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  1. #1
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    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 downpipe, twin 2.5" mid pipes with magnaflow resonators, which merge to a 3" flange.

    Click here to enlarge


    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.


    Click here to enlarge



    • Varex muffler. This is basically a 3" straight through muffler with 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.


    Click here to enlarge

    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 which would be significantly lighter? Keeping in mind that I am going to be installing a Vargas Stage 2 turbo in the next couple of months.
    I don't know if this thinking is correct so I am after your thoughts and opinions.

  2. #2
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    Im not an expert on the n55, but I have done a lot of research on the s54 platform for my exhaust set up.
    resonator- I would not remove them because they help control the droning exhaust sounds that would other wise resonate through the cabin hence the name resonator...
    Muffler- designed to muffle exhaust sounds so if you want a louder exhaust note then you need a more free flowing muffler
    I would recommend going cat-less. It will give you a slight increase in volume (and rasp), but it gives you a much more aggressive tone.
    As far as going with the single 3'' midpipe I doubt there will be any noticeable difference in performance, but it may give you a slightly deeper note ( not louder, just more rumble)
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    I am not worried about drone with the Varex muffler installed, when it is closed it is really quiet.
    When it is open it is straight through, you can literally see all the way through it, so I don't think it's very restrictive.
    Also I have a BMS downpipe which is catless.

    My other option is to just put the muffler delete back on and stop letting the wife in the car Click here to enlarge

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    Haha well yeah I would probably go back to the muffler delete. Honestly I don't think cutting the resonator will be enough for any significant change in sound

    Edit* I don't know if you have a tune or not but I would look into removing the cold start. If your cat-less there's no reason to have the cold start cause it's just meant to warm the cats up faster
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    I would think a full 3 inch setup would be more efficient.
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    Click here to enlarge Originally Posted by Sticky Click here to enlarge
    I would think a full 3 inch setup would be more efficient.
    How is that?

    2x2.5" diameter = 9.817 square inches in cross section, 1x3" diameter = 7.069 square inches in cross section.

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    Click here to enlarge Originally Posted by maxnix Click here to enlarge
    How is that?

    2x2.5" diameter = 9.817 square inches in cross section, 1x3" diameter = 7.069 square inches in cross section.
    what about the air friction\heat caused by 2x2.5" against a single 3"?
    135i N55 DCT / JB4 / CP-E 5" DP / BMS CP / CTS Turbo 7" FMIC / KW V3 / M3 Bushings / M3 Front Control Arms / 3.25" SS exhaust / to be continued...

  8. #8
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    Click here to enlarge Originally Posted by maxnix Click here to enlarge
    How is that?

    2x2.5" diameter = 9.817 square inches in cross section, 1x3" diameter = 7.069 square inches in cross section.
    Considering it is a single turbo it likely is best to not split the exhaust gases like that and go with a single pipe isn't it?
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    Click here to enlarge Originally Posted by Tamut Click here to enlarge
    what about the air friction\heat caused by 2x2.5" against a single 3"?
    Great minds.
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    Click here to enlarge Originally Posted by Sticky Click here to enlarge
    Considering it is a single turbo it likely is best to not split the exhaust gases like that and go with a single pipe isn't it?
    Always this, at least unless you split after midpipe, which I will be doing on mine. I think what he's saying is though which I agree is yes, go single pipe, but do 3.5" instead of 3"
    2009 335i coupe back to stock...for now

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