I did a JDM S5 swap on my S4 I used all S5 wire harneses and ECU. I have a racing beat turbo back exahust and a fuel cut defender. The car idles and has good compression. I took it out for a drive and when ever I go to boost the car just cuts fuel and sputters. If I slowly increase throttle I can get the car to boost a little in 1st and second gear but not even stock boost. I am wonering what could cause this.

 

sounds like the computer is freaking out, this happed to us on a honda project. we popped a couple of one way wiper fluid valves into the manifold hosings right before the manifold pressure sensor and that did the trick. Dunno if that helps or not. gl