This system regulates boost pressure directly based on engine load (throttle position). It is independent of engine RPM, turbo speed, vane position, air filter restriction, exhaust restriction and any engine modifications. No electricity or electronics are involved, though switches and vacuum solenoids instead of valves on the throttle will work exactly the same.
3 stages; 4psi, 10psi and 14.5psi
two roller switches on EGR cam plate (The stock 3-2 valves could be retained with altered positioning)
Manual Air Control Valves- 4LB50, Grainger Industrial Supply
low boost- valves open, not contacting cam plate
med boost- egr cam closes low valve
high boost- egr cam keeps low valve closed, closes medium valve
Small reservoir (vacuum sphere from a chevy) to buffer vacuum pulses from the pump and provide storage volume. (Consistent turbo operation in all conditions.)
A manual on/off switch valve (commonly used on most semi trucks) is used on the dash to "turn off" the turbo during low speed conditions (low loads under 40mph) to maximize economy.
0.8mm orifice is between the switch and vacuum side of the controller to limit return flow to the vacuum system.
Three boost controller valves from Grainger, commonly called a "Dawes device" by the VW crowd. Modified with a copper tube soldered to the outlet for the vacuum hose to connect.
The high pressure regulator has a stronger spring to increase its adjustable range up to 18psi.
A boost pressure signal is supplied from the intake manifold to the underside of the valves.
Vacuum from the supply switch is given directly to the VNT turbo's actuator, closing the vanes.
As boost rises to the set pressure the regulators bleed boost air into the actuator's vacuum hose, causing the actuator to extend and open the vanes so that manifold pressure is equalized with the airflow through the regulator.
The turbo's actuator starts pulling the vane lever at 3inHg, vanes are all the way closed at 11inHg and the vacuum system maxes out at 15inHg.
My controller lives in the cruise actuator's old location, secured by longer bracket bolts, but any location is suitable since its not hard tied to anything on the engine.
The hardware is very durable. As of this writing I have two years of use on the system with no faults. (Update 1/2014, still zero issues)
The last thing I will try is to link the vanes directly to the pedal in an adjustable way regarding movement, then push back the vane lever with up coming boost. The pedal linkage is spring loaded to the vane lever so the boost actuator can push the lever back although the pedal is depressed.
That method has been popular in the VW world. It is obsolete though, it doesn't have any way to limit/vary boost pressure with engine load. Throttle movement only changes how quickly the turbo spools up to maximum boost. Boost will rise and fall significantly with RPM and as a result fuel economy is very poor and turbo response is slow.
Here is an example of that outdated system:
Its only going to be 6CM for about 10-15PSI to get it to light fast, then it will jump to 9CM for 25 - 30PSI then jump to 12CM for 40PSI.
Ramp allows for micro adjustments, PSI is measured in 0.01Vdc increments. When voltage increases with pressure, the controller will make small proportional adjustments to turbo
Jump holds one position until a predetermined PSI is achieved, at that time the controller will move to the next predetermined position. This programming is great for staged turbos.
That is another example of a somewhat recent "solution" by steed. Being set to "steps" causes large jumps in VGT position to fixed points. The result is its little better than a fixed geometry turbine. As soon as it reaches 15psi, the turbo's response is drastically reduced. It is changing position based only on boost output. Boost output in VGT turbos is not based on load, an engine at constant RPM and load can have drastically different boost pressure based solely on vane position. VGTs require load/throttle input to work properly. Without it you end up like steed's system with laggy/jerky response.