With a VGT you can hold the "fins" closed to maintain full boost. For our applications most are setup to release pressure between 12 and 20 psi. As manifold pressure increases and you are nearing the upper limits of your settings, the VGT and VNT turbo will open fins to let off boost. the mad max switch can be installed so that the fins never open and you maintain the boost...

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1983 W123 300D Turbo
2000 F250 7.3 Powerstroke