Subaru Per Gear Boost Compensation


Subaru Per Gear Boost Compensation

Version 1.0



Per gear boost control allows for altering both boost targets and boost (wastegate) controller duty cycle in order to achieve different boost response and characteristics in each gear. Depending on your needs, this can be used to equalize boost levels in all gears, reduce boost in low gears when traction limited, reduce boost in high gears at low RPM to reduce max torque output and so on. Before using these systems, please be sure gears are being properly detected by monitoring Gear position, and operating the vehicle in all gears. If gear ratios have been altered either by internal rebuild or drivetrain component swap, update the Gear Determination tables accordingly (if available). Keep in mind there is a brief delay between shift completion and the ECU registering the new gear position.

0.0% compensation will result in no change. Positive corrections will increase the result value, and negative ones will reduce it. 

Keep in mind there are other factors which still affect both boost target and wastegate duty cycle. Some potential examples are intake air temperature compensation and coolant temperature compensation. Be sure to consider all compensations when working towards a desired target.

Per Gear Boost Targets

02-05 WRX SD ECUs have single cell per gear absolute boost target compensation. (Not available in MAF ECU type)

08+ WRX and STI CCF ECUs allow for boost target compensation based on a 3D RPM vs. requested torque table per gear. (Not available in MAF ECU type)

Compensation of boost target is based on absolute pressure. Lets walk through an example.

Per gear compensations are applied as follows: Pre gear compensation absolute MAP target * ((comp value+100)/100) = Post gear compensation absolute MAP target

If your primary boost target is 20 tapering to 15 psi relative, but in 3rd gear you want to target 5 less psi at peak and 3 less psi near redline here's how you calculate the correction to use:

  • Convert the current targets to absolute pressure: 20+14.7=34.7 psia  and 15+14.7=29.7 psia
  • Next convert your desired 5th gear targets to absolute pressure: 15+14.7=29.7 psia and 12+14.7=26.7 psia
  • Divide the desired gear specific absolute pressure by the primary target absolute pressure: 29.7/34.7=0.85  (I'm rounding to the nearest hundredth to keep this simple)
  • Convert to a percentage by multiplying by 100. 0.85*100=85
  • Since the target table is set up based on a percent compensation, subtract 100 from your percentage to calculate the appropriate percent compensation. 85-100=-15%
  • Next repeat for your redline boost target: 26.7/29.7=0.90     100*0.90=90       90-100=-10%
  • As you can see the corrections at peak boost target and redline are not the same. If you're working with an 02-05 WRX ECU, you'll need to choose a single value and compromise.
  • On an ECU with 3d Boost Target compensation, enter your corrections:

*values shown are the closest possible to the -15.0 and -10.0 we calculated. This will not cause a significant error in our compensation.

    • Then you can interpolate as you see fit for smooth operation:

Per Gear Wastegate Duty

02-05 WRX ECUs have single cell per gear wastegate duty cycle compensation. This table only affects Wastegate Duty Cycle High.

08+ WRX, STI CCF ECUs allow for wastegate duty cycle compensation based on a 3D RPM vs. Requested torque table per gear. This table affects the Low and High Wastegate Duty Cycle tables.

Compensation of wastegate duty cycle is more straight forward, but may require trial and error.

WGDC compensations are applied as follows: Pre gear compensation WGDC value * ((comp value+100)/100) = Post gear compensation WGDC value

In order to achieve our new lower boost targets in 3rd gear, we'll need to reduce wastegate duty cycle. While we can calculate boost target compensations using simple math, adjusting the wastegate duty until our targets are achieved with minimal turbo dynamics correction involves making an educated guess, followed by some trial and error.

In this case, we know we want to decrease boost more near 4000 RPM where peak boost is occurring, and a bit less near redline so I've entered some values to start.

Then you can interpolate as you see fit for smooth operation:

Next you would operate the vehicle in 3rd gear at wide open throttle while datalogging and review results. Watching the turbo dynamics boost error monitor will help determine your error (in psi) from the current target. Adjust wastegate duty cycle compensation and test again until your new compensated 3rd gear boost targets are achieved.

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