Originally Posted by jsedlak
Dyno says a pretty consistent 12-20bhp.
AFAIK, because the cat's main purpose is to make the exhaust "cleaner," it doesn't affect the sound nearly as much as the muffler. From what I've seen on YouTube it seems to make the exhaust throatier more than anything else. The goal of removing the cat is to improve the flow of the exhaust. The catless pipe will be louder than the HFC.
Edit: This is from the stage1 thread:
The main difference between Catless and HFC is that the HFC still tries to pass emissions standards. Therefore it sacrifices some performance gains to make it semi-streetable.
So long as the downpipe doesn't throw codes and your exhaust can pass emissions, you should be okay. IF you're caught though... well good luck.
Anyways, if you're going WOT past a cop in an exhaust modified (even the M-Perf exhaust) 335i then you're probably going to get pulled over with or without the modified downpipe.
As always - correct me if I'm wrong. I'm still a novice in this arena.
Just to see if I am on the same page, the catless down pipe (smaller looking downpipe) would be louder but less expensive; while the HFC down pipe (bigger looking DP) which is more money, will be less loud and more likely to pass a SMOG, is this correct?
I guess because it would be not as loud and more likely to pass SMOG, I think I am leaning to the HFC...