Tech Bulletin - Subaru - Wastegate Frequency & Behavior

Date: 1/17/18
By: Mike McGinnis

Vehicles affected:

• Subaru

Software affected:

• 1.9.1.0-15639 and newer

Firmware affected:

• not applicable

If you have not, and do not wish to alter wastegate frequency from stock, you can skip this entire bulletin. Otherwise, read on.

COBB has exposed the factory wastegate frequency table on a number of Subaru models. While wastegate frequency change does have a predictable result on wastegate solenoid on time, wastegate behavior is not consistent across Subaru models due to differences in the electrical systems. This bulletin is meant to provide additional info to save you time and reduce surprises.

• DIT WRX WGDC (wastegate duty cycle) behavior has been altered from stock in ATP version 1.9.1.0-15639 and higher. This is to correct a concern with the stock Subaru code. The stock code does not account for period correction when wastegate frequency is altered, resulting in longer than expected on times when frequency is increased. We can guess Subaru omitted this functionality because they never intended on altering the stock frequency. COBB has corrected this via custom code.

  • This change will only affect DIT WRX calibrations with non stock (not 10 Hz) wastegate frequency.
  • The code addition (which is already present in stock code for other Subarus) is: Commanded WGDC * (10/WG Frequency) = Result WGDC  
    • This affects WGDC tables and Turbo Dynamics Tables. Compensations are applied as usual to the result values.
  • If you have made a DIT WRX calibration with ATP prior to 1.9.1.0-15639 and re-save it in ATP 1.9.1.0-15639 or higher, our custom code will be applied and boost behavior will change. If you are using higher than stock wastegate frequency, expect boost to decrease. If lower than stock frequency, which we do not suggest, expect boost to increase. Conversely, if you tune a car using ATP 1.9.1.0-15639 or higher, then open and re-save it in a version of ATP prior to 1.9.1.0-15639, the old period calculation will be used so boost behavior will change. For this reason and others, we suggest not going back to using old versions of software.
  • While the wastegate solenoid period concern has been resolved with custom code, boost response as wastegate frequency is changed will still not behave as you might expect. This is due to an electrical system difference on DIT WRX. Please review the next bullet point for details.

• DIT and 02-05 WRX ECUs exhibit similar electrical behavior which results in unexpected boost behavior as wastegate frequency is altered.

  • We have recorded data via oscilloscope and current probe on multiple vehicles. We found the voltage profile and duration is the same across COBB supported vehicles at various frequencies, but the current profile is unique and unpredictable on DIT and 02-05 WRX due to circuitry we imagine Subaru used for flyback protection. The current profile changes significantly with frequency change, causing solenoid latency changes which alter effective duty cycle. To clarify, the ECU is providing low vs. high voltage for the proper cycle time based on the wastegate duty cycle commanded by your tune. However, at 30 Hz, the solenoid closing latency accounts for so much of the total period that almost any non-zero duty cycle may result in overboosting because the valve barely spends any cycle time closed. 

  • If you want to increase wastegate frequency on DIT WRX and 02-05 WRX vehicles, I suggest trying 15 Hz (not higher), but you may have positive results at 20 Hz. You will want to reduce WGDC and turbo dynamics values (by the same amount) to maintain the boost level and PID behavior you desire. As with any boost tuning, we suggest starting with low values and working your way up gradually for safety. Halving WGDC and turbo dynamics values has been a safe starting point when adjusting frequency from 10 to 15 Hz, but your experience may vary depending on the EBCS (electronic boost control solenoid) used.

• 08-14 WRX and STI models have performed predictably at 10-30 Hz while using COBB boost control solenoids. Changing solenoid frequency within that range has not drastically affected wastegate flow. The current profile is stable and unmitigated flyback is allowing solenoid closing to occur more quickly and consistently.

  • You may want to try increasing frequency to 15-30 Hz to reduce wastegate flutter.

• We have not yet tested boost behavior with increased wastegate frequency on 15+ STI or 04-07 STI, 06-07 WRX and other models, but we are confident the stock code calculates wastegate period and on time properly to correspond with wastegate frequency changes.

• Before considering altering wastegate frequency, we suggest confirming the capabilities of the wastegate solenoid being used. Each model has unique characteristics including current draw, opening and closing latency, etc. 
  • The stock solenoids are set to operate at 10 or 15 Hz depending on year, model. You can review the wastegate frequency table value in each stock calibration to verify.
  • COBB's current EBCS offerings have been tested to function properly on each application. COBB EBCS available for Subaru are designed to operate at 10-30 Hz.
  • We have not tested the stock ECU's drive of the EBCS circuit at frequencies over 30 Hz.

We hope this Tech Bulletin helps you to more effectively tune wastegate frequency and correct associated duty cycle and turbo dynamics tables. If you have any feedback, questions, or concerns please let our Subaru Expert Group know.