You must be made aware that tuning the boost control system is the most difficult tuning you will perform on your Subaru. TUNING THE BOOST CONTROL SYSTEM IS ALSO GOING TO TAKE THE LONGEST TIME TO COMPLETE. Although, once you are finished tuning your boost control system you will be very appreciative of the complexity and capability of the OEM boost control system. The OEM boost control system is much faster than any human input so we highly suggest you start with lower wastegate duty cycles than you may need and work your way up from there. The boost curve and the stability of the boost curve must be established in order to allow you to properly tune all other tables from this point on. The MAF signal has a major influence on the ignition advance and fuel curve as this signal is the major component used by the ECU to calculate engine load (and in turn the fueling and ignition calculations).
The OEM Subaru boost control system employs a closed-loop, targeting system for tuning boost. You must first establish your boost targets in the Boost Targets table. The values in the Boost Targets table are in relative pressure, Bar or Psi. Running these boost targets is going to be the primary goal for the ECU. The ECU will start of with using the wastegate duty cycles established in the Wastegate Duty Cycles (Low & High) table(s). Some Subaru ECUs use a single Wastegate Duty Cycles table and some use two, Wastegate Duty Cycles Low and Wastegate Duty Cycles High. If your ECU uses the Low and High Wastegate Duty Cycles tables we suggest you set your Wastegate Duty Cycles Low table ~8% lower than the Wastegate Duty Cycles High table for the corresponding Throttle Position and RPM cells. The ECU will then use the Turbo Dynamics tables to adjust the wastegate solenoid duty cycle in order to achieve the dictated boost target. Other compensatory boost and wastegate tables are also used by the ECU to fine-tune boost for environmental changes, temperature, barometric pressure, etc. Although, these tables should not need to be modified when using the stock boost control solenoid. If the wastegate duty cycle values are too low, you will not achieve your target boost pressure. If the wastegate duty cycle values are too high, you will overshoot your boost targets and potentially damage the engine. We do not suggest you run a wastegate duty cycle of more than 95% to prevent overheating or lock-up of the wastegate solenoid, and to promote the longevity of the wastegate solenoid.
While tuning the boost control system you will want to datalog and/or view your RPM, Throttle Position, Wastegate Duty Cycle (WGDC), Relative Pressure, and Turbo Dynamics values so you can see what your ECU is actually doing to achieve its boost targets. The further your turbo dynamics value is from zero, negative or positive, the further away your engine is from achieving the dictated boost targets. Your boost targets are either too high or too low for that particular RPM and TPS point to achieve your boost target or your wastegate duty cycles are not close enough to where they need to be in order to achieve your boost targets. We suggest you tune so that your turbo dynamics value is a small positive number (8 or less for the DBC 2.0L vehicles, 100 or less for the DBW 2.5L vehicles) across the RPM range. We suggest you tune your boost control tables in this manner so that if you accidentally overload the engine (with additional passengers, going uphill, towing, flooring the car in higher gears at lower RPM, etc.) you have some protection against over-boosting.
If your datalogged turbo dynamics value is a negative value then your ECU will remove wastegate duty cycles to hit your boost targets because the engine is over-boosting; the pressure measured at your intake manifold is higher than what is dictated in the Boost Targets table for the given RPM and TPS values. If your boost is surging up and down or then your boost control is searching because it is grossly overshooting its target. If your datalog shows a negative value for turbo dynamics then the ECU will use the additional authority in the negative portion of the Turbo Dynamics tables to lower WGDC until the target boost is achieved.
If your datalogged turbo dynamics value is positive value then your ECU will add wastegate duty cycles to hit your boost targets because the engine is under-boosting; the pressure measured at your intake manifold is lower than what is dictated in the Boost Targets table for the given RPM and TPS values. If your boost is lower than what is dictated in your Boost Targets table then you will need to increase your WGDC for the given RPM and TPS values. If your datalog shows a positive value for turbo dynamics then the ECU will use the additional authority in the positive portion of the Turbo Dynamics tables to increase WGDC until the target boosts are achieved.
If you are increasing or holding wastegate duty cycles steady and boost is dropping then you have most likely reached the threshold of the mechanical efficiency of the turbo or your exhaust gas back pressure prior to the turbo is too high and is forcing the wastegate valve to open.
If you are having a small boost spike you may need to decrease the WGDC percentage a few hundred RPM prior to the over boosting event to allow the exhaust energy to be released past the turbine wheel.
