How Subaru’s Factory Boost Control System Works

Alex · Aug 29, 2007

Thought you may find this an interesting read, its a cross post rom Christian at Cobb and is by no means "recent"

Christian. said:

How Subaru’s Factory Boost Control System Works

Chapter 1 – Hardware

Turbo - An exhaust driven air compressor which consists of four basic sections or components. The compressor section consists of the compressor housing and the compressor wheel. This section is the inlet or intake for the turbo which compresses the intake charge generating relative pressure (boost). Generally speaking the inlet is always in a vacuum sucking air in and the outlet is pressurized with the intake charge. Next is the center section which contains the bearings, shaft, and the oil and anti-freeze passage ways; the compressor and turbine wheels are also attached to this section. The third section is the turbine section which consists of the turbine wheel and turbine housing. This section also contains a machined by-pass for the wastegate valve to seat against. The last component of a turbo charger is the wastegate valve and wastegate actuator which control the wastegate valves movement.

Wastegate Actuator - A spring/diaphragm based mechanism which controls the movement of the wastegate valve. A turbo wastegate is normally closed, forced shut by a compressed spring inside the actuator canister. As pressure is applied to the nipple of the canister the wastegate shaft moves away from the actuator, opening the wastegate valve.

Wastegate Solenoid Valve - An electromagnetic solenoid which controls the air flow from the wastegate actuator to the turbo inlet. This device is normally closed when no voltage is applied. When a voltage is applied, by the drivers in the ECM, to the wastegate solenoid valve fully opens allowing air to pass through the device.

Vacuum Lines - Rubberized or silicone tubes attached to various components in the engine assembly. For this article we will be concerned with the 6 attachment point and the four sections of vacuum line plumbing and adapters which we will cover in Chapter 2.

Restrictor Pill - A small pill made of brass which contains a precision machined lengthwise hole in the center.

ECU - Also known as an ECM, PCM, EEC, EMS. Engine Control Unit which contains the processors, drivers, and logic which is calibrated to control the relative pressure (boost) via wastegate solenoid duty cycle.

Insert Hardware Picture Here

Chapter 2 – Plumbing

We will break down the plumbing of the factory boost control system into 4 sections of vacuum lines/adapters and 6 attachment points. Please look at the below picture where we have the four basic lengths of vacuum line and the 6 attachment points labeled. Three of these lines are pressurized while the vehicle is under load and the one vacuum line which goes to the turbo inlet tube is under a vacuum which is created by the turbo sucking in air.

Insert Vacuum Line Plumbing Picture Here

Line 1 plumbs the compressor housing to one end of the T-fitting. This line contains the brass restrictor pill.
Line 2 plumbs the wastegate actuator to the opposite on end of the T-fitting.
Line 3 plumbs the middle to the T-fitting to the wastegate solenoid valve.
Line 4 plumbs the other side of the wastegate solenoid valve to the turbo-inlet pipe.

Chapter 3 – Hardware Function

Turbo - The function of a turbo is to compress the intake charge creating a greater volumetric efficiency for the internal combustion motor.

Wastegate Actuator & Wastegate Valve - A wastegate actuator's function is to control the wastegate valve thus the exhaust energy being directed into the turbine housing or by-passing the turbine housing. If the wastegate valve is fully closed, more exhaust energy is directed into the turbine housing the shaft speed of the turbo charger will increase and the relative pressure (boost) will increase, all within the efficiency range of the turbo. If the wastegate valve is open the exhaust energy by-passes the turbine wheel and goes into the downpipe so that the turbo shaft speed decreases or remains constant. This will generally lower relative pressure (boost). Below is a line graph demonstrating the rod movement of the wastegate actuator arm as pressure is applied to the canister nipple.

Insert Wastegate Actuator Movement Line Graph

NOTE: The MORE boost you run the LESS wastegate you need/use. So unless you want to run less pressure than stock and/or have un-tunable boost problems, then I would leave your wastegate, the area around it, the turbine housing, etc. alone.

Wastegate Solenoid Valve - The function of this device is to control the amount of air pressure being bleed away from the wastegate actuator.

Restrictor Pill - The function of this component is to limit the amount of pressurized air flow coming from the turbo compressor housing. The pressurized air flowing out of the compressor housing is limited with this restrictor pill so the wastegate solenoid valve and wastegate actuator are not overdriven forcing the wastegate valve to open prematurely.

Vacuum Lines - The function of the vacuum lines is to plump the pressurized air to the proper components so the Subaru boost control system works properly.

ECU - This is the master device which controls the slave device, the wastegate solenoid valve, so the targeted relative boost pressure is obtained.

The factory boost control system bleeds pressure away from the wastegate actuator to the intake or turbo inlet pipe. With this device set at 0% wastegate duty cycle through the ECM calibration, all of the air pressure generated at the compressor housing will be applied to the wastegate actuator forcing the wastegate valve to fully open. When the wastegate actuator is fully open, the vehicle will run mechanical boost pressure which can be anything from 7-11psi on OEM turbochargers. With this device set at 100% wastegate duty cycle through the ECM calibration, all of the air pressure generated at the compressor housing will be allowed to by-pass to the wastegate actuator allowing the wastegate valve to close. This flow is limited by the size of the hole in the restrictor pill located in the first vacuum line attached to the turbo compressor housing. The wastegate valve will only close as much as it can (taking into consideration that the exhaust gas pressure between the exhaust port and the turbocharger is generally greater than the manifold pressure the turbo is generating) with this exhaust gas pressure pushing on the wastegate valve.

NOTE: If you run a turbo charger beyond is compressor efficiency range it will turn into a flame thrower.

Chapter 4 – Tuning Using the AccessECU StreetTUNER Software

Insert close-up picture of restrictor pill here

Mechanical Tuning
Changing the restrictor pill size. The vacuum lines for the factory boost control system contain a small brass restrictor pill located in the short vacuum line coming off the turbo compressor housing. The middle of this restrictor has a lengthwise hole precisely machined to a certain specification so that it works with the factory wastegate actuator and the wastegate duty cycle settings in the stock ECU. The smaller diameter hole in the center of the restrictor pill, the tendency to boost spike will be higher and the less wastegate duty cycle you will need to run higher boost. The larger diameter hole in the center of the restrictor pill, the boost control system will have less of a tendency to boost spike and the greater wastegate duty cycle you will need to run higher boost. Please verify that the vacuum line coming off the turbo compressor housing (prior to the T-fitting) contains a restrictor pill with a hole machined in the center of pill.

NOTE: The larger diameter wastegate valve a turbo has the more difficult it is to stabilize boost pressure as the valve initially opens. This is also true for greater exhaust gas back pressures created by a smaller A/R on the turbine housing.

Insert close-up picture of a wastegate actuator that has an adjustable arm here

Pre-loading the wastegate actuator arm. Adjustment of the wastegate actuator rod (is the rod length is not fixed and adjustments can be made) will allow some additional tuning as well. If the rod coming out of the wastegate actuator is shortened it will pre-load the spring inside the wastegate actuator increasing the pressure level at which the actuator will open and total boost pressure that turbo can generate. If the wastegate actuator rod is lengthened the actuator will decrease the load on the spring and decrease the pressure level at which the actuator will open and total boost pressure the turbo can generate will decrease.

Electronic Tuning Through ECU Calibration -

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.

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.

Copyright © 2006 Cobb Tuning, Inc.
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How Subaru’s Factory Boost Control System Works

Alex Community Founder Staff Member

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How Subaru’s Factory Boost Control System Works

Alex Community Founder Staff Member

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