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A bleeder valve is simply a pressure divider, analogous to a variable resistor or reostat. The wastegate actuator with its spring is really the regulator. But the MBC alters the feedback signal to the wastegate, so the wastegate just does its thing. Change the MBC, pressure changes. The wastegate does not need to be adjusted. Yes you could adjust it and yes it would alter its dynamic response, but you still adjust the MBC to get the required boost level. So why change it when it is not neccessary.
When the BMC is not a simple bleeder, there is a fixed bleed orifice, and the upstream variable bleed orifice is replaced with a ball and spring, with an adjustment for spring pressure. The ball stops all flow until a certain pressure is achieved, then once it starts to flow, it provides the flow restriction to create the pressure drop in conjunction with the down stream bleed orifice. Some valves have a variable downstream bleed orifice, but the requirement for this is not often seen. It is not easy to do such a two dimentional setup either. Some valves have a both a ball and spring and an upstream fixed orifice. The TurboXS unit has this. I drilled ou the upstream fixed orifice to allow the wastegate to stroke as fast as possible once boost is achieved. This allows the control system to have a faster rise time more suited to the boost rise rates in cold weather. I could adjust the wastegate to a softer setting and then readjust the MBC to suit. What would that do? It would allow the wastegate to stroke open even faster. A lower spring rate in the actuator would do the same. But at some point, you need to worry about the exhaust back pressure on the wastegate valve face area being strong enough to move the valve itself. At this level, analyis and dynamics would actually be quire complex. Note that at 15PSI of boost, the exhaust backpressure at the turbine is probably over 30PSI!
Conventional bleeder valves do create a lot of artifical turbo lag. As boost pressure builds up, pressure is being applied to the wastegate which causes it to start to open before the boost target is reached. This makes the whole thing very sluggish. The BPC control is an electrically controller bleeder valve. And it shares this sluggish characteristic with its MBC bleeder cousin. The Ball and spring and electronic MBC's overcome this problem. The turbos in the Saabs are small and will spool up very fast with the MBX's. You will see....
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