2015 STI Development : Staged Power Package Evaluation


As you may know, one of our public commitments following the release of the new 2015 model year VA-series chassis code Subarus (2015+ WRX and STI) was to thoroughly explore each platform and quantify our findings whenever possible.  Because the 2015 STI retained a very similar mechanical configuration to the 2008-2014 STI (GR-series chassis code), many in the community have anticipated that hardware and tuning options would be identical to the current offerings.  The idea behind our testing was to discard any existing assumptions about how the 2015 STI may compare to its older brothers and make sure that we are providing hardware and tuning solutions that exceed expectations.

Two of the prevalent issues with the GR series WRX and STI have been a “Boost Creep” problem and a fuel system that is at or beyond capacity on cars that are equipped with a turboback exhaust, especially when combined with an upgraded free-flowing intake.  This topic was previously discussed here: Subaru Tuning : A Quick Look into Boost Creep.

The key item of significance here: Airflow increases must be met by increases in fuel flow – AFR is a ratio air against fuel, by definition, and thus they must increase proportionally with each other.  We’ve previously explored how adding an intake increases airflow on the 2015 STI in a previous article, available here: 2015 Subaru STI Development Update – COBB SF Intake Testing.

COBB Tuning’s 2015 STI undergoing dyno testing a COBB Tuning Surgeline in Portland, OR

Goal:  Analyze fuel system performance when attempting to tune for a COBB Turboback Exhaust and COBB SF Intake System on a 2015 STI.  Will the factory fuel system be adequate to support this setup?

Vehicle:  COBB Tuning R&D 2015 STI (#2).  ~900 miles on odometer.  92 octane pump gas.

Method:  In order to gather numerical data for analysis, we collected datalogs from the car under wide open throttle (WOT) dyno runs, in the following tune/hardware configurations:

  • Stock (S#); Stock
  • COBB v340 Stage1 93 OTS (S#); Stock
  • COBB v340 Stage2 93 OTS (S#); COBB Turboback Exhaust
  • COBB v340 Stage2+SF 93 OTS (Experimental) (S#); COBB Turboback Exhaust + COBB SF Intake System

Conditions:  45-48 degrees F, Sea Level elevation.  COBB Tuning Surgeline, Portland, OR.

Background:  In general, while performing aftermarket tuning, a fuel injector is considered to be at their flow limits once reaching ~95% Injector Duty Cycle (IDC).  Beyond this, the amount of “off” or non-driven time for the injectors between injection events is inadequate for allowing the electrical coil within the injectors to fully discharge.  With this, the beginning of the next injection event comes earlier than anticipated, as the injector is already partially charged when the ECU begins driving it for the next injection cycle, and the net result is a flow non-linearity (usually demonstrated as a brief rich dip in air-fuel ratio).  After passing roughly 105% IDC, even with this rich-leaning non-linearity, the total window for injection in terms of time is now simply too short, and an inadequate amount of fuel is delivered to maintained the desired air-fuel ratio, so air-fuel ratios begin to go lean of targets.

What does this mean?  Past ~95% IDC, we mechanically lose control over fueling and air-fuel ratio will not be on-target.  Dangerous lean conditions will result past ~105% IDC.

Things to keep in mind:  Air density is a critical factor.  We intentionally tested in Portland winter months, where temperatures are cool and atmospheric pressures are high.  At elevation and high ambient temps, the concerns mentioned above become reduced as air mass is reduced.  Keep in mind, however, that it was still in the mid 40’s during this testing.   Injector Duty Cycles could easily be 5-15% higher if the testing were completed in a much colder area around the United States, such as New England, which has seen a long period well below frozen this winter.

Results:  OK, time for the relevant stuff.  This chart shows the logged Injector Duty Cycle during a full-throttle dyno run for each of the aforementioned configurations.  We’ve added a red line showing the 95% threshold we would ideally like to stay below.

Graph of Injector Duty Cycle (Y-axis) against Engine Speed (X-axis) for each configuration
Graph of Injector Duty Cycle (Y-axis) against Engine Speed (X-axis) for each configuration

As we can see, the issue is already borderline on the completely stock vehicle.  Subaru targets an extremely rich air-fuel ratio and the 2015 STI has been observed to be the most powerful STI to date; one has to wonder why they have not upgraded the fuel system’s capacity to match this.

With tuning (Stage1), we see that peak values are similar to stock.  We are targeting a leaner AFR but using more boost/airflow to make more power, so overall total injector usage remains similar.

At Stage2, we’re really getting up there.  The fully upgraded post-turbo exhaust hardware facilitates more airflow and we use even more injector as a result.  No NASA scientists needed on this one!  However, while Injector Duty Cycle does definitively now exceed 95% for a good portion of the run, it stays around 100% peak.  We have lost control of fueling, but we probably are not going to induce lean conditions yet, meaning the engine should not be at any significant new risk.

Stage2+SF: Uh-oh!  Now that the intake side has been freed up, we’re using a LOT more injector.  With Injector Duty Cycles peaking around 107%, we can no longer be sure that there is enough fuel available to keep a ball-park safe air-fuel ratio, let alone precise control over it.

