How much do stock cats attenuate noise? - BMW M5 Forum and M6 Forums
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post #1 of 9 Old 20th September 2018, 12:06 AM Thread Starter
thunderegg
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How much do stock cats attenuate noise?

I'm enjoying my new M5 that I picked up from waltzy23 here. It has a catless muffler delete, which my aging ears are finding to be just a little too much, especially in the low frequency range. I have the original cats and would like to get some opinions on whether it would be worth it to reinstall them or not. Has anybody done this and did it make a noticeable reduction in volume?
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post #2 of 9 Old 20th September 2018, 03:07 AM
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I can't speak for the E39, but on my E92 M3 the cats made a large difference in volume. I encountered a similar scenario there and ended up running catless with the stock rear-section, which was perfect for me at the time.

Most members here seem to be in consensus that a muffler delete with stock cats and resonators is a great setup from a sound and volume perspective.
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post #3 of 9 Old 20th September 2018, 10:38 AM
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Originally Posted by thunderegg View Post
I'm enjoying my new M5 that I picked up from waltzy23 here. It has a catless muffler delete, which my aging ears are finding to be just a little too much, especially in the low frequency range. I have the original cats and would like to get some opinions on whether it would be worth it to reinstall them or not. Has anybody done this and did it make a noticeable reduction in volume?
Putting in the stock cats will alleviate the drone. You can go for 200 cell sport cats for something in between.

Get some eisenmann mufflers too and there will be no drone and a exotic beautiful beautiful beautiful sound at high rpm.
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post #4 of 9 Old 20th September 2018, 08:25 PM
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My first e39 m5 came with a muffler delete which I then stupidly decided to decat. It was obnoxious when giving it full throttle, not to mention smelly.

My 2nd e39 m5 had a stock exhaust but was decatted. I then installed Eisenmann race back boxes and again it was obnoxious, and smelly.... fine for an old muscle car, but not a luxury sports sedan imo. I installed Magnaflow 200ci cats and have been happy since.
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post #5 of 9 Old 21st September 2018, 12:09 AM Thread Starter
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Sounds like it'll be worth the small expense to put the stock cats back on. Thanks for the advice!
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post #6 of 9 Old 21st September 2018, 06:51 PM
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Sounds like it'll be worth the small expense to put the stock cats back on. Thanks for the advice!
Definitely a noticeable difference. I run the Medusa headers and Dinan mufflers but I made sure to use the stock cats for I did not want unbearable noise. For me the setup is perfect and sounds incredible with no drone. My wife even likes it and always comments on how nice this M5 sounds. I vote to put the oe cats back in.

Current Stable

2003 E39 M5
2003 E53 X5 4.6is
1985 E24 635csi Euro
2008 P80 Volvo XC90 R-Design Yamaha V8


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Style #65 Rears - four corners
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post #7 of 9 Old 21st September 2018, 10:08 PM
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The smell factor alone would force me to install cats regardless of exhaust volume.



Since you have a muffler delete (and I assume you still have resonators), you should still enjoy the roar from the exhaust pipes out the back without the annoyances of having no cats. Depending on your tastes, you might still find that its still louder than you prefer, so you might start looking into exhaust cans to suit your tastes.

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post #8 of 9 Old 23rd September 2018, 01:25 AM
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Big difference. Sounds better with just muffler delete. I have 100 cell race cats and they smell too
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post #9 of 9 Old 28th September 2018, 10:36 PM Thread Starter
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Picked it up this morning, and yeah, what a difference! No droning, well-mannered at low speed, yet still makes the hair on my arms stand-up when I get on it. Thanks again for the advice!
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