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2015-2017 Subaru WRX STi 2.5L E85 Flex Fuel Conversion Kits

Boost your HP by up to 20% With our Plug-and-Play E85 Kits

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.

No tuning required!


Starting at $61.50 / month with Affirm logo

Truspilot consumer rating 4.7 SKU: U-EF-PLUS
  • Free shipping
  • Delivery time 2-5 days
  • 5 year warranty with registration


The eFlexPlus and the eFlexPro are so easy to install that you can do it yourself in an afternoon and have your car running any mixture of E85 flex-fuel and standard gasoline the same day. Both come with a handy phone app that allows you to monitor the fuel mixture and switch between efficiency and performance profiles, effortlessly. The eFlexPro comes with the additional eFlexTuning software that allows you to tune your turbocharged engine on any mixture of fuel above E50. Order one of these for your 4th generation Subaru WRX STi today and increase your horsepower by up to 20%. Your HP can go from 301 to 370 ponies. That’s not bad for a couple of hours of “work”.


DIY level

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 & ModelSubaru WRX
Engine2.5l / EJ257
Power301 hp
Fully Automatic Flex FuelAlways Optimized Fully Automatic Flex Fuel
Cooler & Cleaner EngineCooler & Cleaner Engine
No Tuning NeededComplete System, No Tuning Needed
Increase PerformanceUp to 10% Increase in Performance
Flex Fuel System VerifiedYour Fuel System Verified

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+

All eFlexFuel devices provide smoother and more seamless drivability than any other flex fuel system in existence. No small feat, our R&D team accomplished this with unrivaled attention to detail, countless long days & late nights, and a unique eye for optimization. The result of this "Velvet Touch" development? Our class-defining eFlexPro 2 & eFlexPlus.

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

More eFlexApp Features

Please confirm device versions features at eFlexApp page

eFlexTuner Main Features

More eFlexTuner Features

Please confirm device versions features at eFlexTuner page

+ Compare products

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
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)13
eFlexPro view for analog data
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



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.

Prop 65 Warning
Warning signNot legal for sale or use on pollution-controlled vehicles

Want $50 off your eFlex kit?

Easy! Sign up for the latest E85 and eFlexFuel News, Giveaways and Specials, and we’ll hook you up with a $50 discount code.

By providing your phone number, you agree to receive promotional and marketing messages, notifications, and customer tservice communications from eFlexFuel. Message and data rates may apply. Consent is not a condition of purchase. Message frequency varies. Text HELP for help. Text STOP to cancel. See terms.