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.


8 Comments

Mach V Dan
Mach V Dan

August 10, 2020

Joseph, your car has direct injection, where the fuel rail pressure is many times higher than in conventional port fuel injection, and the fuel injectors are made of special high-tech materials. There are currently no aftermarket fuel injectors made for your car.

Joseph
Joseph

August 10, 2020

Do they make any injectors for a 2016 wrx premium? I can’t seem to find anything on it

Mach V Dan
Mach V Dan

June 25, 2020

Ralph, we have run plenty of cars on 1050cc injectors with stock rails, and they work fine. I wouldn’t say upgraded rails are required until at least 1300cc injectors. 1300cc would definitely be overkill on 93 octane fuel with a stock turbo.

Ralph
Ralph

June 25, 2020

I’m looking at going the route described above. I’m currently tuned at a Stage 2+ with both intake and downpipe minus the fuel upgrades.

At what point would it be beneficial to actually do fuel rails along with injectors?

And would 1300cc injectors be overkill with, for example, a FBO setup? Assuming stock turbo and pump gas.

Mach V Dan
Mach V Dan

February 17, 2020

Matthew, Cobb calls that setup Stage 3, but the hardware would include the rest of the fuel system — regulator and rails. If you don’t have those, the off-the-shelf (OTS) Stage 3 map will not be quite right. and you should have a custom map developed by your local tuner.

Matthew Dronet
Matthew Dronet

February 17, 2020

On my 18 sti I have the cobb ots stage 2 tune and mods. (Cobb turbo back, heat shield, accessport) I bought an intake and I’m getting injectors and a fuel pump. Has cobb made a tune for these upgrades as a “stage 2” that they can offer if contacted or is the stage 3 ots the answer or will I need all the fuel mods to run the stage 3? Or is a protune the answer? Thanks.

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.

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