I have read posts for years, both on this site as well as the old fc3s.org e-mail list about how you cannot regulate (lower) peak boost pressure by using a boost controller.
I have a friend with an FD that disagrees. He uses a manual boost controller and has his boost set at 11psi.
He claims that he can lower that to 7psi (with the valve completely open).

Here is what I have:
88AE with

DP
cat replacement pipe
Free flow cat back
TID
Cone filter
FCD
Ported wategate (Series 4 turbo)

I am not sure what my peak boost is at this point, (don't want to find out) simply because I don't want to pop my motor.

Is there a restricter pill in the wastegate line?
Can I restrict boost any other way besides putting the Cat back on my car? Can I use a needle valve to restrict boost??

I would prefer not to put the cat back on the car because the wires for the ACV and Port Air Solinoid (needed to run a cat) are completey deteriorated from excessive heat. I would basically have to splice into the engine harness to get them to work.

 

Your friend is actually raising his boost from 7 to 11psi. 7 being the 'natural' boost level that his particular configuration produces. He can't lower his boost below 7 psi with a boost controller.

Rob

 

Your friend is wrong, and his car is broken.
Boost controllers cannot lower boost. This is a simple engineering fact that cannot be disputed. I've explained why before, do a search coz I can't be bothered typing it all again.
FD's run 10psi standard. If your friend's car runs 7psi then something isn't working. Most likely the second turbo isn't being activated (common FD problem). Perhaps you shouldn't have done all those mods to your car if you now think it's unsafe. Do you have a decent boost gauge? Do you know how much boost your set-up is making? Not in the S4, only on the S5 (it's for the S5's bleeder type boost controller). Any airflow restriction (intake or exhaust) will restrict boost and lower power. Consider reinstalling the complete stock intake system. No. (How?) Use an aftermarket cat that doesn't require air injection. Besides, a stock 15yo cat will be completely shot anyway. Don't reuse it.

 

ugh.. guys, listen.. you cannot LOWER boost beyond the mechanical limits of the whole turbo system..

BUT if you have a wastegate that is mechanically sprung at 5.5psi, and no other devices "controlling" that boost besides the boost controller, then YES you can limit it to 5.5psi, as long as the wastegate can keep up.