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Subaru E85 Conversion Guide: Fuel System Upgrades for Performance

Posted by IAG Performance on Apr 26th 2024

Subaru E85 Conversion Guide: Fuel System Upgrades for Performance

Thinking of switching to alcohol-based E85 to increase the horsepower in your Subaru STI or WRX? It can be a real game-changer, as E85 not only boosts horsepower but also offers a multitude of additional benefits; it burns cleaner and cooler than regular gasoline, it has a higher resistance to detonation (knock), and it’s a cost-effective option for race fuel. Before you go and fill up with E85, though, there are a few modifications you'll need to make so that your engine’s fuel delivery system can handle the transition. While a proper setup is crucial for safety and efficiency, navigating the process is easier with the right parts and a little guidance. In this article, we’ll explore the ins and outs of shifting over to E85 (or Flex Fuel) and provide some helpful setup options to maximize your engine’s fuel efficiency and performance.

Photo of gas station at night


Before jumping into the finer details of setting up an E85 system, let’s take a look at E85 and its characteristics. E85, commonly referred to as flex fuel, is a blend of ethanol and gasoline. The ratio of ethanol to gasoline can vary depending on the region, the manufacturer, and even the time of year it was produced. This is because ethanol is a type of alcohol made from grain crops like corn and sugarcane; the varying composition of the fuel can affect the blend ratios as well as the final product. Another factor to consider is that manufacturers often reduce the ethanol content in winter to help with cold starting. On average, E85 contains anywhere from 51% to 85% ethanol, so keeping track of the ethanol content will be crucial for a successful setup.

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E85 fuel requires approximately 30 to 35% more volume than regular gasoline for your engine to run properly. Since OEM Subaru fuel pumps can’t keep up with the flow demands that flex fuel requires, a high-flowing ethanol compatible fuel pump will be a necessary upgrade to handle the additional volume of fuel. Ethanol compatible fuel pumps must also incorporate the use of specialized o-rings, sealed electrical wiring and augmented housing components to prevent corrosion and premature failure due to exposure. AEM offers several solid options that meet all these requirements; their 340LPH E85 Ethanol Compatible In Tank Fuel Pump works with ethanol fuels up to E100, methanol fuels up to M100, as well as all types of gasoline. Another thing to keep in mind when upgrading pumps, especially dual pumps is that the OEM wiring may not be sufficient to deliver enough amps to keep the pump running at peak efficiency. We recommend a Fuel Pump Hardwire Kit when upgrading your fuel pump to avoid damage that can occur without upgraded wiring.

Click here to view our selection of E85 compatible fuel pumps-

AEM 340 LPH E85 Ethanol compatible in-tank pump.
Photo display of Injector Dynamics fuel injectors.


Since E85 requires approximately 30% more fuel for your engine to reach the same output levels as with gasoline, it stands to reason that your fuel injectors will also need an upgrade to accommodate the increased fuel flow. Stock WRX injectors are 420cc, while STI injectors are about 560cc. Given that stock WRX injectors typically have a capacity of 420cc, and STI injectors are around 560cc, upgrading to injectors with capacities around 546cc or more for WRX and 728cc or more for STI would be necessary to ensure adequate fuel delivery. Keep in mind that you don’t want to run your injectors at 100% duty cycle, so it’s a good idea to go with injectors slightly larger than required for increased headroom. For most applications under 500whp, the ID 1050 Injectors are a great choice. If you’re shooting for more power you’ll want to go even bigger. Remember, larger injectors can affect driveability and idle characteristics, so it’s important not to go bigger than necessary. The Injector Dynamics’ ID2600 Top Feed Injector Set, for example, triples the stock cc numbers, while offering corrosion-resistant internals and long-term compatibility with ethanol and methanol. These are only recommended for those looking to make the highest power levels possible.

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Since the ethanol content of one tank of E85 can vary to the next, or can even be blended to adjust the octane level, it’s necessary to calibrate the fuel map to adjust to the percentage of ethanol being run through the system. Testing the ethanol levels to account for fluctuations in fuel composition sounds like a daunting task, but these days, there’s a multitude of options to set yourself up for success. One of our favorites is the COBB NexGen Ethanol Sensor Kit, which measures the ethanol content level of the fuel being pumped into your engine and then sends that data to the ECU to be used in a Flex Fuel calibration, with a little help from COBB’s Accessport.

Click here to learn more about COBB’s engine management systems for Subaru-

Photo of Cobb Nexgen Ethanol Sensor, installed.
Photo of COBB Accessport, lifestyle image


COBB’s V3 Accessport will give you the ability to toggle between your saved maps and make quick adjustments on the fly, even while your engine is running. This single control module makes engine management easy, store up to 100 OTS (Off The Shelf) maps, or access custom maps from your COBB tuner. Whether you’re adjusting your fuel calibration to account for high ethanol content, or to gain a bit of power off the line, or just turning it down for the daily commute, the Accessport removes all the guesswork in your fuel calibrations.

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Piecing together an E85-compatible fuel system doesn’t have to be a difficult undertaking. IAG’s Fuel System kits provide everything you need to successfully feed your engine with enough fuel to meet your horsepower demands. These kits come with pre-fitted fuel lines, so they don’t require you any cutting or hose assembly. The lines are constructed of chemical resistant PTFE, and won't deteriorate over time when exposed to ethanol.

Shop our selection of specialty fuel lines and filters, click here-

IAG Performance Fuel System Kit, display photo.
Photo of AEM fuel pressure regulator.


The IAG Fuel System Kit comes with an Adjustable Fuel Pressure Regulator, but if you're upgrading with separate components, an adjustable regulator offers precise fuel pressure control and can withstand exposure to ethanol. AEM’s high-flowing, ethanol compatible regulator is a great option that is capable of supporting enough fuel flow for engines up to 1000 horsepower.

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E85 has approximately a 105 octane rating, which when combined with its superior cooling properties, work together to provide a higher resistance to detonation and engine knock than regular gasoline. This will allow you to tune your engine for advanced timing, and ultimately, more power. This is where an experienced tuner can come in handy. A professional tuner familiar with E85 can set up multiple maps to account for the variables posed by E85, ensuring your engine runs safely and efficiently while also giving you that extra edge of horsepower.

From routine maintenance to horsepower upgrades, we've got a great selection of parts for your Subaru-

Photo of engine with aftermarket fule lines installed.

Switching over to E85 requires a bit of research and effort, but it can be well worth it for the performance gains. We’re hoping we’ve provided some useful options to reconfigure your fuel system, while also making it a little less daunting. From high-flow fuel pumps to specialized injectors, we carry a full selection of the specialty parts needed for a successful E85 setup, but if you have any questions about any of the recommended upgrades, feel free to give us a call at 1(410)840-3555 or shoot us an email at

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