E85 Flex Fuel gas pump

Flex Fuel for the WRX? Ethanol for big power gains.

Where do you go when your WRX is already stage 2 or stage 3 and you want more power? Of course, there's always the option of a built motor and bigger turbo, but there's a lot more power to be had before going to that extreme. How? With Flex Fuel of course! Flex Fuel is a blend of gasoline and ethanol, sometimes referred to as E85, meaning 85% ethanol content. Running on E85 or some other ethanol blend is a great way to add power to any turbocharged car.

How Do I Run My WRX on Flex Fuel?

With the 2015+ Subaru WRX, there are a few different options if you want to run your car on Flex Fuel. You can have a tuner, either through an e-tune or on a dyno, make you a tune for E85 or some other ethanol blend.

This is less than ideal, however since it means you'll need to mix gasoline and Flex Fuel to the correct blend yourself in order to run the tune safely. The ethanol content of Flex Fuel at the pump can vary, so this can require testing the ethanol content of the fuel and calculating how much gasoline should be added.

Unfortunately, since Cobb's Project Green Speed update, you'll no longer be able to tune Flex Fuel kit with Cobb's software. Alternative tuning solutions do exist, but the easiest option for the 2015+ WRX is now using multiple tunes for different levels of ethanol content. For a true Flex Fuel setup, you'll either need to flash the ECU with an alternative solution or upgrade to an aftermarket ECU. See our full guide on the update for everything you should know.

WRX Flex Fuel Kits

We're going to be discussing the various options for Flex Fuel kits for the WRX. These kits include an ethanol sensor and connect to your ECU. With this installed, your tuner can make a custom tune for your car that adjusts based on the measured ethanol content. You won't ever have to worry about the exact percentage of ethanol that's in your fuel since the car will be tuned to adjust for anything.

Cobb is one of the biggest names in the Subaru tuning community, with their AccessPort as a must-have who anyone who wants to modify their car. Cobb Tuning was founded in Rockwell, Texas in 1999, and all of their engine management products are made for turbocharged cars.

Running the car on Flex Fuel, Cobb saw gains of around 33 horsepower in their testing running with 74% ethanol. The sensor adds a monitor to the AcessPort, so you can see your fuel's ethanol content in real-time. Your car will need custom tuning to run Flex Fuel after installing this kit. You can either work with an e-tuner or take your car to a tuning shop with a dyno (otherwise known as a "pro tune"), but there are no off the shelf tunes for Flex Fuel, but it's well worth the investment!

Cobb's Flex Fuel Kit dyno testing results,
showing 315 horsepower on 74% ethanol
Red - 15% Ethanol
Blue - 48% Ethanol
Green - 74% Ethanol

Delicious Tuning Flex Fuel Kit

One alternative to Cobb's Flex Fuel kit is one by Delicious Tuning. Their business focuses on easy to install Flex Fuel kits for a variety of vehicles. They make kits for a variety of other brands, including Ford, Porsche, Nissan, and Toyota.

Like Cobb's offering, the Delicious Tuning Flex Fuel Kit has an ethanol sensor that reports the ethanol content of your fuel to the ECU. It allows you to monitor the ethanol content of your fuel using a smartphone app, instead of on the AccessPort like Cobb's kit does.

For some, it may be preferable to have this monitor on an app, but if you have their AccesPort plugged in all the time, you'll probably prefer monitoring it there. The kit also includes a fuel pump, something most people will want to upgrade anyway when running ethanol. The Cobb kit doesn't include a fuel pump.

After installing the kit, you need to have a custom tune made to allow you to use Flex Fuel. There are many e-tuners who will tune for this, and many people have great results, but generally, people recommend getting a dyno tune if there's a tuning shop close to you. Delicious Tuning has their own e-tuning service, making it a convenient choice.


The two main options for adding Flex Fuel support to your WRX are the kits from Cobb and from Delicious Tuning. Either one is a great choice coming from a big name in tuning, so it just comes down to a few differences. The Cobb kit doesn't include a fuel pump, and with the Delicious kit, ethanol content is monitored with a smartphone app.

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