2018-2021 Subaru WRX STi 2.5L E85 Flex Fuel Conversion Kits
2018-2021 Subaru WRX STi 2.5L E85 Flex Fuel Conversion Kits
eFlexPlus
Guaranteed E85 Performance, No Tune Required
In 2015, Subaru decided to focus their Rally Car efforts on the WRX STi sedan and we are here for it. The 4th and current generation of WRX STi still gets its speed from the 2.5L turbocharged 4-cylinder, capable of 310 horsepower but its suspension got a major overhaul. Reminiscent of the 2000-2007 Impreza WRX STi, the current generation has a much tighter turn with quicker steering and super sharp reflexes for sailing through the curves. Boost the horsepower by up to 20% with one of our WRX STi E85 flex fuel conversion kits and you have a real track contender.
- Drive on E85, Gasoline, or any mixture of the two
- Fully Automatic: No tuning, changing maps, or adjustments required
- “Under-the-hood” installation that can be completed in an afternoon that doesn’t touch your factory ECU, OBD, or emissions system
- Fully reversable: No soldering or permanent modification required
- Will not trigger a Check Engine Light (CEL) or Malfunction Indicator Lamp (MIL)
- Use our smartphone eFlexApp to choose between Eco, Dynamic, & Sport Driving Modes, adjust cold-start fueling, monitor Ethanol content, Duty Cycle, and more
- We verify your vehicle’s E85 compatibility, including fuel injectors, fuel pump, fuel lines, and any modifications done
- Everything needed to DIY install & run E85 is included
- Make more power, lower operating temperatures, and reduce emissions on E85!
The eFlexPlus E85 flex fuel kit automatically adjusts fueling, whether you have E85, gasoline, or a mixture of the two in your tank. eFlexPlus is the easiest way to properly convert your vehicle to a true flex fuel, E85-and-gasoline capable vehicle. Looking for greater ethanol fueling capabilities? Check out our patented eFlexPro system via the drop-down to learn more.
+ Have an aftermarket wideband?
If YES, we strongly recommend our eFlexPro system. Your aftermarket wideband can be connected directly into the eFlexPro harness to fine-tune fueling and dial in ethanol AFR’s with our free eFlexTuner Windows software. Select the eFlexPro drop-down to learn more.
+ Vehicles with larger injectors or big power-adders
Vehicles with upgraded injectors and/or big power adders (aftermarket forced induction, cams, etc.) are recommended to go with our eflexpro kit for detailed fine-tuning and fueling capabilities.
Starting at $61.50 / month with 
SKU: U-EF-PLUS - Free shipping
- Delivery time 2-5 days
- 5 year warranty with registration
Installation
Plug&Play installation with some modifications - An experienced mechanic will install in 2-3h. For first time DIY installation, expect to double the time needed.
Technical specs
| Make & Model | Subaru WRX |
| Years | 2018-2021 |
| Engine | 2.5l / EJ257 |
| Power | 310 hp |
Always Optimized Fully Automatic Flex Fuel
Cooler & Cleaner Engine
Complete System, No Tuning Needed
Up to 10% Increase in Performance 
Your Fuel System Verified+ Why use E85?
- PERFORMANCE: E85 is the best bang-for-buck performance upgrade you can make without compromising your vehicle’s ECU programming. Depending on application, max HP gains can be up to 20%, though the real gains are felt throughout the entire powerband.
- SAVINGS: E85 is lower cost than gasoline. While your MPG may be impacted when running E85, your cost per mile will end up cheaper with this locally produced fuel.
- ENVIRONMENT: Modern day E85 is significantly cleaner and lower carbon than gasoline. Using E85 instead of gasoline will considerably reduce your carbon footprint and help protect the environment.
+ How is vehicle compatibility confirmed?
We confirm full compatibility with eFlexFuel systems and E85 for all kits that we offer. If your vehicle is approved and a kit is available, this means the injectors, fuel pump and fuel lines will be compatible with E85 once an eFlexFuel kit is installed. With over 65,000 eFlexFuel customers worldwide, we pride ourselves on our E85 and flex-fuel expertise.
+ Where does the increased horsepower, torque, and performance come from?
- E85 is approximately 30% oxygen by weight, while normal gasoline is anywhere from 0-3.5% oxygen. Greater fuel oxygenation means more fuel and more complete combustion, leading to more power.
- Ethanol absorbs heat during vaporization, leading to reduced air temperatures when the air-fuel mixture ignites. This charge cooling effect creates a modest power increase while supporting ethanol’s knock resistance.
- The higher chemical octane of E85 reduces the risk of knock. When combined with ethanol’s charge cooling effect, E85 is highly knock resistant, with virtually zero knock under normal operating conditions; the same cannot be said for gasoline. As a result, the vehicle’s ECU does not need to retard ignition (pull timing) while running E85.
- Ethanol burn properties affect cylinder pressure, increasing power:
- There is less ignition delay (from spark to burning) with ethanol.
- Ethanol’s flame front propagation is faster.
- These burn properties result in cylinder pressure rising faster, increasing efficiency and power.
