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      02-20-2014, 03:13 AM   #1
Nugget
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2011 BMW 135i  [10.00]
Twin 2.5" vs single 3"

I am not really happy with my current exhaust setup, the car is a 2011 135i with a N55.

I have a BMS catless downpipe, twin 2.5" mid pipes with magnaflow resonators, which merge to a 3" flange.




I have gone through three axleback sections

-Maganaflow muffler which droned and made a whistling noise under WOT

-3" muffler delete, sounded good, no drone, but sounded horrific on cold start and my wife hated it so it's gone.


-Varex muffler. This is basically a 3" straight through muffler which has a butterfly valve, when the valve is closed the exhaust has to go through a separate chamber which makes it very quiet. There is more info here. This is pretty handy for when I want it quiet but IMO it's not loud or deep enough when it's open.



I was thinking about taking the resonators out of the mid pipes, will this make it louder or just raspier?

Which led me to thinking if the exhaust goes down to a 3" axleback anyway would it make a difference to performance if there was just a single 3" midpipe?

Obviously this would sound different and be significantly lighter. Thoughts, opinions on sound and performance?
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      02-20-2014, 07:02 AM   #2
ShocknAwe
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taking the resonators out will make it raspier for sure. Single 3" midpipe would further reduce resistance. Dunno on sound.
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      02-21-2014, 12:57 AM   #3
Gray330iForSale
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Any time you consider taking a resonator out of a car I want you to think of that guy in the Honda Civic. It's the one with that echoing buzzing sound that sounds like death. That guy removed his resonator.
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      02-21-2014, 09:12 AM   #4
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Most single piped designed I've seen for 135/335's are 3.5. There are a few like Cobb that use a single 3". If I remember the measurements right a 3" should give about the same flow as the factory mids, slightly less than the aftermarket mids at 2.25 and the 3.5's flowing the most. But in the real world where the manifold is the real restriction and all the mids flow into the same 3" rear section I don't think any of the sizes really matter. And keep in mind most cat backs are only adding what around 5HP?
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      03-02-2014, 04:49 AM   #5
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3" is smaller than twin 2.5 iirc. Would want to go a 3.5. I havent done the math though.
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      03-02-2014, 04:56 AM   #6
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Quote:
Originally Posted by Gray330iForSale View Post
Any time you consider taking a resonator out of a car I want you to think of that guy in the Honda Civic. It's the one with that echoing buzzing sound that sounds like death. That guy removed his resonator.
All the guys (including me on my 135i) who have/had the BMW PE with the Y-Pipe have also removed their resonators...

THE PE IS ONE OF THE BEST SOUNDING EXHAUSTS OUT THERE.
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      03-02-2014, 01:46 PM   #7
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Quote:
Originally Posted by flinchy View Post
3" is smaller than twin 2.5 iirc. Would want to go a 3.5. I havent done the math though.

There is drag near the walls of the pipe so that doesn't tell the whole story, but you get the general idea.

Twin 2.5" is probably the best for ground clearance for most street cars. A single 3.5" would be more appropriate for a race car as it will weigh less and support probably more power than you would ever need (600+).

OP: Sounds like your midpipe is the problem if none of three mufflers sounds good to you...
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      03-03-2014, 06:48 PM   #8
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2011 BMW 135i  [10.00]
Quote:
Originally Posted by Freon View Post

There is drag near the walls of the pipe so that doesn't tell the whole story, but you get the general idea.

Twin 2.5" is probably the best for ground clearance for most street cars. A single 3.5" would be more appropriate for a race car as it will weigh less and support probably more power than you would ever need (600+).

OP: Sounds like your midpipe is the problem if none of three mufflers sounds good to you...
Good info thanks.
Ground clearance is another good point, my midpipes are tucked up pretty tightly and I have still managed to scrape the resoanators a few times, I wouldn't say the car is slammed either.
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