How each system works:
2 port system “Bleed System”:
If the turbo outlet was connected by pipe straight to the wastegate actuator the wastegate actuator would always open at its spring rate (normally 0.5 BAR) to achive higher than wastegate ‘spring rate’ pressure. You need to bleed air away from the actuator to prevent it from opening the wastegate at its spring rate pressure. To do this on the 2 port system a ‘t’ piece is inserted in the middle of the pipe that connects the turbo outlet straight to the wastegate actuator, then a pipe is run from this ‘t’ piece to the solenoid.
What the solenoid does is open and close a port to allow a controlled bleed (governed by the ECU-solenoid duty cycle) out of the pipes to keep the air pressure within the pipes below the 0.5 BAR that opens the wastegate. Without the restrictor pill in the first pipe between the turbo and the ‘t’ piece the air passing from the turbo into the pipes cannot be bled off quickly enough to prevent the wastegate actuator from opening the wastegate. The result of this would be poor boost response, so what the restrictor does in effect is cause a pressure drop and slows everything down to allow the solenoid to deal with a much lower/slower volume of air.
The smaller you make the restrictor hole the greater the pressure drop and the easier it is for the solenoid to keep the pressure in the system below 0.5 BAR (spring rate pressure).This will result in higher boost output, a larger restrictor hole the smaller the pressure drop will be and the harder it is for the solenoid to keep the pressure in the pipes below (spring rate pressure) which will result in lower boost output. Do not adjust the restrictor hole size unless you know what you are doing.
3 Port System “Direct Interrupt Bleed”:
Unlike the simple bleed system that 2 port solenoid uses, the 3 port uses a direct interrupt bleed setup. What this means is that the feed from the turbo outlet duct has to pass through the solenoid to get to the wastegate actuator. This is a much more stable way of controlling boost than the 2 port system. How it works is similar to the 2 port as in the idea is to prevent the wastegate actuator being moved until your desired boost level is achieved. To do this the solenoid again bleeds air away from the system using a controlled bleed ( governed by the ECU-solenoid duty cycle).
The restrictor in the bleed/return pipe determines the rate of bleed, a large restrictor hole has a high bleed rate making it easier for the solenoid to keep the system pressure below the actuator spring rate pressure which will result in a higher boost output. A small restrictor has a low bleed rate making it harder for the solenoid to keep the system pressure below the actuator spring rate pressure which will result in a lower boost output. Please do not adjust the restrictor hole size unless you know what you are doing.