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The following article explains the various boost related maps in your Subaru ECUs and their purpose.

Boost Targets

Boost is a closed-loop system. You set the boost targets to the values you would like to see the car achieve. These values then serve as a guideline to the ECU to tell it if the car is currently over/under boost. From there, you need to modify the wastegate duty maps to achieve the boost limits you set. If you set the wastegate duty too low, then you will not achieve your targets, whereas if you set it too high, you will get boost osciallations as the car tries to remove duty cycle to get back down to the target.

Maps Affecting Boost Targets

Turbo Dynamics

Then there are other maps that will affect how quickly the car will ramp up the wastegate duty cycle (turbo dynamics - burst or coarse). Based on how far you are from your programmed boost target, this map will control how aggressively the wastegate duty cycle will change.

Once your boost has come up to target (or more precisely you have hit your wastegate duty cycle target), the turbo dyanmics - proportional or fine map comes into play. If you go over or under boost from this point, this map controls how quickly the ECU will react and how much off target boost must get before the ECU will modify the wastegate duty cycle.

Compensation Maps

If that weren't enough, there are a bunch of compensation maps for boost and wastegate duty cycle that will alter your boost targets, depending on intake temp, barometric pressure, etc. So if you aren't seeing the values you programmed, this could be the cause.


Finally, other changes in the tune can affect boost as well. AVCS can change spool characteristics as well as the VE/compression of the motor, so that will affect boost. Also, different AFRs can affect boost as well. Leaning the car out during the spoolup period will add more heat to the exhaust gas, causing the turbo to spool faster.

List of stock boost control maps

The stock boost control for MT is using the following tables :

  • a boost target 3D map (absolute pressure target)
  • an initial duty 3D map (% solenoid duty to start with)
  • a maximum duty 3D map (% solenoid not to exceed)
  • a proportionnal continous compensation 2D table (% duty correction/boost delta error)
  • a positive integral 2D table (% duty error learning/positive delta)
  • a negative integral 2D table (% duty error leaning/negative delta)
  • an atm. pressure based duty compensation 2D table
  • an intake air temp. based duty compensation 2D table
  • a coolant temp. based duty compensation 2D table
  • a gear compensation table (based on 5 gears boost % correction, even with 6MT)

Testing it out

Boost is really the hardest part to get right. After you get all of this correct for a WOT pull, then you get to check things like how boost reacts at mashing the throttle at 3000 rpm, 4000 rpm, going 1/2 throttle at 5000 rpm, a run through all of the gears, etc. There are always compromises to be made. My car will spike a few psi if I got WOT in 5th gear, but I never do that, so I prefer the boost response in the lower gears.


To make a long story short... The boost target 3D map is the target. The initial wastegate duty is the map providing % duty applicable for a certain rpm and tps (these are the axis applicable for all 3D boost tables, even if scaling can be different), this duty is then adjusted by the ECU to stick to the target boost, within the max duty values applicable in the other 3D, with regard to the proportional continuous rules and to the error learned in the integrales...

On top of this basic routine, you can find some additional corrections, as listed above (I think most of these are explicit)...

Information source: [crazymike] OpenECU thread MY05 OSECU Custom Stage 2

Page last modified on July 18, 2006, at 08:34 PM
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