Originally Posted by RollingHard28
How much HP does each downpipe add? Also can you explain the difference between the two?
How much louder would this make your car once you add it? Is it going to be super noticeable to authorities, or would it still be low key?
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:
Originally Posted by brentmeister7592
Yes, it gives is a PERFECT not in my honest opinion. Just slightly louder and more aggressive, and since there is no more cat the turbo spool is very apparent
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.