If you drive a Subaru WRX, Subaru STI, or Forester XT, switching from pump gas to E85 is one of the most talked-about upgrades in the turbo community. Ethanol blends can unlock impressive power gains, but they also change how your engine operates mechanically.
Before making the switch, it’s important to understand what actually changes inside your engine and what supporting modifications become necessary.
Explore compatible performance parts on our dedicated Subaru WRX, Subaru STI, and Forester XT pages.
Why E85 Makes More Power
E85 has a higher effective octane rating than pump gas, which allows for more aggressive ignition timing and higher boost levels. Because it resists knock better, tuners can extract additional horsepower safely when the rest of the system supports it.
However, ethanol contains less energy per unit volume than gasoline. That means the engine must inject significantly more fuel to make the same power.
Because ethanol allows more aggressive timing and boost, it’s critical to understand safe pressure limits. Review our guide on safe boost levels for WRX & STI before increasing PSI.
Fuel Pumps Become the First Limiting Factor
Since ethanol requires roughly 25–30% more fuel volume, the factory fuel pump often becomes a restriction as power increases. Running lean under boost is one of the fastest ways to damage a turbocharged engine.
Upgrading to an ethanol-compatible pump like the AEM 340LPH Ethanol-Compatible Fuel Pump or the AEM 320LPH Fuel Pump Kit ensures adequate fuel delivery under higher boost levels. For a full breakdown of what the AEM 340LPH supports and when it becomes necessary, the AEM 340LPH fuel pump guide for WRX and STI covers everything. For a complete look at the COBB flex fuel kit and how it works on both FA20 and FA24 platforms, the COBB flex fuel kit guide for WRX and STI covers the full setup and what it requires.
You can browse additional upgrades in the Fuel Delivery category. STI owners planning a fuel system upgrade alongside an ethanol setup can find the full upgrade path in the STI fuel system upgrades guide. Managing crankcase pressure becomes increasingly important when running higher ethanol blends at elevated boost. The best oil catch cans for WRX and STI covers which systems work best on flex fuel and high-boost Subaru builds.
Flex Fuel Monitoring Is Critical
Ethanol content varies from station to station. Without proper monitoring, your ECU may not adjust fueling correctly, increasing risk under load.
Systems like the Subaru NexGen Flex Fuel Upgrade Kit allow the ECU to compensate for ethanol content in real time when paired with proper tuning.
Professional calibration through the EFI Tuning category is mandatory when switching fuels.
Boost Levels Often Increase on E85
Because E85 resists knock more effectively, tuners often increase boost pressure. Upgraded turbochargers such as the Boost Lab TD06SL2 20G Turbocharger can take advantage of ethanol’s higher knock resistance. For a full breakdown of what the Boost Lab TD06SL2-54X delivers on both the WRX and STI platform and whether it fits your build, the Boost Lab TD06SL2-54X guide for WRX and STI covers power numbers, fitment, and supporting mod requirements.
Supporting hardware from the Forced Induction category must be balanced with proper fueling to maintain reliability. For a complete breakdown of every STI turbo upgrade option by power goal on ethanol, the best turbo upgrades for Subaru STI guide covers where to start and what to budget.
For builds running an external wastegate setup alongside an ethanol build, the Turbosmart external wastegate guide for WRX and STI covers what it does and which setup is right for your build. Before pushing past stock limits on ethanol, understanding what the stock EJ257 can actually handle is critical. The stock STI power limits guide covers EJ257 limits and what breaks first. For STI owners comparing the EJ207 and EJ257 platforms before committing to an ethanol build, the EJ207 vs EJ257 guide covers what STI owners should know. For owners comparing the EJ257 to the newer FA24 platform, the EJ257 vs FA24 guide covers what Subaru owners should know.
Increased Cylinder Pressure and Engine Stress
While E85 can make more power safely when tuned correctly, it also increases cylinder pressure. Higher torque levels place additional stress on rods, pistons, and bearings.
If you are planning an ethanol setup beyond stock internals, upgrading to a closed-deck short block becomes the safer long-term path. Consider proven options like the IAG 950 EJ25 Closed Deck Short Block for serious street builds, the IAG 1150 Closed Deck Short Block for higher horsepower goals, or the IAG Stage X Billet Short Block when you want maximum strength for high boost builds.
