remove cats or resonator?

[shibby170]

Hey guys. I have been the proud owner of a 2005 Z4 2.5i for the past three years. She has been so good to me. But a few days ago I bid her farewell, and welcomed into my life a gorgeous 2007 Z4M Roadster. I plan on keeping her for a long time, and slowly try to transform her into the most beautiful lambo killer ever.

My first step will be freeing up the airflow of course. Now, as much as I want to get the air free flowing, the sound does matter. I honestly want to keep as much of the rasp and tinniness out of the sound, and keep the drone down. On my 2.5I, after switching out the muffler, the only thing I removed was the secondary resonator, I kept the first resonator and the cats. The sound was gorgeous, loud, minimal drone, and literally zero unwanted rasp or tinny sounds.

It looks like the z4m stock exhaust has one resonator in the x-pipe and the two unmonitored cats before the headers. I believe if I go straight pipes from the headers to my muffler of choice, I'll end up with a raspy sound. I want to keep either the x-pipe resonator or the mid-pipe cats.

Does anyone know keeping which one will keep out most of the rasp? To free up airflow, id really like to get rid of the cats, but I'm afraid they may be keeping a lot of the rasp at bay. I've been researching for days and can't seem to figure out the answer.

[fullerra]

Hey Shibby

Welcome to the forums!

It has been my experience that the more free flowing your system is, generally speaking, the more rasp you are going to get.

If horsepower is your main concern, eliminating one of the 2 sets of cats is the best way to go, with headers>section 1. That is both in terms of HP and rasp however.

What would probably work for what you want is aftermarket headers, stock section 1, and a "quiet" set of mufflers like Stromung.

I hope that helps!

[jadnashuanh]

Depends on if this car will be driven on the street.

Removing the cats, if done by any shop is a federal violation and a big fine. You cannot modify the emissions system. Now, whether removing any resonator or muffler may or may not be, depending on local noise ordinance laws (often not enforced). In NYS, getting a legal emission check, may be impossible, and an inspection, seeing them gone (they're supposed to check!) should immediately fail the vehicle. Now, can you find someone who may overlook that? Probably, but if you're caught in some other way, they could be fined and lose their certification and ability to perform work.

[speddmn]

Replace section 2 before you remove the header cats

I did and the rasp wasn't really there at all - now with headers and a magnaflow in section 2 going straight back to the tips it's raspy and obnoxious but I like it anyhow

[shibby170]

Originally Posted by fullerra

Hey Shibby

Welcome to the forums!

It has been my experience that the more free flowing your system is, generally speaking, the more rasp you are going to get.

If horsepower is your main concern, eliminating one of the 2 sets of cats is the best way to go, with headers>section 1. That is both in terms of HP and rasp however.

What would probably work for what you want is aftermarket headers, stock section 1, and a "quiet" set of mufflers like Stromung.

I hope that helps!

So you think that I should keep section 1 with the second set of cats, and second 2 with the resonator, correct?

Originally Posted by speddmn

Replace section 2 before you remove the header cats

I did and the rasp wasn't really there at all - now with headers and a magnaflow in section 2 going straight back to the tips it's raspy and obnoxious but I like it anyhow

You don't think I should replace section 1 before section 2?

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I was also wondering, do you guys know if the stock piping is 2.5" in diameter like my 2.5i was, or if it is different?

[Spreadsheet]

Rebuttle to "jadnashuanh - Removing the cats, if done by any shop is a federal violation and a big fine. You cannot modify the emissions system."

I don't believe this is correct, at least not for every state in the USA. My BMW dealer installed my SuperSprint Headers and Hi-Flow Catalytic Convertors. They even have a specific Service Advisor who specializes in aftermarket modifications on staff. Given this, I doubt they are worried about losing their certification.

Only stock exhaust part in my Z4MC is the X-Pipe and Resonator, Have SuperSprint header and cats with Magnaflow cans. It is a bit loud, but the sounds it makes are glorious. Have had civil servants follow me on the street and highway, but never been stopped so far, so must not be over the top.

Good luck with your mods.

[shibby170]

Originally Posted by Spreadsheet

Only stock exhaust part in my Z4MC is the X-Pipe and Resonator, Have SuperSprint header and cats with Magnaflow cans. It is a bit loud, but the sounds it makes are glorious. Have had civil servants follow me on the street and highway, but never been stopped so far, so must not be over the top.

Good luck with your mods.

Thanks...I think I'm going to follow the same route. The only stock component I'm going to keep is the resonator.