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2006 M5
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Discussion Starter #1
Fam, I wanted to get some feed back on a couple options I plan on doing with the stock S85 headers and needed your input.

1. What's the CPSI of the stock S85 primary header Cat?

2. Would you gut the primary Cats and run 100 or 200 CPSI cats in the midpipe to cut down on exhaust fumes entering the cab?

3. Would you cut the stock primary Cats out of the headers and weld in 100/200 Cats in their place and run Catless mid pipes to also eliminate excessive exhaust fumes from entering the cab.

Also, can you cut the stock primary Cats out without removing the headers?

Thanks in advance
 

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Most of the answers depend on your end goal, are you looking for max power gains or just louder sound?

A lot of people gut the primary cats (you don’t have to remove headers to do this) and keep the secondary cats. This keeps the fumes down but there will be a smell. Don’t know the cell count but the major restriction occurs at the primary cats.


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Discussion Starter #3
Makes sense, my main goal is to extract as much hp as possible without sacrificing a ton of civility. Meaning, I'm leaning toward gutting the stock headers to unlock some additional hp&tq but don't want to cause my kids to pass out from c0 poisoning every time I stop at a light and I know there isn't a way to eliminate 100% of the exhaust fumes once you gut the headers.

Noise isn't that much of a concern, I can always weld additional resonators to tone it down if necessary.

Would you go with 100 or 200 cpi replacement secondary Cats?
 

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I have the RMS decats with V bands paired up with secondaries. Doesn't bother me and my family at all.
 
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Really? That's encouraging. Are your secondary cats stock or high flow aftermarket? Thanks
Check out RMS headers. Ryan the owner is here on the board.
 

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2006 M5
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Discussion Starter #9
OP... do you have to deal with passing an emissions test?
Unfortunately yes. Here in Northern Virginia we do a obd test. Visual inspection is not really an issue here. I think we're allowed 1 emission monitor in the "not ready" mode and still pass.
 

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I currently have the headers gutted, kept the secondary cats and installed a tune. Mufflers are stock. Car sounds perfectly balanced....loud on the outside, no drone on the inside and barely any fumes in cabin.


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Discussion Starter #11
I currently have the headers gutted, kept the secondary cats and installed a tune. Mufflers are stock. Car sounds perfectly balanced....loud on the outside, no drone on the inside and barely any fumes in cabin.


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That setup sounds like the best option for me as well. I think i'll just use a deep Hole saw or Spade bit to hollow out the primary's then weld in high flow cats in place of the secondary's. Not sure if I want to drop $3000+ on headers right now, as a 3.91 Diff is going to be my next big up grade. Also compiling a stockpile of preventative maintenance parts too. Should be fun! 😂
 

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Really? That's encouraging. Are your secondary cats stock or high flow aftermarket? Thanks
My secondaries are actually stock. But I plan on removing those as well..
 
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