This or a test pipe? I haven't really found a cut-off for my car, if anyone knows where, help me out. My car is turboed, so would the cut-off mess with the air/fuel?

 

From what I've read, if you tune your car with the cutout open, your air/fuel ratio leans out slightly if you hit boost with the cutout closed. Just don't redline it with the cutout closed and you should be fine.