Ok, got a problem. My friends car, when he boosts in 1st up to 6500 RPM it feels fine, but then after that it feels like there is backpressure, like fuel cut off almost and you can hear the exhaust puttering like backfiring. In 2nd gear, at around 4500 RPM and up, the car putters. What could be the problem?

When we disconnected the charge pipe so that there is no boost, the car runs fine "N/A". But then we connected the charge pipe back and it does the same crap.

Could it be fuel issues? Messed up/sticky FMU?

 

you do realize when you disconnect the charge pipe, you are spinning the turbo to oblivion, right?

My guess says it's either too rich, or your spark gap isn't closed enough

 

With the charge pipe disconnected, there isn't enough load/exhuast gases to spin the turbo to oblivion.

 

could a sticky FMU cause this?

 

Wha??? When you disconnect the charge piping, you're just not allowing the engine to inhale any boost. Your STILL placing load on the engine just as you would normally. Load is not a byproduct of boost, boost is a byprodct of load. And yes, without some kind of restriction (ex. an engine in which to force air into), the turbo is spinning some ungodly speed while you run the car hard, unless you remove the turbo from the exahust stream therby not sending any exahust gasses through the turbine.