2015-2020 STI Fuel System and Staged Upgrades

Cobb Stage 3 PackageIn this post I'm going to address some limitations with the factory fuel system on the 2015-2020 Subaru WRX STI. In the old days, you could bolt an intake and a turbo-back exhaust (consisting of a downpipe and a cat-back exhaust) to an STI, and then install the Stage 2 map from your Cobb Accessport, and you'd be all set. The stock STI fuel pump and fuel injectors provided adequate fuel flow for the Stage 2 hardware.

The current STI (2015-2020) is closer to the edge in terms of fuel supply, and if you install both a turbo-back exhaust AND a high-flow intake system, the car will run out of fuel supply. We know this because we can watch the fuel injector duty cycle, and in this state the car will run 100% injector duty cycle even before red line in higher gears. If you run short of fuel supply you could find yourself with a lean fuel mixture in the combustion chamber under boost. That's a recipe for detonation and/or melted pistons -- either way it could destroy the engine.

So what should you do? You could run just a turbo-back exhaust, with no intake. On this car, that's what Cobb Tuning calls Stage 2. You can buy a complete kit for that setup HERE. Or, if you really want to do the full exhaust plus the intake, you'll need to add upgraded fuel injectors and fuel pump. We highly recommend the Injector Dynamics 1050x injectors, and the AEM 340 lph fuel pump. Finally, the Cobb fuel pressure regulator kit is a must. There's no off-the-shelf map for this exact setup, so custom tuning would be required.

If you want to go whole hog, there's a complete Cobb Tuning Stage 3 Power Package including all those upgrades -- including the intake, exhaust, and all the fuel upgrades mentioned above -- plus upgraded fuel rails and AN lines. Cobb offers an off-the-shelf Stage 3 Accessport map for this setup, so you would not need custom dyno tuning.

To review: Unlike with older Subarus, the 2015-2020 WRX STI is nearly tapped-out in terms of fuel supply, and if you install both a high-flow intake and a turbo-back exhaust, you will run out of fuel. In order do prevent that you can run an intake ONLY, or a turbo-back exhaust ONLY, but not both together on the stock fuel system. Or you can upgrade the fuel system with larger fuel injectors and fuel pump, and then you'll be free to install all the intake and exhaust upgrades you want.

Happy modding.


2 Comments

Mach V Dan
Mach V Dan

December 12, 2019

Anthony, you really only have to upgrade the injectors and fuel pump at this level. We would highly recommend the Cobb Fuel Pressure Regulator kit, though, which helps prevent resonance issues with the larger fuel injectors.

Anthony
Anthony

December 12, 2019

Hello. I have a 2017 STI and my setup is Tomei headers, Tomei up and downpipe, custom catback exhaust and a blow off valve. All with a Stage 2 Cobb Accessport tune. I also have a Cobb air intake but I haven’t install it yet because I need to upgrade the fuel system. My question is, do I have to upgrade the injectors and the fuel pump or do I have to upgrade the entire system?

Thanks in advance.

Leave a comment

Comments will be approved before showing up.