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Using E85 in a Subaru WRX/STI EJ25: A Guide

November 8th, 2023 | 4 min read

By Alan Jackson

There is a lot of hype around using ethanol, or E85, in your Subaru WRX or STI. There are many benefits to it but it needs to be done correctly for a safe, effective increase in power and torque. Knowing how to make it work is the first step in setting up your WRX or STI for success. 

What is E85?


E85 is an ethanol based fuel blend available at certain public gas stations that is supposed to be 85% ethanol and 15% gasoline. The reason you can’t just fill up your EJ powered Subaru and go is because E85 requires about 30 to 35% more volume than regular gasoline to run right. E85 was intended to reduce the consumption of fossil fuels, but due to its much higher octane and oxygenating ability, it has quickly become an excellent source for cheap and easy to find race gas. With it becoming more and more readily available, switching to E85 has never been easier to do. And thanks to advances in tuning and the aftermarket, it has never been easier to make power running it in your WRX or STI.

What Parts Will I Need?

Since so much more fuel is required when running E85, the first limits you are going to reach are with the hard parts. Fuel pumps can’t flow enough to support 30% more fuel reliably and need to be replaced. In most cases, fuel injectors that can be maxed out on regular gas, do not have the headroom to flow an additional 30% of fuel and will also need to be replaced. When sustaining more fuel flow, maximizing the efficiency of your fuel delivery system is a must and an upgraded fuel pressure regulator is necessary. There are plenty of other items that you can upgrade as well in order to cover all bases, but these three parts are the main contenders and the most necessary to upgrade. When upgrading these parts you also have to keep in mind what your pump gas fuel demands will be. If you are running a bigger turbo, then you need bigger injectors, regardless. Whether you are making 300hp or 600hp on regular gasoline, if you plan to switch to E85, you will need at least 30% more fuel headroom to do so.

Want to know WHY you need so much more E85 fuel when compared to gasoline?  See our blog post about how E85 works and what makes it such a great performance fuel - The Magic of E85.

Injector Size Gas Horsepower E85 Horsepower
Factory (560cc) 360 280
1000cc 650 500
2000cc 1300 1000

As you can see in the table, you need much bigger injectors to push an E85 fueled car to higher horsepower.  The benefit is that you can make that power with a bigger margin of safety for your engine.  It's much easier to make power on E85 if you have the appropriate fueling modifications.



So, you are just three mechanical part upgrades away from being able to run E85.  That’s not so hard… Hold up a sec, we are missing something…

Do I Need To Tune My Car?

At this point, you must address the tune! Your ECU won’t know what to do with itself when all of the sudden the mechanical parts are flowing so much more fuel through the system. And, not only do you have to adjust the tune for the bigger injectors in order to make sure they operate properly on regular gasoline, you also have to adjust for the ethanol content. Depending on the percentage of ethanol, a different amount of fuel will need to be delivered and the ECU on your EJ powered WRX or STI has no clue what is in your tank.


This is where a skilled tuner comes in handy. A skilled tuner will be able to look at the car on a dyno and adjust the fuel until it is just right, calibrating everything to run efficiently and clean giving you a nicely running car that is now capable of considerably more power on this E85 magic fuel. Unfortunately, it's still not that easy… 

Ethanol_Content.jpgThe problem is that the ratio of ethanol to gas in the gas stations that sell it is extremely inconsistent. You will find that ethanol content varies from station to station and it even varies at the same station with different truck loads containing different ethanol content. You will also find that winter blends are often closer to E70 and lower in order to aid with cold starting (something E85 is known to have trouble with). Then you will get summer blends that can be anywhere from E70 to E85 or higher (fingers crossed). That is a 15% variance and is something that definitely needs to be calibrated for.

What If I Can't Find E85?

Another issue is that E85 is not always available and you might have to switch back to regular gasoline. So what do you do at the changeover? There will be a period of time where you are still pushing ethanol through the fuel lines and will need to be on the E85 tune. Also, when you fill up with gasoline that is already E10, there will still be some ethanol in your tank and will increase this ratio enough, in some circumstances, to still require tuning.

This sounds a lot more complicated than it is.  Fortunately for all of us ethanol lovers, Flex Fuel really takes the last remaining hassle out of going fast with E85. Flex Fuel allows an ethanol sensor to interface with the car's ECU and will make all these adjustments on the fly for you.

Cobb_Flex_Fuel_Kit.jpgOne such kit is the Cobb Tuning Flex Fuel Kit. It integrates with the ECU and the Cobb Tuning Accessport and monitors ethanol content. This sensor sends a signal to the ECU and allows it to add or subtract fuel and ignition timing based on the amount of ethanol in the system. It will need to be properly calibrated by a tuner, but once everything is dialed in you don’t have to worry about getting a bad batch of ethanol. The ECU will be able to adjust accordingly as the mixture changes and give you worry free operation and power.

 E85 is a great way to pick up power on any car.  When calibrated correctly, it can even allow you to make more power with more safety built in. If you are considering a switch from regular gasoline to an inexpensive and easy to find race fuel, E85 is a great choice.  Just make sure you set your car up properly so that you can truly enjoy the benefits of ethanol.


This post was originally published in May, 2016 but has been updated to reflect current parts and accurate information!