The COBB flex fuel kit is the most popular way to run E85 properly on the WRX and STI. It adds an ethanol content sensor to the fuel system that communicates with the Accessport in real time, allowing the car to run any blend of E85 and pump gas without requiring a separate tune for each fuel type. This guide covers how the system works, which kit fits your platform, and what the full setup requires for the Subaru WRX and Subaru STI.
What the COBB Flex Fuel Kit Actually Does
Running E85 without a flex fuel kit means committing to a dedicated E85 tune. Every time the fuel blend changes, filling up with pump gas when E85 is not available, for example, the car is running an unsafe calibration until it gets retuned. That is not a practical solution for a street-driven build.
The COBB flex fuel kit solves that problem. An ethanol content sensor reads the percentage of ethanol in the fuel and sends that data to the Accessport. The Accessport adjusts fueling and timing in real time to match the blend. The car runs correctly on straight E85, straight pump gas, or any mix of the two without any manual intervention. Our E85 vs pump gas guide for WRX and STI covers what E85 actually changes on the platform and whether the switch makes sense for your build.
Sensor Kit vs CAN-Based Upgrade: What Is Different
COBB uses two different approaches for flex fuel depending on the platform generation. Understanding the difference before buying is important.
The sensor kit approach adds a physical ethanol content sensor inline in the fuel system. It reads the fuel blend directly and sends that data to the Accessport via a wired connection. This approach covers most WRX and STI generations and is the standard flex fuel solution for EJ and older FA platform builds.
The CAN-based upgrade is specific to the 2022-2025 WRX. Instead of a physical inline sensor, it uses the CAN bus communication architecture of the newer FA24 platform. The result is a cleaner, more integrated solution designed specifically for that generation. Our post on the COBB CAN flex fuel upgrade for 2022-2025 WRX covers that platform in detail.
Which Flex Fuel Kit Fits Your WRX or STI
COBB Tuning and compatible brands make platform-specific kits for every WRX and STI generation. Here is the breakdown by platform.
For the 2022 to 2025 WRX, the COBB CAN Flex Fuel Upgrade for WRX 2022-2025 is the correct solution. It integrates with the FA24 platform architecture and is the complete kit for that generation.
For the 2018 to 2021 WRX, the Subaru CAN Flex Fuel Upgrade and Fuel Pressure Kit for WRX 2018-2021 is the right fit. It includes the fuel pressure kit alongside the flex fuel sensor for a complete solution on that generation.
The 2015 to 2017 WRX takes the Subaru CAN Flex Fuel Upgrade and Fuel Pressure Kit for WRX 2015-2017, the same complete kit approach sized for that generation.
WRX owners on the 2011 to 2014 platform need the Subaru Nexgen Flex Fuel Upgrade and Fuel Pressure Kit for WRX 2011-2014.
STI owners on the 2008 to 2021 platform have two options depending on the build. The Subaru Nexgen Flex Fuel Ethanol Sensor Kit for STI 2008-2021 covers the sensor kit approach. Owners who also need updated fuel rails should look at the COBB Fuel Rails and Line Kit Package for STI 2008-2021 for a complete fueling solution.
What Else the Build Needs for E85
The flex fuel kit is the sensor and communication layer. It tells the Accessport what blend is in the tank. But running E85 properly also requires the fuel system to supply the increased fuel demand E85 creates.
E85 requires approximately 30 percent more fuel volume than pump gas to achieve the same air-fuel ratio. At stock power levels the stock fuel system can sometimes keep up. As power increases, the injectors and fuel pump become the limiting factor. A proper flex fuel setup accounts for both the sensor kit and the supporting fueling hardware.
The WRX fuel system upgrades guide covers the full fueling picture, injectors, fuel pump, and supporting hardware, for builds running E85 at different power levels. Our fuel pumps vs injectors guide breaks down which upgrade matters most depending on the power target.
Power Gains From Running E85 With a Flex Fuel Kit
E85 has a significantly higher octane rating than pump gas. Higher octane allows more aggressive ignition timing without knock. More timing means more power. On a properly tuned WRX or STI with a flex fuel kit, E85 typically delivers 20 to 40whp more than the same car on pump gas depending on the platform and existing modifications.
The gains are most significant on builds that are already knock-limited on pump gas, high boost setups where the ECU is pulling timing to protect the engine. E85 removes that limitation and lets the tune run the timing the engine can actually support. Understanding what EFI tuning does for your WRX explains exactly how timing and fueling interact and why E85 unlocks more from the same hardware.
Find the Right Flex Fuel Kit for Your WRX or STI
The right kit depends on the platform generation and what the rest of the fuel system looks like. Confirm the model year before ordering and make sure the fueling hardware can support the increased demand E85 creates.
Browse the full range of flex fuel kits for WRX and STI to find the right option for your platform, or explore the complete WRX and STI fuel system upgrades hub to see how flex fuel fits into the full fueling picture. For everything COBB makes for the WRX and STI platform, browse the COBB Tuning lineup for WRX and STI.
0 Comments for “COBB Flex Fuel Kit for Subaru WRX and STI”