As mentioned earlier, this would only get worse as ambient temperatures go further downwards.  While mid-40’s is definitely cold for all of the Arizonians reading along, Michigan residents are likely nodding along in full agreement.  Much of the U.S. sees sustained temperatures well below 40 F during their colder periods of the year.

Summary:  So, where does this leave us?  In short, it means that we (COBB Tuning Subaru Experts) are not comfortable offering “Stage2+SF” as a released stage package for the 2015 STI.  We know this is likely to disappoint some and confuse others, especially since the package is still offered for the GR vehicles.   However, not all is lost.  Thanks to our partnership with Injector Dynamics, we are well-positioned to now offer OTS packages that include fueling system upgrades, which have typically been “custom tune only” territory for Subarus.  Be on the lookout for an upcoming COMPLETE Stage3 package for 2015 STI that includes fuel system solutions in addition to full turboback and intake hardware!

54 responses to “2015 STI Development : Staged Power Package Evaluation”

  1. So how is the 2015 WRX Intake/Airbox Combo coming along? With the last blog update I thought it was being released in February. Oh well, just excited to see the power gains you guys pull off with it. Was real curious on the ballpark power gains you are expecting with the intake on the FA engine.

    • Hey Kevin, we’re still working on our intake design for the 2015 WRX and are hoping to have something available in the near future. As has been true with the Subaru platform for some time, the power gains from simply adding an intake are marginal. Our main goal with Subaru intakes is to reduce pre-turbo restrictions allowing the turbo and engine to make power more efficiently.

      [email protected]

  2. When do you think Stage 3 will be release for the 2015 STi, I’m happy to work with you on testing – I’m up in the Northeast

  3. Will the fueling upgrades include the Cobb regulator kit? How about adding the EBCS to the 2015 OTS maps as well?


    • We will definitely offer a Staged Power Package that includes the FPR kit. Not sure about the plans for OTS support for EBCS, the factory solenoids are quite good on the newer cars so an upgraded part is not needed right away.

      [email protected]

  4. Hi,

    I just bought my first sti, and I was wondering what would be the best bet for intake and exhaust from cobb ? Its a 2008 hatch. Ive read that I have to have it tuned even if its just a Cobb intake. Is this true?


    • Hey James, you’ll definitely need to have the car tuned whenever making changes to the intake. Any time you alter the way air flows over the MAF sensor the ECU will need to be properly calibrated. We do offer Off The Shelf maps that will support our COBB SF Intake and the AEM Cold-Air Intake systems for your vehicle. If you were to add an Accessport into the mix you would have those maps as a solution!

      [email protected]

  5. What type of power are you looking at getting in your preliminary tests with the Stage 3 Package for the 2015 STI?

  6. What are the numbers for the fuel pressure required with just the air intake cobb access port tuner and blow off valve stock STI exhaust. I ordered my access port tuner and I am in colorado at 5500 feet. we only have 91 octane here and I would be putting the cobb air filter and box with a stage 1 tune at 91 octane here in Colorado at 5500 feet. do you think i will need to upgrade my injectors and fuel pump to handle that small of a change
    thank you

    • Hey Nick, you should be just fine with that setup in your location. As you’re probably aware, there is a bit less oxygen at that altitude so the fuel requirement is also a touch less. Our Stage1+SF map should be just fine for your car. If you’re concerned at all, feel free to send a datalog over to [email protected] for us to evaluate.

      [email protected]

  7. Where can i find the stage 2 maps for STI 2015 with a AEM cai, I have the access port with a DP stock intake for now and have the CAI waiting for a map

  8. So what i gather here is that I should hold off getting any kind of turbo back exhaust parts for my 2015 STI if I leave my Cobb SF intake on with Accessport until there is a MAP and fueling resolution…correct?

    • Correct. The stock fuel system can handle an intake OR turboback exhaust, but not both. We’re finalizing some details with our Stage 3 Power Package right now and should have that available here in the next month.

      [email protected]

  9. So I have a 2015 STI with an accessport +Cobb SF intake…what you are sayin its NOT safe to add a Cobb downpipe with stock exhaust without a fuel pump upgrade which in the works…?

    • Correct, the factory fuel system does not have the capacity to support both an intake and upgraded downpipe. We are developing a Stage 3 Power Package that will include fuel system upgrades along with a downpipe upgrade.

      [email protected]

      • I remember awhile back you were telling people that the 2015 STI could either support a SF intake, or a Down pipe, but not both due to some fuel problems. Has this problem been resolved.

  10. How is the development on the stage 3 package looking? I know it was previously stated you were hoping for a March launch date. Is that still on track?

  11. Hey guys I want to know what the fuel upgrade parts are going to be needed as far as injector brand and size and fuel pump and lines?

    • We made some adjustments to the fuel pump we will be offering however I believe testing has been completed on the latest revision. Hoping to see a release by the end of this month!

