I'm pretty sure I already know the answer to this but I wanted to feel out your guys opinion. Car is 1JZ-GTE swapped with stock twins.

I recently threw a Turbosmart single stage MBC onto my stock twins. I T'd the wastegate lines to one side of the MBC and then blocked off the rear turbo pressure fitting and connected the front line to the other side of the MBC.

Before the MBC I would religiously hit 12psi. No more, no less. I have a massive front mount intercooler with 3" piping. Also a 3" Ypipe and downpipe. Stock catback.

After the MBC, even with it turned off, I hit boost cut at just under 14psi. In theory, with it off I should be at wastegate pressure and should only be seeing 12psi.

Now, I know the first answer will be "check your lines/wastegates"

I took the MBC off entirely and just ran my new hose configuration alone without the MBC in place.
The result? 12psi on the nose. Just like baseline.

Conclusion: The presence of the MBC alone causes a 2+ psi increase?? Thoughts???

 

possible, longer vacuum lines, or smaller vacuum lines will both cause increases in boost.

 

I tried to rule out that possibility by removing the MBC and directly connecting the pressure source to the wastegates. Even with the new lines and longer lengths the pressure was steady at 12psi just like with the stock lines.

My theory is that the MBC T itself is probably causing too much of a restriction in air flow and not allowing the wastegates to open, which is bizarre with all the praise for how high quality Turbosmart is. Unless this is just a common known phenomenon associated with running MBC's

 

Yeah it does happen. In my uncle's 1jz S14, we installed a blox MBC, and it went from 10 psi to 12 without adjusting the MBC.

 

Had the same thing happen with the MBC i had installed, My solution EBC and BCC lol.

Maybe check the lines to make sure you dont have any pinhole leaks.