- Turbocharged engines also benefit from higher exhaust gas flow from E85, resulting in quicker turbo spool.
DIY Virtual Load Sensor Tuning
Easily & safely create custom ethanol fuel maps based on your own vehicle's performance & modifications, without tapping into external load sensors - all from your laptop. Now you can augment your Ethanol fueling yourself, without touching your factory ECU and without jeopardizing engine safety.
Experience the VelvetTouch+
VelvetTouch+ is the first-class signal processing technology developed by eFlexFuel. This technology ensures that the eFlexFuel system works in any engine, environment and conditions without interfering in any way with the operation of the vehicle. It also ensures that there is no signal interference that could disrupt vehicle systems. VelvetTouch+ allows you to convert your vehicle to E85 in complete safety. Whether you drive on ethanol or gasoline.
Supercar Technology in your Daily Driver (or Weekend Warrior)
By installing the eFlexFuel kit, you're in good company. eFlexFuel conversion kits utilize technology found in the Koenigsegg Jesko, Dodge Challenger SRT Demon 170, and other top-tier performance vehicles. This tech is used to get the most out of E85, Gasoline, or whatever blend of the two you have in your tank.
Industry Leading, Free 5-Year Warranty (With product registration)
Nothing says, "We stand behind our product," like our free 5-Year warranty. We know what our kits are made of and we aren't afraid to put our money where our mouth is. You will not find a warranty like this anywhere else.
Expertly Engineered, Designed & Manufactured in Finland
Expertly Engineered, Designed & Manufactured in Finland. We have developed our hardware & software in-house in Pirkkala, Finland, since day one. Before shipping, we confirm every single control unit's functionality before shipping.
Technical Specs & Features
- Precise Ethanol Content real-time measured over 100 times per second
- Advanced signal processing filtering system in all input signals
- Shock-Proof & Weather proof casing
- High Quality Wiring Harness with snap in connectors (no soldering required)
- Ethanol Tested & Approved fuel accessories (line, clamps, quick-connectors)
- Bluetooth 5.2
eFlexApp Main Features
Dynamic Gauge Screen
with Key Information
Choose Your Driving Mode
Dyno Proven Power Gain
Real-time Analog Data
for Sensor Inputs
Log History
with Real-Time Parameters
Please confirm device versions features at eFlexApp page
eFlexTuner Main Features
3D Fuel Maps
With eFlexFuel's Pro models, fuel maps can be observed in the 3D graphics window. The view can be rotated freely and selected fuel map cells are highlighted over the 3D map. The color-coded fuel map makes analysis simple.
Data Logging
All running parameters of the control unit can be captured and saved to a log file. Choose which parameters to display including date range. eFlexPro and eFlexMoto Pro log files can be analyzed in-depth with the eFlexTuner. Color-coding helps distinguish data points. (For example, the fuel mixture or the injector duty cycle.)
Log File Analysis
eFlexPro and eFlexMoto Pro log files can be analyzed in depth with the eFlexTuner. Data can be taken from an entire session or a specific timespan. The data is color-coded, which helps to distinguish e.g., the fuel mixture or the injector duty cycle.
Please confirm device versions features at eFlexTuner page
+ Compare products
 |  | |
|---|---|---|
| Features | ||
| VelvetTouch+ | ||
| Free 5-Year Warranty (with registration) | ||
| Ethanol Content real-time measured over 100 times per second | ||
| Advanced signal processing filtering system in all input signals | ||
| Shock-Proof & Weather Proof casing | ||
| High Quality Wiring Harness with snap in connectors (no soldering required) | ||
| Ethanol Tested & Approved fuel accessories (line, clamps, quick-connectors) | ||
| Bluetooth 5.2 | ||
| Analog Wires for External Sensors | ||
| eFlexApp | ||
| Ethanol content | ||
| Engine RPM | ||
| Duty cycle | ||
| Fuel temperature | ||
| Ambient engine temperature | ||
| Injector activity | ||
| Cold enrichment | ||
| Device status | ||
| Driving mode selection | ||
| Cold enrichment custom setting (created with eFlexTuner) | ||
| Save RPM based enrichment with driving mode selection (created with eFlexTuner) | 1 | 3 |
| eFlexPro view for analog data | ||
| eFlexTuner | ||
| RPM. based 2D fuel map | ||
| Fault codes | ||
| Data logging | ||
| Fuel map adjustment | ||
| Log file analysis | ||
| 3D fuel maps | ||
| Sensor linkage | ||
| Cold start adjustment | ||
| Virtual Load Sensor Tuning |
+ What's included
+ Installation
+ Returns
+ Warranty
Testimonial
Installation
What's included in the eFlexPro E85 flex fuel conversion kit
Customer Comments
Note: If your car is modified you can use E85 with eFlexFuel. If this is the case, please list all the main engine modifications on the checkout page. Our technical team will ensure that your setup is fit for E85.
* Power and torque increase depends on your engine technology and/or your custom built of the engine.
Not legal for sale or use on pollution-controlled vehicles