Subaru Flex Fuel Explained

Subaru Flex Fuel Explained

Apr 15, '21

In this blog post we talk about Flex Fuel specifically for Subaru 2.0 Liter 16 bit ECU platforms. For the 32 bit platforms this is a straight forward process using the Cobb Flex Fuel Kit with the Cobb Accessport.

What is Flex Fuel?

Flex fuel is the ability to run varying amounts of ethanol in your fuel system. You can Flex-able the type of fuel in your car. The amount of ethanol varies from area to area and time of year so if the ECU can adjust for these changes the car will run better. It also helps in situations where E85 is not available (IE a road trip to another state) and standard fuel must be run for a period of time.

Why would you upgrade to Flex Fuel?

Ethanol blended fuel, typically referred to as E85, has anti knocking properties that allow for more horsepower similar to higher octane (IE difference between 87 and 100 octane).

How can I upgrade to Flex Fuel for my 2.0 Liter Subaru?

The simplest way to get this working is to grab up the 04-06 Cobb Flex Fuel Kit and utilize a Open Source tuning platform called Carberry. We have a plug and play conversion harness that will adapt the TGV input to the rear 02 input if required or preferred depending on the ECU (JDM STi ECUs do not have a TGV input)

Why do I need the whole Cobb kit? Can't I just hook up a content sensor?

Due to the lack of Flex Fuel support from Cobb for this application we get a fair number of people trying to purchase a content sensor only to try to keep the cost down and asking us to wire it in. The reason this won’t work is because the flex fuel content sensor uses a digital signal but the ECU is only able to accept an analog signal which are not compatible with each other. To make the flex fuel sensor work, a conversion is needed for the signal. The reason the Cobb Flex Fuel Kit is more expensive than the content sensor is the conversion box required to convert the digital flex fuel signal to an analog signal the ECU can understand. While we tend to lean toward the Cobb kit since it is simple and plug and play, the other route is to go with a Flex Fuel Gauge like an Innovate (or any other Flex Fuel conversion kit) and utilize the analog output from the gauge to tie into the TGV or Rear 02 to the ECU can read it. The gauge analog output is doing the same digital to analog conversion as the Cobb conversion box.

What if I'm running standalone ECU?

This is the scenario in which the content sensor is all that is needed because the content sensor be wired to output directly to a digital input on the standalone ECU.