E85 and what you need to know

Running E85 has become commonplace in the performance sport compact community. It is an affordable option for a high-octane fuel. In this article we are going to explain why E85 can make more power than pump gas and what is all needed to run it. We will also go over the benefits of Flex Fuel.

E85 has a typical octane rating of around 100-110. That’s 7-17 more octane points than 93, 9-19 more points than 91. This is one of the big reasons you can make more power with E85. Turbocharged Subaru engines, as well as other brands, are knock limited from the factory on 91/93 octane. So, increasing the octane allows us to run more ignition timing, higher boost levels and leaner AFR. The higher boost, advanced ignition timing, along with leaner AFR’s are what makes the car have more power when running E85. You also get the benefit of cooler charge air temps as ethanol absorbs heat better than the regular pump gas when it changes from liquid to vapor.

What’s all needed to run E85?

Depending on what car you have you will determine what parts are needed to run E85.

  • 2002-2014 WRX, 04-2021 STI, 05-12 LGT/OBXT, and 04-13 FXT.- Upgraded fuel injectors, fuel pump, and a tune.
  • 2015-2021 WRX- Upgraded in-tank fuel pump and a tune.
  • EVO VIII, IX, & X- Upgraded fuel injectors, fuel pump and a tune.
  • 2013-2020 BRZ/FRS/86 all you need is a tune.

One downside is E85 requires up to about 50% more fuel then regular 91 octane. Pump gas stoichiometry is 14.7:1, whereas the stoich of E85 is 9.862:1. This difference in stoich is where we get the almost 50% increase in fueling, 14.7/9.862=1.49 or 49% increase. Most pump gas already contains 10% ethanol so we are really going from a stoich of 14.131:1 to 9.862:1. That’s still a 43% increase fuel needs. This increase in fueling will drop your MPG. E85 typically costs much less than 91 or 93, so the drop in MPG is usually offset by the cheaper cost at the pump. The rest of the fueling system has been shown to handle the higher ethanol content with no ill effects, unless the vehicle sits for a very long time.

A side note, when most people talk about their AFR when running E85 they are actually using the pump gas stoich scale. So, when they say they are running 11.7:1 they are actually running 7.85:1 in the E85 stoich scaling. I know that makes it a little confusing. For someone doing the math to figure out what size injectors/fuel pump you need to make X amount of power, knowing these differences are critical to get the right size fuel flow for your power goal. We always use the pump gas stoich scale when tuning as it’s just simpler to use only one scale across the board.

What’s the difference between a flex fuel tune and a E85 tune?

We hear a lot of people clumping together flex fuel and E85 tunes, saying they are the same thing. While they both end up making the same power. A flex fuel tune will achieve the power with much more precise and consistent fueling and power output. One downside of E85 is the ethanol content will range from 51-85% here in the Midwest. That can change fueling by as much as 20%. It’s typical to see E60-E70 in the winter and E70-E85 in the summer.

With an E85 only tune we tune your car with the ethanol content that is in your car. We provide you with 2 maps so you can flash between summer and winter blend E85. While this works well even small changes in ethanol content can have big results. With a 5% change in ethanol, your fueling at full throttle can end up 3% richer/leaner. If your tune is set up for E75 and you pump in E80 your AFR can go from hitting the target of 11.7:1 down to 11.35:1. Alternatively, if you pump in E70 and are tuned on E75 it will lean out the AFR by 3%. That 3% will make your full throttle fueling to go from 11.7: to 12.05:1. You can see how important it is for the ECU to know what ethanol content is in the fuel tank since it is ever changing.

Enter flex fuel tuning. With flex fuel we monitor ethanol content via an ethanol sensor wired into the stock ECU. Knowing the ethanol content allows us to set up the power output of your engine. We do this via different boost, AFR, and ignition timing settings based on that ethanol content. With that we first tune your car on pump gas or the low ethanol, usually E0-E10. Then tune on high ethanol, usually E60+. After that the ECU’s software blends the two tunes together to get desired AFR, ignition timing, and boost depending on the ethanol content.

On all the EJ engines we tune the ethanol portion to make the same power from E60-E85. We haven’t found much for power gains above 60% so there is no need to find “good E85”. Your winter or summer blend E85 from all stations will make your peak power.

On the FA20 or 2015-2021 WRX we tune max power to range from E50-E70. Since the FA is direct injection, we have a very small window to inject all the fuel compared to a port injected engine like the EJ25. Running straight E85 can max the stock fuel injectors so we don’t recommend it unless you have upgraded your high-pressure fuel pump and larger direct injection injectors or auxiliary port injectors and controller.

The BRZ/FRS get the same treatment as the EJ engine since they are both port and direct injected. We tune them to make the same max power from E60-E85.

On the EVO’s we tune the ethanol portion to make the same power from E60-E85

This 2017 STI is tuned to 19psi on 93oct and 21psi on E85. These are the typical power gains you can expect going from 93oct to E85.

We can tune the following for full flex fuel

All Subaru’s currently require the Delicious Tuning flex fuel kit, except when running carbarry. More information can be found here.

  • Manual transmission 2004-2021 STI, 2006-2021 WRX, 04-08 FXT, 05-12 LGT/OBXT with a V3 Accessport
  • 2002-2005 WRX with the Delicious Tuning Kit or Carbarry Rom Via Opensource
  • 2013+ FRS/BRZ with EcuTek
  • Now newly available for the EVO 8 & 9 on the stock ECU!
  • EVO X Flex Fuel with Delicious Tuning kit or Tephra V3 Flex Rom
  • Stand Alone ECU’s on E85 or flex fuel if the ECU has the capabilities

We can also tune all of these options on straight E85 including the Evo 8 & 9. We have also been able to tune some automatic transmission Subaru’s with flex fuel capability. Contact us for more information.

We have done extensive testing so all of our flex fuel or E85 tunes will start all year round. Even on those -30 degree Minnesota mornings.

Ethanol Calculator

Gas tank size
Gasoline Ethanol Percentage
E85 Ethanol Percentage
Target Ethanol Mix
Current Fuel Level Percentage
Current Ethanol Mix
E85 to Add
Pump Gas to Add
Resulting Mix
0.0% Ethanol
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