Subaru Accesstuner Tuning Guide Supplement
For 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.
OTHER RESOURCES
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 SUBARU MODELS
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.
CONTACT US
COBB Authorized Protuners are welcome to contact us with any additional questions/concerns or when in need of troubleshooting assistance.
ECU LOGIC CHANGES
LIMITS
The 2.4L WRX has the following notable limits:
Boost reading - this is 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 that the factory sensor is a combination unit that also includes the IAT manifold sensor).
Boost targets - this is limited to a max of about 61.5 psig as sea level when the Boost Targets Max. Limit table is raised from its 24 psi limit.
Load limit - load is limited to just under 4.0 g/rev.
IPW limit - At higher load with richer fuel targets, a hard-coded IPW limit can be hit that will cause IPW to go static in the 7.x ms range and the car to run leaner than expected. This is the same limit that was present in prior DIT that was addressed via the CCF Gen 2 features on the 15-21 WRX. Those features have not yet been applied to the 2.4L WRX ECU.
THROTTLE MAPPING
The 2.4L WRX has some unique changes related to requested torque:
Requested Torque tables are split by the estimated gear position (1st through 6th). A new Gear Position ESTIMATED Req Torque monitor tracks the unique gear position monitor used for table switching.
When transitioning between gears, the Requested Torque (Gear Transition) table is used.
Due to hard-coded limits, the maximum Requested Torque value is capped at ~350 n-m.
TUNING STRATEGIES
MISCELLANEOUS
LIGHT LOAD KNOCK
During testing, we observed that certain 2022 WRX vehicles can demonstrate very significant knock corrections under very “light duty” conditions. This typically occurs when:
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
When this behavior begins:
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 its maximum negative value allowed by the calibration
It will generally demonstrate itself as large spikes specifically in the “Knock Sensor Noise Level Cylinder 3” monitor
These spikes can be reconciled against the “Knock Sensor Level Threshold Cylinder 3” monitor
Any time that the Noise Level exceeds the Threshold, an FBKC increment will occur
Our testing indicated that:
The behavior is not consistent across cars but will be easily observed on the stock (or Stage0) calibration for those cars that are impacted
This issue is not specifically worsened or alleviated by changes in octane; it was observed on 91 octane fuel up to 96 octane fuel
It is unlikely that the noise is being generated by legitimate knock events and that it is mechanical/external noise (or another anomaly)
Many FA24 vehicles, including Ascent, also exhibit the noise spikes on Cylinder 3, but often to a reduced maximum, effectively reducing or entirely negating the severity of the issue
Potential consideration for further investigation:
The VB WRX 6MT features a mechanical vacuum pump for the braking system and brake booster, as does the Ascent
This vacuum pump is attached to the passenger side cylinder head and the back of the intake camshaft “behind” Cylinder 3, which could potentially contribute to the excessive noise being detected on that cylinder
The VB WRX CVT does not utilize this hardware and instead uses an electric brake booster configuration; the cylinder head is fitted with a blanking plate at the factory for this configuration
We were unable to reach a specific conclusion on if this component may play a factor in the issue
Our Stage1 maps have a series of small changes included that help mitigate the frequency of these light load knock events, though larger changes may be needed depending on the vehicle and the specific behaviors observed.
IMAGE: 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.
