3inch dump into high Flow cat then splits into 2x2inch pipes into a little muffler then out the back.
it had no cat first and was doing the same thing and i got told to put one in because it didn't have enough back pressure.
it is a pain in the a$$
could not say my tuner sets it up.
their is also a r33 with the same turbo and same problem and he is using a blitz dsbc spec r