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

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 some additional info to save you time and reduce surprises.

• DIT WRX WGDC 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 only guess Subaru had the frequency table present, but never intended on altering the stock value. COBB has corrected this via custom code.

  • This will only affect DIT WRX calibrations with non stock (not 10 Hz) wastegate frequency.
  • The change that has been made to correct this is: Commanded WGDC * (10/WG Frequency) = Result WGDC 
    • This affects WGDC tables and Turbo Dynamics Tables
  • 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.
  • While the wastegate solenoid period concern has been resolved with custom code, boost response as wastegate frequency is changed will still not behave like some other Subarus and vehicles. This is due to an electrical system difference on DIT WRX. Please review the section below for details.

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

  • We have scoped multiple vehicles for voltage and current, and determined the current profile changes significantly with frequency change. This results in solenoid latency changes which alter effective duty cycle. 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 would result in overboosting because the valve barely spends any time closed.

  • If you choose to increase wastegate frequency on these vehicles, I suggest trying 15 Hz (not higher). You will want to reduce WGDC and turbo dynamics values  by the same factor to avoid boost increase and maintain the boost level you desire. As with any boost tuning, we suggest starting with low or even zero WGDC values and working your way up gradually for safety. 

• 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. 

  • 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 as wastegate frequency changes on those vehicles.

• Different solenoids are capable of being driven at different frequencies.
  • 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 for Subaru are designed to operate at 10-30 Hz.  Please refer to your solenoid supplier for specifications.

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