      [email protected]

  12. I know that the stage 3 is not out yet for 2105 sti, but are the fuel components available to purchase now. I don’t see the fuel pump under the products.. would like to purchase products now so I have when stage 3 is released

  13. I was wondering what would be included in the stage 3 power package? I know the upgraded fuel pump/injectors/regulator and downpipe is included, but will it be like the other packages that come with the AP + intake/tbe or anything? Trying to plan ahead but I’m sure if I should wait for the package to buy any parts or if I can piece some stuff together now. Thanks!

        • We’re shooting for this to be available here in the next couple of weeks. We’ve made a last-minute change to a boost reference line for the FPR kit and need to verify that on car before releasing.

          [email protected]

          • what all with the updated kit include/require? intake/downpipe/exhaust/injectors/fuel pump/fuel rail?

            what if you have the SPT catback exhaust?

          • The Stage 3 Power Package will include an Accessport, SF Intake/Airbox, Turboback Exhaust, Fuel Injectors, Fuel Pump, and Fuel Pressure Regulator Kit. You would be fine to continue using the SPT catback exhaust, you would just want to purchase the rest of the Stage 3 components separately.

            [email protected]

  14. This package is terribly overdue, iv have been patiently waiting since mid march and still have no idea when to expect this to be released…. maybe brand switch may be in order.

    • Hey Michael, I definitely understand your frustration. We’ve unfortunately been dealing with some supplier issues with a few key components for the fuel system upgrades that are required for Stage 3 Power Package.

      [email protected]

  15. Hey there, I’ve got a couple of questions for you:
    1. Would one be able to run the stage 3 map with the SF intake/box, fuel injectors, fuel pump, FPR, and JUST the downpipe (i.e. just do the downpipe and not the rest of the turboback)?
    2. Do you guys plan on making an AOS for the 2015 STi?
    Thanks in advance for your response!

    • Hey Philip!

      1. Traditionally using a factory catback with other bolt-ons has been okay. We haven’t done any testing with that configuration, however after speaking with our lead calibrator it would likely be just fine. If anything, you may fall a touch short of target boost with the factory catback being marginally more restrictive than an aftermarket system.
      2. We don’t have any current plans to develop an Air Oil Separator. We currently recommend the IAG unit, a number of us have them on our personal vehicles and they perform flawlessly.

      [email protected]

  16. So Excited for the stage 3! You guys rock at keeping the roll subaru and I cant say thank you enough! I guess the only way i can fully say thanks is to buy your stage 3 kit!
    keep up the amazing work COBB guys!!!! 🙂

  17. Will an aftermarket exhaust manifold be compatible with the Stage 3 map? Any updates on the release?

    • Hey Joe, our Stage 3 OTS Map will not be compatible with an aftermarket exhaust manifold, you would want to have the car custom tuned for that configuration. We are very confident in being able to release the full Stage 3 Power Package and OTS map in the next couple of weeks.

      [email protected]

    • Hey Brian! We are wrapping up development of updated Stage 3 OTS maps and Stage 3 Power Packages. The upgraded packages will include new fuel rails, lines, and the recently released 1050X fuel injectors. I would expect those to be available this Fall.

      [email protected]

  18. So I have been running stage 2 with a Cobb intake and the aftermarket exhaust for almost a year and I nor anyone else has had any problems, or at least noticed any. Do you guys think the car is at risk at all and what do you guys think is the best route of the few options I have?

    • Hi Jonathan,

      It’s certainly a risk to drive a car that is potentially demanding well over 100% IDC at high load. Naturally, the severity will vary from car to car and location to location but it’s not something I would gamble on. There are only two solutions. First would be to upgrade the fuel system and other supporting parts to run our stage3 setup (or some kind of custom setup with a ProTune). Second, I would temporarily revert to the stock intake until you are ready to upgrade other parts later.
      [email protected]

  19. Hi Adam,
    New owner to wrx/sti last 3 months.
    I’m getting a borla catback soon and will get an accessport and have it mapped to stage 1.

    When I eventually decide to go to stage 3, I get the required kit you sell, cobb air and the highflow catted downpipe seperately I can then get it all fitted and tuned, is that correct please?

    • Hi Adam,

      Congrats on the new Subaru!
      Ultimately the answer is yes, but there are a few details to keep in mind. Your Borla CBE should fit with our downpipe assuming it fits well with the stock downpipe. Also, since you already have a cat-back and Accessport. Our full stage3 package will include those and can only be sold as a complete unit, but we do sell our fuel system components as their own packages and the intake/airbox as a combo. So you can still save a few dollars by getting those together but just makes sure you compare to the full stage3 so you don’t miss anything.

      Let me know if you have any other questions.

      [email protected]

      • Hi Adam,
        Thanks for your reply.
        What horsepower estimation atw would you think I expect on the Access port stage 3 on the stock 2018 ej257 sti motor? And I would expect it to last for at least 70,000 miles reliably, as I wont thrash it. Thanks in advance.

        • Hey Adam,

          Based on environmental conditions (temp, humidity, elevation, etc.) and fuel type, I would expect to see a Stage 3 setup produce right around 300whp and around 330wtq. With proper calibration and maintenance, this setup should definitely be reliable! For more info on power gains and our fueling options, check out this release article: Subaru STI Stage 3 Packages and OTS Maps.

          [email protected]

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