NOTE: With porting a wastegate, you are trying and make the wastegate valve function potentially work better which means that your turbo is going to lower boost super fast when the wastegate door/valve opens or not run as much boost as it was engineered to. If you make your wastegate react quicker then boost will be very difficult to stabilize and reach peak #s at an earlier RPM. If you make the wastegate flow better, then the exhaust energy your turbo needs to make and maintain boost will have less opportunity to flow across the turbine wheel. Generally speaking, air/pressure/exhaust gases will always flow along the path of least resistance. Not bashing, just trying to give you a different perspective.
The remainder of this document is intended to demonstrate how to measure the wastegate pre-tension using a vacuum pump. Your WG actuator arm may need to be adjusted (shortened for more pre-tension and lengthened for less pre-tension) in order to achieve proper wastegate actuator movement. These measurements are all static measurements which do not take into account the effects that exhaust gas back pressure prior to the turbo (temperature, exhaust restrictions, etc.) will have on the movement of the wastegate valve.
The wastegate actuator is the bronze item with the “W” stamped on it in the above picture.
While the turbo installed on the vehicle, we suggest you expose the wastegate actuator rod so you can easily see the wastegate rod movement. Now you will need to remove the stock vacuum line attached to the WG actuator. Replace this line with the vacuum line from your vacuum/pressure pump.
A standard Mityvac™ vacuum/pressure pump can be used.
Plumb the vacuum line so it goes directly from the pump nozzle to the wastegate actuator nipple.
Be sure to set the pump so it is in pressure mode.
Now you can slowly pressurize the wastegate diaphragm; at some pressure level, you will see the wastegate rod move. Below is a graph of the WG rod movement for a 2006 WRX MT.
As you can see the WG rod begins to aggressively move at ~7psi. The pre-tension measurement for the WG actuator would be ~7psi, which means if you connected this actuator directly to a high-pressure source on the turbo compressor housing, your vehicle would produce around ~7psi of turbo boost (manifold) pressure.
To summarize, you have A few mechanical options for fine-tuning your boost control system.
Option 1 = Restrictor Pill Sizing, you can change the size of the restrictor pill employed in your system.
- A smaller restrictor pill will allow the system to generate greater boost pressures with the same or less WGDC, the trade-off is that the smaller restrictor pills can potentially induce boost spikes. These spikes can sometimes be tuned out by greatly lowering WGDC a few hundred RPM before the boost spike occurs.
- A larger restrictor pill will force the system to use more WGDC to achieve boost, which makes the conditions safer when you lose a section of vacuum line and the turbo goes into an overrun condition.
Option 2 = Wastegate Actuator Pre-Tensioning, you can change the amount of pre-tension on your WG actuator.
- Creating greater pre-tension will allow the system to generate greater boost pressures with the same or less WGDC, the trade-off is that the greater pre-tension can potentially create a phenomenon known as “boost creep” by not allowing enough exhaust gas energy to by-pass the turbine housing. On rare occasions, this boost creep condition may be tuned out by greatly lowering WGDC or setting the WGDC to zero at higher RPM.
- A larger restrictor pill will allow the system to use more WGDC to achieve boost, which makes the conditions safer when you lose a vacuum line and the turbo goes into an overrun condition.
Below are the various plumbing diagrams for the different types of electronic boost control systems. We will start with the more common, internally wastegated system in which the wastegate assembly is a design element of the turbocharger itself. Subaru employees a bleed-type boost control system (as opposed to an interrupt type) for controlling the internal wastegate systems. The logic present in the Subaru ECU is capable of tuning either internally or externally wastegated systems as long as they are properly set up. Please refer to the below diagrams to make sure your boost control system is mechanically set up in the proper manner.
The operating voltage for most EBC solenoids is usually 12 Volts, and the polarity is unimportant since the solenoid will require +12V power and the other wire is grounded by the ECU. When the solenoid is energized, ports 1 & 2 are connected allowing air to flow between them; when de-energized, ports 2 & 3 are connected allowing air to flow between them. Please take this port diagram into account when reading the below plumbing instructions.
NOTE: As a rule of thumb, you can generally only create turbo boost pressure which is twice your mechanical wastegate spring pressure through electronic wastegate manipulation. In other words, if you have a 7psi wastegate spring (in your external wastegate) or you have internal wastegate that is pre-tensioned to 7psi then you should only be able to create around ~14psi of peak boost pressure by locking down your EBCS @ 100% WGDC. Of course, your actual results may vary based on how well you have located your external wastegate, or how well the internal wastegate is ported, what size of restrictor pill you are using, what your turbine A/R is, etc.