have a look on boostcontroller.com
here is a snipit from there, i just read a few pages and i am enlightened, and have learnt a few things.
manual boost controller The MBC, which is essentially a valve with an adjuster on it, has two hoses. One hose is installed at a location that provides an accurate boost source. The location that provides an accurate boost source could be any number of placesÂ… a nipple on the compressor housing of the turbo, or another place in the intake system between the turbo and the intake manifold (basically, any location in the "charged" portion of the intake system). Different boost sources have different virtues, so turbo enthusiasts choose varying places to tap their boost sourceÂ… we recommend that enthusiasts of the novice level follow the manufacturer's recommendation as to where to tap an accurate boost source. The other hose is installed at the Wastegate Actuator. In the picture below, the red arrow is pointing to the port on an actuator where the hose is attached. The Wastegate Actuator is the device that opens and closes the Wastegate, typically by means of an internal diaphragm, which cases an arm to move, which opens and closes the Wastegate
With ball-and-spring types, a spring-loaded ball is used to block this delivered boost "signal", until the desired boost level is attained. It is at this point, that the delivered boost pressure is strong enough to push the spring-loaded ball toward the spring and out of it's seat, allowing the signal to pass, and reach the Wastegate Actuator. The boost pressure then presses against the Wastegate Actuator's diaphragm, causing its arm to move, so that the Wastegate is opened. The opened Wastegate then allows the exhaust gases to divert away from the spinning turbine, thus preventing the turbo from boosting higher than the desired level.
The MBC is adjusted by turning a knob (or other adjustor), which varies the load on the spring inside the MBC. By adjusting it so there is more load on the spring, you are 'raising the boost" because more boost pressure is required to move the ball off its seat before the signal can pass to the Wastegate Actuator. By contrast, lessening the load on the spring allows the boost signal to more easily unseat the ball and continue on its voyage to the Wastegate Actuator, so by backing the adjustor away from the spring, you are "lowering the boost". The Joe P MBC, and all Hallman Manual Boost Controllers, are ball-and-spring type MBCs.
With bleeder types, a valve simply "bleeds" off some of the boost pressure that it receives. It always allows some boost pressure to reach the Wastegate Actuator, but the boost pressure that the Wastegate Actuator receives is always less than the level of boost pressure in the charged portion of the intake system (or the boost level delivered to the bleeder-type valve) because this kind of MBC basically is a controllable boost leak. Since the Wastegate Actuator does not receive the "full boost signal", it only opens the Wastegate when the amount of boost that gets past the "leak" is sufficient to force it open. The bleeder-type MBC is adjusted by changing the size of the leak. Closing the leak down lowers boost level, because more of the boost signal then reaches the Wastegate Actuator, opening the Wastegate sooner. Opening the leak wider raises the boost level, as more boost is released to the atmosphere, as opposed to being delivered to the Wastegate Actuator as a boost signal; so the opening of the Wastegate is delayed.
The ball-and-spring type MBC is clearly superior over the bleeder type MBC. The ball-and-spring type does not leak any boost (at least prior to reaching the desired boost level), and then sends an immediate signal to the Wastegate Actuator to open the Wastegate upon reaching the set level. The bleeder type, by design, is actually a boost leak. Typically, for maximum performance, it is best if the charged portion of the intake system has no leaks while it is in a charged state (AKA "under boost") but the bleeder type valve is allowing boost pressure to be released before attaining the desired boost. So, like your Mom or Dad yelling, "We're not paying to heat the outdoors" when you left the door open as a kid, you are in a sense now "paying to boost the outdoors" with a bleeder type MBC. Boost leaks are counter-productive and you pay the price of using a bleeder type MBC by blowing out energy from your closed, charged-air system that would have been better used to cram more air into the intake manifold. Also, Wastegate operation is less efficient with a bleeder type MBC, as some boost pressure is always allowed to reach the Wastegate Actuator. This results in the Wastegate being partially opened at times before the desired boost level, and results in the Wastegate opening more slowly when the desired boost level is obtained.
