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Series: 2015+ Subaru WRX Modification (Pt.2 - Intake / Bypass Valve / Turbo Inlet)

February 08, 2021

Series: 2015+ Subaru WRX Modification (Pt.2)

Hello and welcome to my Op-Ed on the 2015+ Subaru WRX! This is part 2 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 Intakes, Bypass Valves, and Turbo Inlets. 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.

Disclaimer

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.

Intentions

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.

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Modifications and Preferred Brands

Intake

As the primary point entry for air into your engine, an intake is very important. An intake can either encourage healthy airflow, or restrict it. A good intake will also protect against heat and water as the vehicle is driven through a variety of different conditions. The OEM intake is sufficient for most common builds. However, people prefer the turbo noise to not be muffled. Hence, an intake is one of the most common modifications out there.

Some key considerations to make when sourcing an intake are as follows. What is the pipe diameter? Pipe diameter will either encourage or restrict air flow depending on pipe size and bend geometry. Is the MAF (Mass Air Flow) sensor housing different? The MAF housing will affect how air is calculated as it passes the sensor. How does the intake control temperature and protect against hydro-lock? An intake with a heat shield is the best way to protect usable air from radiant heat. It also can protect against water as it splashes up into the engine bay.

Tuning changes for an aftermarket intake generally revolve around properly scaling the MAF sensor to ensure that the proper amount of air is being accounted for as it enters the engine. If this sensor is improperly calibrated, the ECU will compensate by adding or removing fuel, as necessary, as determined by the measured AFR (Air Fuel Ratio) at the wideband O2 (Oxygen) sensor during closed-loop operation. Not accounting for an intake during tuning can result in elevated fuel trims and generally erratic fueling which can lead to bigger issues down the line.

For intakes, I would recommend either the COBB Big SF or GrimmSpeed StealthBox units. I have had great results testing with both of these units. The COBB unit is a bit more robust. However, the GrimmSpeed unit has a better fit and finish, in my opinion. The GrimmSpeed unit also has a clever design that eliminates any guesswork when placing the air filter. This means that there is more consistency from one setup to the next.

Subaru Big SF Intake System WRX 2015-2020

GrimmSpeed StealthBox Cold Air Intake Black - 15-20 WRX

Bypass Valve

A bypass valve is important, especially for the FA20DIT which utilizes a fully recirculating air flow path to accurately account for all air in the system. Instead of venting excess air into the atmosphere like a traditional BOV (Blow Off Valve), the BPV (Bypass Valve) will recirculate excess air back into the intake tract. This allows for the correct amount of air in the system to be preserved, especially during transient operating conditions. The OEM BPV is sufficient for most applications. However, it is plastic and prone to failure over time.

Some key considerations to make when sourcing a BPV are as follows. Does the unit recirculate 100% of the excess air? If it does not, then I would look at a unit that does. My experience with VTA (Vent To Atmosphere) setups is minimal. However, from a conceptual standpoint, if air is accounted for and then lost, the engine will be expecting more air than it will receive. This will cause a rich condition and is not optimal for most situations. Does the unit leak? A BPV that leaks can cause a wide variety of operating concerns as the vacuum in the system is no longer preserved.

Installing a fully recirculating BPV requires no general tuning changes. However, installing a VTA unit may require an adjustment to the VE (Volumetric Efficiency) table(s). Nevertheless, it is best to discuss the application of a hybrid or full VTA charge pressure release system with your preferred tuner.

For BPVs, I would recommend the COBB LF unit. The Boomba unit is adequate as well. However, I’ve heard more horror stories about the Boomba unit vs. the COBB unit. I personally run the Boomba unit and have had no issues thus far.

Boomba Racing Bypass Valve - 15-20 WRX / 14+ Forester

Turbo Inlet

A turbo inlet is a critical component that allows air to transfer smoothly from the intake to the turbo. By design, the unit should flow as freely as possible and provide an ample seal between the components it connects. For this reason, some flexibility and resilience is important. The OEM inlet is sufficient for most applications. However, it is plastic and prone to failure over time. It is also designed to lightly tumble air as it enters the compressor housing of the turbo. This may or may not be desirable depending on your specific situation.

Some key considerations to make when sourcing a turbo inlet are as follows. Does the unit flow air laminarly? Laminar (non-turbulent) flow will be more efficient and predictable overall. Does the unit provide an ample seal from the intake to the turbo? If not, then a vacuum leak will be introduced. Is the unit resilient? If not, it may fail over time due to heat and stress.

Installing a turbo inlet requires no general tuning changes. However, it is best to share a post-install log with your preferred tuner so that any potential vacuum leaks can be identified and corrected.

For turbo inlets, I would recommend the Perrin unit. There’s not much competition on the market at the moment and the wireframe/silicone design is perfect for most setups.

Perrin Turbo Inlet Hose w/ Nozzles - 15-20 WRX

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Tune in next week (02-15-2021 @ 12:00 EST) where I will be giving an in-depth overview of Electronic Boost Control Solenoids, Turbochargers, Intercoolers, and Charge Pipes. 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.

Cheers!

-Clint