I made a manual boost controller and will be installed by tee-ing in the wastegate rubber hose. Before installation, what should I set the MBC opening pressure to initially before dialing it in by trial and error with the boost gauge?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Unless you have modified the wastegate, I would set it just below factory settings to make sure you have control of the wastegate. This would be under 9 psi.

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What's the purpose of the MBC - are you trying to limit boost, turn the wastegate into an on/off switch or fool the wastegate into opening later?

Sixto
87 300D

This was from April 2011. The MBC didn't increase the boost one bit, I think it was due to a weak wastgate spring. I got the wastgate cover off and made a clamp and discarded that PITA cover flat spring so I can easily do the multiple adjustments. I ended up having to max out the adjustment and achieved 12 psi of boost. The engine is peppier and it runs awesome!

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked