of energy source) | Mandatory (notable modification) |
| Indicative price | From €859 (kit + installation) | From €490 incl. VAT at RS-TRONIC |
Which solution should you choose for your profile?
The conversion kit is suitable if...
Your vehicle has indirect injection and compatibility is confirmed
You want a physically reversible solution with no intervention on the ECU
You have no performance gain objective, only access to E85
Remapping is preferable if...
Your vehicle has direct injection (GDI, TSI, TFSI, etc.)
You want to combine E85 conversion and performance optimisation
You prefer an intervention directly in the original ECU, with no additional component
You want a certified, custom mapping with a guaranteed return to original
In both cases, all RS-TRONIC dealers offer both services. When you book your appointment, the technician analyses your vehicle and recommends the most suitable solution.
E85 conversion with RS-TRONIC: what's included
RS-TRONIC is a network of over 80 dealers specialised in engine remapping and E85 conversion throughout France. Each service includes:
Vehicle analysis and compatibility check before intervention
E85 remapping from €490 incl. VAT, E85+ (with Stage 1) from €590 incl. VAT
Certified mapping, developed by RS-TRONIC engine engineers
Free return to original configuration guarantee for 5 years
Workshop appointment, with no excessive waiting time
FAQ: your frequently asked questions
Entry-level non-approved kits may seem cheaper to buy, but approved kits with installation commonly reach €600 to €1,200. E85 remapping at RS-TRONIC starts from €490 incl. VAT, with a broader scope of service.
No, the two solutions cannot be combined on the same vehicle. You must choose one or the other. Remapping completely replaces the kit's logic since it acts directly on the ECU.
These three brands are the main players in the approved kit market in France. Flexfuel Company and Biomotors have the most extensive networks of certified installers. eFlexFuel is recognised for the compactness of its kits and its mobile app.
The choice mainly depends on compatibility with your vehicle (each brand publishes a compatibility list) and your installer's recommendations. At RS-TRONIC, the technician guides you towards the best choice for your vehicle.
The best way to know the savings you can achieve through E85 conversion or ethanol remapping is to run the simulation directly on our dedicated page.
No, unless your vehicle is natively FlexFuel from the factory (some Ford and Volvo models, a few Renault and Peugeot models). For the vast majority of petrol vehicles on European roads, an adaptation is required: either an approved kit, or a remap. Using E85 without adaptation on an engine not designed for it exposes you to risks of malfunction.
Engine remapping constitutes a notable modification under article R321-16 of the French Highway Code. It involves mandatory administrative procedures: declaration of modification within the month following the intervention, along with vehicle registration update. In France, this procedure is carried out directly online on the ANTS website. In other European countries, similar procedures apply through the local registration authorities.
RS-TRONIC, through its dealer network, supports its customers in understanding these regulatory obligations, in accordance with the brand's general terms and conditions of sale.
The conversion kit aims solely at E85 compatibility: no power gain is expected, and that's its purpose. Remapping, on the other hand, can simultaneously integrate E85 management and optimisation of engine parameters.
At RS-TRONIC, the E85+ package (from €590 incl. VAT) combines ethanol mapping and Stage 1, to combine fuel economy with improved torque and power.
Ethanol has a higher heat of vaporisation than petrol, which can make cold starts more demanding on a poorly adapted engine. At RS-TRONIC, ethanol mappings are developed to operate optimally in all seasons.
One point to watch, however: in winter, even with optimal tuning, low temperatures combined with the variable quality of E85 from one station to another can slightly extend the start time by a few seconds. To remedy this, simply add a few litres of unleaded petrol (95 or 98 octane) to the tank, which facilitates cold starting without compromising the benefits of ethanol the rest of the time.
The condition of the ignition system (spark plugs, coils) must also be good to ensure reliable starts with E85, regardless of the season.
Both solutions are likely to impact the manufacturer's warranty. The manufacturer may refuse to intervene under warranty on a modified vehicle, even after returning to original configuration. This is a reality to take into account regardless of the solution chosen.
RS-TRONIC covers its own service: free return to original configuration for 5 years.
Conclusion
Conversion kits and ethanol remapping are two valid solutions for running on E85. Remapping offers a broader compatibility range, an intervention with no physical modification of the vehicle, and the possibility of combining fuel savings with performance gains.
The conversion kit remains a relevant option for those looking for a physically removable solution.
At RS-TRONIC, both solutions are available throughout the network. The right choice is made on a case-by-case basis, depending on the customer's preference, vehicle, injection type and objectives.
Book an appointment with your nearest dealer for a free eligibility analysis.