Thanks for the reply, I was thinking the stock cats are the biggest source of back pressure so just sticking them on the end of the headers is great for passing emissions but the bottleneck is still there no?
Here's the combinations I have been through so far:
1) Replace stock suitcase with Magnaflow 12599 and delete reso's
result: good sound, nice and deep, but drone at certain rpms.
2) Add obx longtubes
result: stinky, vibrations, sound level okay, drone not as noticible, probably due to volume. Low end torque seems to be lacking. Right tail pipe turns black.
3) add stock cats back in to the mid-pipe, with a magnaflow 12867 behind each cat, remove large magnaflow
result: loud as hell, start up and cruising sound pretty good but any kind of acceleration and its starts blating and farting, pretty obnoxious. Low end torque still missing even with stock cats in, but on the highway it feels like I'm pulling harder from 80-120km/h. I'm thinking the stock cats dont provide enough back pressure with this set up.
I feel some stumbling at highway speed and one of the O2 error codes comes in constantly so I limp back home and put the stock tune back in until I can sort it out.
4) add flowmaster 40's in where the stock resonators were
result: quiet, maybe too quiet. No drone, nice deep resonance. Low end torque definitely back, even on stock tune I can feel the difference. My muffler guy didnt get the tips lined up right so I'm a little pissed and I'll try to do it myself, but generally he's pretty good. I think I'm putting his kids through college.
Up next: No emissions testing to deal with, so I'll probably have straight pipes sewn in between the headers and rear axel, get rid of the magnaflows and replace stock cats with high flows. I figure this will give me a little more volume with the same tone. Still have to put johan's rear O2 delete tune back in and add O2 extensions.