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Series: 2015+ Subaru WRX Modification (Pt.5 - Engine Builds / Downpipe)

March 01, 2021

2015+ Subaru WRX Modification (Pt.5 - Engine Builds / Downpipe)

Series: 2015+ Subaru WRX Modification (Pt.5)

     Hello and welcome to my Op-Ed on the 2015+ Subaru WRX! This is part 5 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 Built Blocks and Downpipes. 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

Built Block

     For bigger builds, the power capacity of the stock block and internals may be reached. In this case, an upgraded internal setup will be required. Torque is typically capped at 350-375 WTQ (Wheel Torque) in order to preserve the integrity of the connecting rods. OEM FA20DIT rods are especially thin to help reduce oscillating mass and increase engine efficiency. As a result, they are the weak point in many cases. An engine build can also help increase the durability of other internal components, such as the pistons or the crankshaft, without sacrificing engine cooling and the amount of power than can safely be made. An upgraded valvetrain may also be a worthwhile consideration for some applications. However, we will not be discussing that topic in this editorial.

     The only key consideration to make when sourcing a built block is as follows. What are the power goals? You should source your internals or built block packages based on power goals. Certain packages will be rated for certain applications. However, It’s always a good idea to over-build as it can save you a lot of money in the long run.

     I have not worked with any aftermarket built blocks 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 an engine build, I’d recommend browsing the offerings from IAG and seeing which package best meets your requirements. I would advise against replacing internals only. Unless you really know what you’re doing and have the tools to support the process, leave the guesswork and assembly out of the equation. You’ll thank yourself later.

15+ WRX Short Blocks 



     As the primary exit point of air from your turbo, the downpipe is tasked with diverting exhaust flow away from the turbo as quickly and efficiently as possible. Air can only enter the engine as quickly as it can exit it. For this reason, a free-flowing downpipe is integral to optimal engine performance. However, the downpipe is also a critical emissions component, as it houses the catalyst. The catalyst uses a combination of platinum and rhodium to facilitate oxidation to remove nitrogen atoms from nitrogen-oxide molecules in order to reduce harmful emissions. The OEM downpipe houses one catalyst and one resonator but features less than optimal geometry.

     Some key considerations to make when sourcing a downpipe are as follows. Will I pass emissions? Depending on your county and laws, this may be something to consider. I recommend looking into how your area conducts emission testing before making the choice to modify your catalyst.

     Upgrading the downpipe does require a few tuning changes. For certain applications, some fault codes may need to be suppressed in the calibration file and the wastegate duty will need to be re-calibrated in order to account for the loss in back pressure.

     For a downpipe, I’d recommend either the COBB or GrimmSpeed catted units. These brands offer superior build quality and excellent flow characteristics. This is especially apparent in the GrimmSpeed unit which is made in the USA and boasts a superior fit and finish. The catalysts on these units are high-flow, yet still very efficient at converting pollutants.

GrimmSpeed Catted J-Pipe 6MT - 15-20 WRX 


     Tune in next week (03-08-2021 @ 12:00 EST) where I will be giving an in-depth overview of Cat Back Exhausts, Air Oil Separators, and Chassis Bracing. 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.