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Building a MBC
The theory is this: Boost enters the MBC, the ball bearing is held down with the tension of the spring. Once boost reaches a certain level the spring will start to give-way and the ball bearing will raise. Boost will then be able to exit the MBC and enter the wastegate where it will open the swing-arm. The longer the wastegate stays closed, the faster your spool is. Since an open wastegate means less exhaust propelling the turbine.
Building a MBC is easy-breezy. A trip to a local hardware store will get you everything you need.
1 - Allen Head Bolt
1 - 1/8" Nut
2 - 1/8" Slip-On Brass Fittings
1 - Ball Bearing
1 - Spring
1 - 1/8" Female Brass Tee
Make sure the ball bearing makes a good seal in the passageways of the brass tee. Otherwise boost will travel around the ball and it will not work so good. Assemble as shown in the diagram below.
The MBC cannot lower boost less than what the wastegate spring is rated for.
For good measure buy a few springs of various tensions. They may have to be trimmed to provide to right load against the ball bearing. A few short runs out on the street will let you know, back the adjustment (allen head) out all the way to put as little load on the ball as possible. If you boost too much, trim the spring a few coils. Do this until you get it just right, then tune accordingly. Presto, $10 MBC.
If you have trouble controlling high levels of boost try a small bleeder hole (1/32") on the outlet side:
The bleeder hole is not large enough to affect the signal to the wastegate but it gently vents off some excess pressure in the line which causes boost spikes and such.