The way this system works is as follows. There is a canister (wastegate actuator) with a diaphram in it. On one side of this diaphram is a spring. A bar attaches to the diaphram on the same side and attaches to the wastegate door. On the other side of the diaphram in this container are two holes with a nipple on each. A hose from one nipple goes directly to the compressor housing which produces pressure in the canister.

When the pressure in the canister (your boost level) exceeds the push of the spring it pushes the diaphram back, compressing the spring and at the same time causing the bar to move. The bar pivots the wastegate door and opens it, diverting exhaust flow away from the turbine wheels. and lowering boost. The other nipple attaches to a vacuum hose which runs upto to a solenoid (one of two mounted on the front of your intake manifold)

This is the wastegate actuator solenoid. When the stock ECU determines that it would like more boost it causes this solenoid to begin pulsing and bleeding air away from the canister. As the pressure is bled away from the canister it lowers the pressure in it and the spring is able to push the diaphram back, which pulls the wastegate door closed or keeps it shut and more or all exhaust is directed to the turbines which then produce more boost. With the solenoid shut once spring tension is overcome the wastegate will open about 7 psi or so.

Here is where the problem lies in attempting to use the stock solenoid to control boost at higher levels. In order to run boost levels up in the 13-15 psi range the solenoid which was designed to bleed enough air for 10-11 psi must increase its bypass amount. So the duty cycle of the solenoid is increased to try and bypass a much larger volume of air then it is designed to bypass, much like there is cap to how much fuel an injector can pass. The result can be as follows, the solenoid locks open and the boost will skyrocket, or the solenoid begins to fluctuate at its limit of pulse which will cause the boost to jump up and down.

This should also explain the purpose behind the jet or orrafice in the pressure hose going to the canister (actuator) it restricts the airflow to the canister. Smaller the orifice the higher the boost will be. No orifice and the bleed line is not capable of bypassing enough air no matter what and the wastegate shoots open right away and very low boost. Someone on the net just did this recently.

So this is my reason for using a manual bleed valve and it should explain how it works as well. Instead of the bleed hose leading to the solenoid it goes into the car and connects to an accurate bleeder valve. The more I open the valve, the more air is bypassed and the higher the boost will be. With the valve closed it will open the wastegate once the spring is overcome (about 7psi).

The other aftermarket boost controls operate on a different basis. They control boost by haviing a second solenoid that regulates the air to the canister (actuator) and I would assume plug the nipple leading to the stock solenoid. So in any case this is why I use the manual control. It is accurate and not prone to electrical glitch, can be controlled while racing the car with a twist of the wrist and best of all cost $50.

IMPORTANT: Please note that all the methods of boost control are subject to one potential failure. If the bleed hose or nipple cracks or comes off your boost will shoot off into oblivion and kiss the motor goodbye because the wastegate will stay closed.