Subaru Accesstuner Tuning Guide Supplement

2.4L DIT WRX

Introduction

This document outlines some of the unique engine control unit (ECU) logic and tuning strategies relevant to Subaru's 2.4L DIT WRX. It is not the intention of this document to outline every difference but rather the most notable differences (known to date) that can impact your ability to tune effectively. ECU logic for the 2.4L WRX is very similar to the Subaru Ascent and other DIT models.

Associated Information

Please refer to the other DIT tuning supplement guides for additional information not described here (where applicable):

Subaru Accesstuner FA24DIT Tuning Supplement
Subaru Accesstuner DIT Tuning Guide Supplement

Applicable Vehicles

The following Subaru models are applicable to this document:

• 2022 Subaru WRX 6MT

Table Descriptions

Besides this document, an additional resource for ECU logic details are the table descriptions shown in the Accesstuner software. These will show up in the lower left corner of the software for the current table that is selected. If not visible, make sure the "Table Description" check box is selected in the View menu. Additionally, the Help menu in the software contains links to documents that list all of these table descriptions as well as all of the data monitor descriptions.

Ecu Changes

When compared to other FA engine vehicles here are some of the notable differences in operation you can expect

Limits

• Boost Reading (Map Sensor Values)

  • Currently limited to a max of about 61.5 psig at sea level, not including the hardware limit for the factory MAP sensor (which likely is limited to about 24 psig at sea level). We have included the MAP calibration tables to account for the installation of an aftermarket MAP sensor. Keep in mind if you choose to replace it that the factory MAP sensor includes an IAT sensor to read air temperature in the manifold.

• Boost Target

  • Boost targets can go to a maximum of ~61.5 psig as sea level when the Boost Targets Max. Limit table is raised from its 24 psi limit.

• Load Limit

  • Limited to just under 4.0 g/rev.

• Injector Pulse Width (IPW)

  • At higher load with richer fuel targets it’s entirely possible to hit a hard-coded limit for IPW. This will cause the IPW to go static in the 7.x ms range which can cause the car to run a different target than intended (leaner than expected).

• Requested Torque

  • Internal values in the ECU cap the requested torque value at 350 N-m

Throttle Map

The 2.4L WRX has some unique changes related to requested torque:

• Requested Torque

  • Tables are split by the estimated(calculated) gear position (1st through 6th). This ecu utilises a new monitor (Gear Position ESTIMATED Req Torque) which tracks the unique gear position monitor and is used for table switching.

  • Due to internal ECU limits, the maximum Requested Torque value is capped at 350 N-m.

• Requested Torque (Gear Transition)

  • This table is used when the car is transitioning between gears.
    
    

Tuning Strategies

Light Load Knock

During testing, we observed that certain 2022 WRX vehicles can demonstrate very significant knock corrections under very “light duty” conditions. We were able to identify a few conditions that can cause this to occur.

• The driver reduces throttle input after an acceleration event, but does not lift off of the accelerator entirely, such as when reaching desired speed after leaving a stop light or entering the freeway.

• Engine Speed is between 2000 and 3000 RPM

• Calculated Load is below 0.5 g/rev

Some symptoms of this behavior can be

• Feedback Knock Correction (FBKC) can rapidly increment negative into large absolute values

• It will often continue until the driver inputs change, such as increasing or decreasing accelerator position, or until the FBKC value reaches the maximum negative value allowed by the calibration.

• If you keep an eye on the monitor Knock Sensor Noise Level Cylinder 3 you will typically see large spikes in the value.

• If you compare these spikes against the Knock Sensor Level Threshold Cylinder 3 monitor you’ll be able to see that any time the noise level exceeds the threshold, there will be an increment in the FBKC
  

While this behavior was not seen to be 100% consistent across all vehicles, it will be fairly obvious on the stock (or Stage0) calibration if the car does have this issue. We did not see any change (positive or negative) with either higher or lower octane fuel (we tested from 91-96 octane).

It is unlikely that the noise is being generated by legitimate knock events and that it is mechanical/external noise (or some other anomaly yet to be uncovered). Many other FA24 vehicles such as the Ascent also can exhibit the noise spikes on Cylinder 3 but often to a reduced level, effectively reducing or negating the severity of the issue we see on the VB WRX.

Mitigating the Light Load Knock.

Our Stage1 maps have a series of small changes included that help reduce the frequency of these light load knock events, though larger changes may be needed depending on the vehicle and the specific behaviors observed.

Below you can see a Visual representation of the issue occurring on an in-house test vehicle during early R&D efforts. This is on the stock calibration and a vehicle running 92 octane located in Portland, OR. The 3rd graph shows the noise spikes (red) exceeding the threshold (white), the 2nd graph shows FBKC incrementing downwards when those spikes occur, and the 1st graph shows the benign conditions under which it is occurring (Calculated Load below 0.5 g/rev, APP below 15%, etc.). While not shown in this graph, Cylinders 1, 2 and 4 do not demonstrate the noise spikes at all during this event.