[Jec1]

A post in the for sale section prompted me to contemplate this-

I currently have the stock downpipe on my car, live in NY with OBD2 scan and drive my car daily.

Originally I bought an AR design catless off the forum, never installed it and sold it on the forum.

About a month ago, I bought an AR Design catted again off the forum, and still haven't installed it. I was planning on installing it this spring along with a minicat so I can get readiness set and squeak through inspection before a CEL is thrown.

I saw a post on another forum dating back a while but haven't seen any other discussion on subject topic or comparison photographs.

I understand that the N55 x drive has secondary cats in the midpipe while the RWD does not. I also understand that no OEM catalyst will come close to the flow of a 200 cell race cat.

That being said, for those of us on the fence about going with an aftermarket downpipe, is the x drive downpipe cat any smaller in size than the RWD? Matt from Camber Toe seemed to think so in a post dating back several years, making the analogy that the N55 x drive followed a similar model to the n54 emissions setup, while the n55 rwd threw one big cat in the downpipe to handle emissions.

Understanding that cell count will likely be the same regardless, is there any advantage, even if minimal, in flow and backpressure for those of us with n55 rwd vehicles to switch to the n55 x drive downpipe?

[jwzimm]

Well they do have different part number according to realoem.com

335xi: http://www.realoem.com/bmw/showparts...81&hg=18&fg=10

335i: http://www.realoem.com/bmw/showparts...81&hg=18&fg=10

I am curious about this as well as it would imply that removing the secondary cats on my car would have a good effect.

[Chicanery]

+1.. I'm also curious to know.. emissions aside, would going catless on a RWD 335i and installing a vibrant resonator be the best way to get the most flow/hp with the most sound/rasp deadening?

[Jec1]

^ No idea, but I've confirmed that the answer to my question is likely a yes, with the caveat that any increase in flow will still be notably less than what catless or an HFC will achieve.

I don't have the pictures saved on my office desktop, but physically the catalyst on the x drive downpipe is notably smaller, and the cell count also is less. I guess it could be thought of as bridging the gap between the stock downpipe and a high flow cat, but it should have no issues passing emissions and likely won't have the deterioration issues that high flow cats are notorious for. They are also likely way cheaper than a high flow cat at a junkyard.

[Chicanery]

Is it safe to assume the xi dp could be directly swapped in for the stock n55 rwd dp?

[Jec1]

Originally Posted by 335inthe305

Is it safe to assume the xi dp could be directly swapped in for the stock n55 rwd dp?

From everything I could tell, yes. The mid pipes are the same length and dimensions so the downpipes should be easily interchangeable.

Whether there is any tangible benefit, most likely not. I don't think there will be any more noticeable power from the swap even with a retune. If anything, the reduction in heat and back pressure is likely so incremental at identical power levels that the effect wouldn't be noticed until long after most of us have sold the car.