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Series: 2015+ Subaru WRX Modification (Pt.10 - Ethanol Sensor / LPFP / HPFP)

April 05, 2021

2015+ Subaru WRX Modification (Pt.10 - Ethanol Sensor / LPFP / HPFP)

Series: 2015+ Subaru WRX Modification (Pt.10)

     Hello and welcome to my Op-Ed on the 2015+ Subaru WRX! This is part 10 of a 10 part series that plans to encompass all of the information you will need to make informed decisions regarding the best upgrade path for your vehicle.

     This week, I will be giving an in-depth overview of Ethanol Sensors and Low / High Pressure Fuel Pumps. This overview will consider the purpose of each component, how to account for it in a calibration file, as well as what specific brands I recommend.


     The statements made in the article below are subjective and based on my knowledge and experience gained from working with the VA WRX platform.  Please refer to your preferred tuner, or a trusted vendor, for recommendations on your specific set up.


     This Op-Ed is intended to help the user better understand how certain modifications affect the engine and performance, what tuning changes are necessary to account for these modifications, and what brand(s) I recommend for each modification category. This is NOT intended to be a tuning guide. I am NOT an expert in ANY category that will be discussed.


Modifications and Preferred Brands

Ethanol Sensor

     The ethanol sensor is the component that measures ethanol content in the fuel system. It is responsible for ensuring that the ECU understands ethanol content in order to properly adjust the flex fuel calibration on the fly. There is currently no factory option for an ethanol sensor. However, there are many aftermarket options available.

     An ethanol sensor works by allowing the fuel to flow through the gaps between the inner and outer electrodes as an AC voltage is applied to the electrodes. The relative permittivity of the fuel changes with the ethanol content, which leads to a change in the capacity accordingly. As the capacitance increases with increasing ethanol content, the oscillation frequency in the system will decrease and vice versa.

     The analog frequency output is tied to a microprocessor which determines the capacitance by working backwards from the measured frequency. Due to the correlation between ethanol’s permittivity and the fluid’s temperature, the microprocessor corrects the capacitance for temperature variations in the fuel according to the output from an NTC (Negative Temperature Coefficient) thermistor. In addition, errors caused by the different conductivities of gasoline and ethanol are corrected.

     The frequency of the single-wire output of the microprocessor to the ECM indicates the percentage of ethanol in the fuel. If required by the OEM the pulse width of the signal will indicate the fuel temperature. The ECM adjusts the injection according to the blend of the fuel, which leads to a stoichiometric combustion.

     Installing an ethanol sensor does require a few tuning changes. The sensor must be activated and properly calibrated before it can be used and trusted for accurate readings.

     For an ethanol sensor, I’d recommend the Flex Fuel Kit from COBB.

Subaru Flex Fuel Ethanol Sensor Kit - WRX 2015-2020 


Low Pressure Fuel Pump


     The LPFP (Low Pressure Fuel Pump) is responsible for drawing fuel from the fuel tank towards the engine. The factory LPFP does not support ethanol usage in saturations greater than 10%.

     A LPFP works by first drawing fuel into the pump through an inlet tube and filter system. The fuel then exits the pump through a one-way check valve which maintains residual pressure in the system when the pump is not running, and is pushed toward the engine through the fuel line and filter. The fuel filter traps any rust, dirt or other solid contaminants that may have passed through the pump. Fuel then flows to towards the engine.

     Installing an aftermarket LPFP requires no general tuning changes.

     For a LPFP, I’d recommend the AEM unit in conjunction with the DW Installation Kit.

AEM 340LPH Fuel Pump (E85 Compatible) with DW Install Kit - 15-20 WRX 


High Pressure Fuel Pump

     The HPFP (High Pressure Fuel Pump) is responsible for pressurizing the fuel and forcing it into the engine. The factory HPFP does not support ethanol usage in saturations above 60%. Even with ethanol saturations below 60%, it is always advised to cycle a full tank of pump fuel for every two tanks of the preferred ethanol blend. This will increase the lifespan of the HPFP.

     High-Pressure fuel pumps are mechanical and are typically driven by a camshaft. A lobe on the camshaft pushes a follower or roller that moves a piston. The piston in the pump has two cycles, suction and compression. The solenoid on the side of the pump controls how much fuel is compressed during the compression stroke. During the suction cycle, the solenoid will allow fuel from the low-pressure side of the fuel system to enter the pump. As the piston starts to travel upwards, the solenoid will remain open. The fuel is then pushed into the low-pressure side of the fuel system when the solenoid is open. When the solenoid is closed, the low-pressure and high-pressure sides of the fuel system are isolated.

     If there is a small load on the engine, the solenoid will remain open longer and a smaller amount volume of fuel is compressed. If there is a large load on the engine, the solenoid will close sooner and a higher volume of fuel will be compressed. The length of time that the solenoid remains open will determine how much fuel reaches the injectors.

     I have not worked with any aftermarket HPFP units at this time so I cannot speak on what tuning changes are required to run them. For situations like this, please contact your preferred tuner.

     For a HPFP, I’d recommend the Nostrum unit. I would recommend this unit to anyone interested in utilizing full E85 or 85+% ethanol blends in their vehicle.

Nostrum High Pressure Fuel Pump HPFP Kit w/ Free Shirt - 15-20 WRX


     Thank you for reading my Op-Ed on the 2015+ Subaru WRX! I do hope you enjoyed the series as much as I enjoyed publishing it. Please reach out to me if you have any questions or would like more insight on any of the discussed topics.