Tech Bulletin - Targeting High Boost Levels

Date: 10/09/19
By: Mike McGinnis

Vehicles affected:

• Subaru

We are excited to see you pushing the boundaries of the Subaru platform and want to assist with part of that process, targeting high boost levels. While the Boost Targets table may seem to limit you, here are some tips to target even higher boost levels.

As your foundation for high boost projects, we suggest using a CCF ECU type when possible. If CCF is not available for the vehicle, the SD ECU type may allow for higher boost targets than the MAF ECU type.

Next, here are vehicle specific tips to target high boost levels:

2.5L Turbo Subaru ECUs supported by COBB Tuning to date:

• Some 2.5L turbo ECU types allow a value up to 48.75 psi in the Boost Targets table, some 64.99, some 75.
• In order to target more, you can employ a table called Boost Targets Compensation (Coolant Temp). Positive values increase boost targets, making them higher than the Boost Targets table alone can allow.
• The Boost Targets Compensation (Coolant Temp) goes up to 99%, allowing for significantly higher boost targets. When using this compensation, remember it works off absolute pressure, using 1 atm (14.7 psi reference). For example, if you are working with one of the few cars limited the most (to 48.75 psi Boost Targets table value), a 99% compensation would support up to 111 psi boost target.
• The math is as follows:
  • (Boost Targets Table Value + 14.7)(Boost Targets Compensation (Coolant Temp) Value / 100 + 1) - 14.7 = Boost Target
  • (48.75 + 14.7)(99 / 100 + 1) - 14.7
  • (63.45)(1.99) - 14.7
  • 126.27 - 14.7
  • 111.57 psi boost target
• Keep in mind other compensations may be active i.e. IAT, by gear, but my goal here is to show how you can target high boost levels using this single compensation table.
• Compensations CAN be combined in extreme cases if further increase of boost targets is required.

02-05 WRX:

• The COBB SD ECU type allows increased Boost Targets table values, up to 49 psi. In order to target boost beyond 49 psi, the Boost Targets Compensation (Barometric) and Boost Targets Compensation (Intake Temp) tables can be used.
• The math works just like the example above in the 2.5L turbo section, with 99% in the Boost Targets Compensation (Barometric) table bumping the 49 psi boost target limit up to 112 psi of boost.
• Keep in mind other compensations may be active i.e. IAT, by gear, but my goal here is to show how you can target high boost levels using this single compensation table.
• Compensations can be combined in extreme cases if further increase of boost targets is required.

DIT Subarus: 

• The DIT WRX v4.0 Gen2 CCF ECU type raises the factory boost target and monitoring limits from ~24 psi (relative at sea level) to a new limit of ~87 psi (relative at sea level) or from ~39 psi (absolute) to ~100 psi (absolute).
  • All of the extended boost tables can be found in the "COBB Custom Features" → "Boost Extend Tables" folder in Accesstuner. These tables can also be found in their traditional folder locations, and within the "Flex Fuel Tables (Group A)" and "...(Group B)" folders if they are flex fuel enabled
  • You will also need an aftermarket MAP sensor with the appropriate higher range (and correct sensor calibration in the map) in order to read/target beyond the factory limits.
• For other DIT ECU types that do not have this extended boost feature, the ECU hard limit for boost targets and boost monitoring is ~24 psi (at sea level):
  • You can rescale the MAP calibration and all MAP based tables in similar fashion to what was required to work around the old 02-05 WRX MAF limit.
  • For example you can scale a 4 bar MAP sensor for 2 bar range, or 5 bar MAP sensor for 2.5 bar range so you won't exceed the 2.68 bar limit. If you skew the MAP sensor scaling in this manner, actual boost targets would be double the Boost Targets table value (if no other compensations are active), allowing 4 or 5 bar range of targets, limits.
  • Keep in mind this would impact everything that uses a MAP reading so all associated tables would need updated.
  • This method could also be employed on the Gen2 ECU type in a similar manner if its higher limit of ~87 psi is still not enough.

Boost Limits:

When targeting or otherwise operating at high boost levels, you may not be able to set the boost limit high enough to avoid achieving it during intended operating conditions. In this situation, you can use the fuel cut delay table to delay or essentially disable the boost limit. That said, usable limits are relatively high and will rarely be insufficient (DIT aside for now). Example usable boost limits are:

• 02-14 WRX, 04-17 STI, 04-13 Forester XT, 05-12 Legacy GT - 80 psi
• 18-19 STI - 90 psi
• 15-19 WRX - 24.3 psi (planned to increase via custom code in the future)

We hope this Tech Bulletin helps you achieve your high boost and power goals. If you have any feedback, questions, or concerns please let our Subaru Expert Group know.