2.5'' Downpipe Options
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This is a discussion on 2.5'' Downpipe Options within the Engine Modifications forums, part of the Tech & Modifying & General Repairs category; How man full catless downpipes that taper to 2.5'' are there really on the market. It seems that not a ...

  1. #1
    Registered User SubyDude05's Avatar
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    2.5'' Downpipe Options

    How man full catless downpipes that taper to 2.5'' are there really on the market. It seems that not a whole lot of people make these, all i can really find are 3''. I want to eliminate both cats in my downpipe but also have this system bolt up to my Blitz Nur Spec RX exhaust which tapers to 2.5'' at the downpipe.



    I am not super intersted in going full 3'' exhaust now becasue I only want to take my car stage 2, and i have read that if your are pushing under 300-350 hp 2.5'' is fine.



    I also dont want to buy a whole new exhaust system because I just bought this Blitz RX catback system and really like it.



    I have also been told that I can modify by Blitz exhaust to fit a 3'' flange, but that would requier a few hourse of a shop's work and at 100/hr thats not cheap...



    Thanks
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    Registered User MiLaB21's Avatar
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    im not really sure of any exhaust that taper, i would just get the 3inch and hack up the catback to fit to the 3inch downpipe, wouldnt be too much work
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    Scoobysport's downpipe tapers to 2.5"

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    Master Baiter EJ257's Avatar
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    Cobb DP will mate to OEM CBE
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    ray

    what is beter a 3" bell or 4.5" bell on a 08 wrx

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    DUB
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    or you could get a catless shorty that tapers then get a 2.5" race pipe.

  8. #7
    Registered User jpjester's Avatar
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    Quote Originally Posted by ramonsalvador08 View Post
    what is beter a 3" bell or 4.5" bell on a 08 wrx
    3 Inch, 4.5 is just extreamly unnecessary and you will loss a ton of back preasure.
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    The Default One SeattleJeremy's Avatar
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    Quote Originally Posted by ramonsalvador08 View Post
    what is beter a 3" bell or 4.5" bell on a 08 wrx
    Ray, I think you're confused. There is no such thing as a 3" bell.

    jpjester, There is no such thing as a 4.5" exhaust system. It's only 4.5" at the turbo.

    There are three types of Downpipe inputs (the part that attaches to the turbo).
    1. Bellmouth (4.5")
    2. Divorced (4.5" or 3" + 2")
    3. Non-bellmouth (3" pipe)

    Here is Invidia's explanation on JCSpeeds' website
    Divorced vs. Bellmouth:
    Divorced: A divorced wastegate downpipe has a plate the protrudes into the turbo to separate exhaust flow coming out of the turbo to give reduce turbulence. This is good for stock and smaller turbochargers; the protrusion does not work with some aftermarket turbos (i.e APS).
    Bellmouth: This design has a wide-open bell at the turbo, which is better for larger turbochargers with more flow. It will work fine with a stock or small turbo, but it is designed for scenarios where wastegate turbulence is not an issue.

    The diagram to the right is a stock WRX turbo, so you can see how the turbocharger interacts with the Bellmouth and Divorced Invidia downpipe designs.
    This thread has pictures of the Non-bellmouth type.
    Engine Performance FAQ [[[READ ME]]]

    Edit : Sorry for the Hijack.
    The Cobb downpipe tapers to 2.5".
    Last edited by SeattleJeremy; 11-05-2008 at 01:41 PM. Reason: More info.

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    Registered User SubyDude05's Avatar
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    Well the only problem with the Cobb is the price and the fact that it is catted. Is there any real advantage to get it chopped to a full 3'' rather than searching more for a full catless 2.5'' tapered downpipe?

    I know there are MANY more full 3'' downpipes to choose from, and the 2.5'' tapers are more expensive as well...

    Yall think its worth it to chop it to 3''? And what type of shop should I look for in my area? I would not trust just any old muffler shop to start chopping up my exhaust...
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    Registered User teflon_jones's Avatar
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    Quote Originally Posted by SubyDude05 View Post
    Well the only problem with the Cobb is the price and the fact that it is catted. Is there any real advantage to get it chopped to a full 3'' rather than searching more for a full catless 2.5'' tapered downpipe?
    .....
    Yall think its worth it to chop it to 3''? And what type of shop should I look for in my area? I would not trust just any old muffler shop to start chopping up my exhaust...
    Cobb also sells an uncatted version for $100 less. What exactly are you going to "chop" to 3"? I don't understand how you propose to modify your exhaust.....
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    Master Baiter EJ257's Avatar
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    Quote Originally Posted by teflon_jones
    What exactly are you going to "chop" to 3"? I don't understand how you propose to modify your exhaust.....
    I'd assume he was referring to cutting the section of the exhaust that tapers to the 2.5" donut flange and welding a new (non-tapered) piece on, with a 3" flat flange.
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  13. #12
    Registered User TurnWRX's Avatar
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    Quote Originally Posted by SubyDude05 View Post
    Well the only problem with the Cobb is the price and the fact that it is catted. Is there any real advantage to get it chopped to a full 3'' rather than searching more for a full catless 2.5'' tapered downpipe?

    I know there are MANY more full 3'' downpipes to choose from, and the 2.5'' tapers are more expensive as well...

    Yall think its worth it to chop it to 3''? And what type of shop should I look for in my area? I would not trust just any old muffler shop to start chopping up my exhaust...
    u will get more top end going full 3 inch, but is it worth modifying your catback? Probably not. Going to a muffler shop to chop off the 2.5 inch area and welding a 3 inch won't be terribly expensive (probably run u around $100 give or take).

    I would just leave it as is and find a tapering downpipe.

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