Stock boost control
The boost supply to the factory wastegate solenoid comes from the back of the plenum/collector. The last nipple under the plenum supplies the wastegate solenoid, and directly to the wastegate actuators.
Boost pressure presses on the wastegate acuators to open them, against base spring pressure.
The way that boost is raised is by venting some of this pressure back to the inlet side of the rear turbo. The more pressure bled off, the higher the boost pressure will be.
The factory solenoid line has a restrictor in it. The restrictor is marked by a yellow ring on the boost line. This restrictor limits the amount of bleed off from the wastegate solenoid. The more pressure you bleed , the higher your boost pressure.
Do you see where this is going ?
The problem with the stock boost control setup is the stock turbos. If you run the boost pressure too high, you will lose a ceramic exhaust wheel. Normally aspirated GT-R's - ones that lose an exhaust wheel- are slow.
If you add a restrictor to the small line under the plenum, the one that supplies the wastegates, depending on the size of the restrictor you can run a lot of boost. With our setup a 1.0 mm restrictor will gave us about 24 psi of boost.