Higher torque output also increases drivetrain movement under load. Upgrading to performance options from our Engine Mounts category can improve stability and reduce unwanted engine movement at higher boost levels.
For higher horsepower builds, upgrading to a closed-deck solution like the IAG 950 EJ25 Closed Deck Short Block or the IAG 1150 Closed Deck Short Block becomes the safer long-term path. For entry-level street builds, the IAG 550 EJ25 short block guide for WRX and STI covers forged internals for street builds up to 550 BHP. For mid-level builds, the IAG 750 EJ25 closed deck short block guide covers what it supports and who should build it.
On the cooling side, running ethanol at elevated boost increases heat output significantly. The Mishimoto aluminum radiator guide for WRX and STI covers what it does and when to upgrade. For 2015 and newer WRX owners specifically, the Mishimoto aluminum radiator guide for 2015+ WRX covers what changes on that platform and when to upgrade. On the exhaust side, the Invidia stainless downpipe guide for WRX covers the divorced wastegate and high flow cat explained.
On the fuel side, for builds requiring more fuel volume than the standard drop-in pump can deliver on ethanol, the AEM 340LPH 65mm fuel pump kit guide covers why serious builds skip the drop-in. For a complete look at what the stock fuel system can handle before upgrades become necessary, the stock WRX and STI fuel system limits guide covers exactly where those limits sit. For builds requiring upgraded fuel rails, lines, and a regulator alongside injector upgrades, the COBB fuel rails, line kit, and regulator guide for STI covers what the kit includes and when it becomes necessary. For builds requiring even more fuel volume, the AEM 380LPH inline fuel pump guide for WRX and STI covers why serious builds step up to the inline pump. On the drivetrain side, ethanol builds running higher torque need a clutch that can handle it. For STI owners running the ACT HD kit, the ACT HD Street Sprung clutch kit guide for Subaru STI covers what it is and who should run it. For STI owners needing more holding capacity, the Competition Clutch STI Stage 2 guide covers when you need more than the ACT kit.
If your goals demand maximum strength, the IAG Stage X Billet Short Block is built for high boost applications when paired with the right fueling, turbo setup, and tuning.
If you are pushing higher torque on ethanol, understanding stock component limits is essential. Our FA20 and FA24 stock power limits guide explains where factory internals begin to reach their threshold.
Hardware upgrades become equally important when running ethanol at higher boost levels. ARP head studs are one of the most critical additions for preventing head gasket failures under the increased cylinder pressure ethanol builds produce. The ARP head studs guide for WRX and STI covers why they matter and which kit to run.
For STI owners running ethanol on the EJ257, understanding the most common failure points is equally important. The most common Subaru STI engine failures guide covers the EJ257 platform in detail and what supporting mods prevent the most common issues under boost.
Explore more built engine options from IAG Performance if your goals exceed stock limits.
Before upgrading internals, read our short block vs long block breakdown to determine the right configuration for your goals.
Supporting Reliability Upgrades
Higher boost and increased cylinder pressure also increase crankcase pressure and oil vapor. Installing an upgraded separator such as the IAG Air Oil Separator (AOS) helps manage blow-by and keeps the intake system cleaner under higher load.
Browse additional crankcase ventilation upgrades in our IAG Performance Oil Catch Cans collection.
Even exhaust flow improvements, like unequal length headers (with modern replacement options if certain models are out of stock), contribute to overall efficiency when running ethanol blends.
Is E85 Worth It?
For many WRX and STI owners, E85 delivers noticeable gains in horsepower and torque when paired with the right supporting modifications. However, it is not a “plug-and-play” upgrade.
Fueling capacity, tuning quality, turbo efficiency, and internal engine strength must all be considered together. When approached as a complete system, E85 can safely unlock the next level of performance.
If you’re planning the switch, start by evaluating your fueling setup, tuning strategy, and long-term horsepower goals before increasing boost.
Higher horsepower levels also increase engine temperatures. Our WRX and STI radiator upgrade guide explains when additional cooling becomes necessary. STI owners addressing exhaust flow as part of an ethanol build can find the full breakdown in the Subaru STI exhaust upgrades guide, which covers downpipe, catback, and turboback options for the EJ platform.