Controlling all the Boosts
What is a Boost Controller?
Boost controller is a device to control boost pressure produced by the engine in turbocharged engines by impacting the pressure seen by the pneumatic and mechanical actuators.
Types of Boost Controllers?
Their are typically two types of controllers. One is Electronic Boost Controller (EBC) and the other is Manual Boost Controller(MBC). EBCs are either solenoid or stepper motor based. Most factory EBCs are solenoid based. They control boost by bleeding out the boost pressure from line in between the boost source and wastegate actuator. MBCs are typically mechanical ball and spring. They too bleed out boost pressure out of the line. Not really used in OE applications but are very popular in the aftermarket scene for their ease of use.
How does Subaru boost control work?
The stock unit is an electronic 2 port boost controller. Its uses a 2 port bleed type system that uses a restrictor pill in the line going from the turbo inlet nipple to the T.
** Removal of the restrictor is not reccomended at all. It will cause very low boost levels regardless of any tuning. Make sure the pill is in the line when working around turbo or if the lines were ever removed**
The way the stock solenoid works is that the solenoid is open to bleed all boost pressure away from the wastegate actuator to bring boost pressure up. And once boost pressure is achieved than the solenoid closes to send boost pressure to wastegate actuator to open the wastegate. This works quite well on stock applications. But because of the way the T line has the boost nipple and the wastegate actuator in the same straight line the actuator will always see some pressure and will crack the wastegate especially when the throttle is stabbed and pressure is built up quick. This reduces boost response.
Why use Aftermarket boost controllers?
Aftermarket 3 port boost controllers are an popular upgrade in Subaru applications. They convert from the stock bleed style to interrupt style of boost control. So it gets rid of the T line and replaces it with 2 individual lines. One port to turbo boost nipple, second port to wastegate actuator and last one back to stock inlet location. The solenoid stays closed to achieve assigned boost pressure and opens to reduce boost pressure. What this does is that the wastegate actuator does not see any pressure at all when the turbo is spooling. This gives you maximum response from the turbo as the wastegate actuator never sees boost pressure before achieving set boost pressure as it would in the stock bleed style.
Manual Boost Controllers should not be used in Subarus. They cause whats called Part Throttle Full Boost. MBC causes the boost come on full boost even when the throttle is not depressed 100%. It may cause the car to go lean and cause some knock. The Fueling and Timing tables in Subarus are based on RPM and Engine Load. If the tables were RPM/Manifold absolute pressure based than it would have no issue running an MBC. Another disadvantage is MBC do not allow any compensation to air temp, engine temp, humidity or barometeric pressure(altitude).