New downpipe causing big over-boost and rough turbo
 

We just replaced worn out stock cats with a down-pipe and pre-silencer on my 88 FC3S Turbo. The car starts and runs fine however when you activate the turbo it seems the car is over-boosting and the boost gage shows max levels. It doesn't cause a sudden surge of power, instead it is almost as if the engine is fluctuating between getting max boost and then boost gets cut off, causing it to run extremely rough when turbo is applied. We tried leaving the waste-gate completely open but the problem still occurs.

It seems as though the absence of back-pressure after removing the cats is causing the turbo to go haywire but we aren't quite sure what is causing the problem. Hopefully the turbo isn't fried. I guess we could try re-installing the old cats to see if the problem still exists, but that isn't ideal because I want to stick with the aftermarket downpipe, and at this point just finding out what the problem is starting to cost alot of $.

Any ideas on what could be causing this? Would a fuel cut controller, aftermarket boost gage / controller help? Any insight or thoughts are MUCH appreciated. Thanks in advance.

Yes a fuel cut defender will help & an after market boost gauge will allow you to see where you are at. By changing out the exhaust parts the boost is now 1-2 psi higher after the reduction in back pressure. The fuel cut defender will prevent the fuel cut to the rear rotor as the boost increases.