The cats should eliminate a good bit of the rasp. My concern would be with the pipe diameters - wouldn't the M3 section 1 be larger than the 330 section 2/3? That step down would cause a lot of backpressure.

 

Yes, the 330i exhaust is smaller than the M3 one, so I'll use an adapter from 55mm pipes to 50mm pipes.

As it is for a 330i engine, I don't think this will be a huge deal.

 

So long story short, there is 1 resonator less, as it is replaced by M3 cats. Will this affect my sound? I'm asking this because I really HATE a raspy or drone exhaust sound, and want to keep the sound as close to stock as possible. I rather ask in advance than after the work is done, only to have to start over again.

In case there is any rasp or drone, would it help to remove section 2 and weld in 2 AR25 resonators? As in, are they better than the OEM resonator?

well of course it will be different sounding. you're going with tube headers where the other manifolds had bulbous cats. but the "euro" M3 cats are where the US M3 has one single (not dual) resonators. being a catalyst convertor with a honeycomb cell design inside it will muffle the soundwaves some. sticking with the stock 330 rear muffler will also be a major factor in rear sound. the tube headers though will contribute to front firewall cabin sound.
it's really hard to say if what you'll hear is too much drone for you. it's still an M54 engine and not an S54.

 

Wait I'm confused now. Which Schmiedmann headers are you purchasing? The ones for the M54 are 50mm:

https://www.schmiedmann.com/en/product/16694-new?product=SCM5254

 

https://www.schmiedmann.com/en/bmw-E46/1943790-new?product=SCM5254MANI
Afaik this was the one I have. Indeed it says 50mm pipe. There must be something wrong. Gonna double check the number on mine. Otherwise I'm ****ed and bought the wrong one. Thanks for verifying this!

 

Pardon me. Inside diameter is 50mm indeed. I measured thte outside (cone) because I needed to find a flange to fit over it.
To going from 50 to 55 to 50 is a really bad idea?

 

I mean if you already have the parts and they bolt up then you could try it but if it's going to require welding new flanges on anyway then I would try to use your current 50mm piping to mate to the headers

 

No can do, as I need cats for emission control. Unless I cut out the first resonator on my OEM exhaust and weld in some aftermarket cats. But I think I'm gonna go for OEM exhaust & manifold for now, sell the M3 cats, and wait until I found all the parts I need to properly connect the schmiedmann headers to some 50mm cats to the rest of the exhaust.

Thanks again friend for the input!

 

That's probably your best bet. The Schmiedmann headers look to be great (I have a set on backorder right now). I'm planning to do the same - replace the first resonator with a magnaflow 94008 catalytic converter.

I actually have a setup right now with eBay headers and two magnaflow cats welded in after, before the first resonator:

It's not a great setup, though, since the headers are crap and the cats are way oversized for the engine, so I'm getting backpressure there too. Ran pretty good before I put the cats in though!

 

I'm aiming towards something similar. Normally I would say that headers are a waste of money, for only a little HP gain, but M54 stock headers are restrictive as hell --> fact. So whatever it is you're changing to, it's probably gonna be better.
I'll just keep everything stock for now, make sure to get my engine running decent after the swap, and then I can start gathering parts for an exhaust upgrade without busting the bank.

I already sold my M54B30 headers, but B22 headers are the same according to realoem.

 

I realize this post is two years old but I have to comment on your statement about headers being a waste of money for little hp gains, other than going to forced induction headers will get you more hp than any other mod, 20 hp(+/-) is not a "little" hp gain. You're up in the night bro.

 

2 years later: I went for a full Alpila B3 exhaust instead. Now debating if I should add schmiedmann short headers. According to a friend this is a noticable increase in power, but I don't want to make it louder, and since the "walls" are thinner, I'm not sure what this will do.

 

I have the schmiedmann headers sitting in my garage and can confirm that they use pretty thick walled pipe. Not sure if they're as thick as the OEM manifolds, but they're a far cry from the tinny eBay headers.