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  1. #1

    J-pipe modifications

    I know this is kind of an old topic, but I don't think this particular mod has been discussed much. What is the advantage of cutting the J-pipe as short as possible? I used to have the flare on my j-pipe, but after reading all of the stories and theories behind blown waterboxes with flared j-pipes I cut the flare off. But is there an advantage to shortening my J-pipe more than I have (about 2-3" after removing the flare). Here are pics of one kenshin89 has for sale. Is it worth doing this? Why or why not. Thanks.
    Last edited by RXP-REX; 03-26-2010 at 11:32 AM.


  2. #2
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    The blow through people were having with flares was a result of gutted and shortened waterboxes, plus some rerouting of water that cooled the pipe itself...

    A gutted AND shortened waterbox is not the setup you want with a flare...If the box was left full length, with the middle internal baffle intact, then a FLARE is your best friend...Same thing with a Greenbox and the new Riva box...

    Trimming the pipe to ANY length will slightly disturb the low end torque response of the motor. The pipe is tuned to a specific length by BRP to match the waterbox. Therefore flaring simply creates a more direct expansion location to help draw the exhaust gases from the path and decrease backpressure, without disturbing the tuned length.

    If you trim it, you might as well trim it so that the pipe is flush with the inlet on the waterbox. Make sure that you are centered in the hole so that you get proper waterflow for cooling of the waterbox...

    On a side note, I ran a custom box with flare for a whole summer before I released information about it...I'm going on 3 season with the same setup, no issues

  3. #3
    Quote Originally Posted by Danny View Post
    Trimming the pipe to ANY length will slightly disturb the low end torque response of the motor. The pipe is tuned to a specific length by BRP to match the waterbox. Therefore flaring simply creates a more direct expansion location to help draw the exhaust gases from the path and decrease backpressure, without disturbing the tuned length.

    If you trim it, you might as well trim it so that the pipe is flush with the inlet on the waterbox. Make sure that you are centered in the hole so that you get proper waterflow for cooling of the waterbox...
    I just didn't want to take any chances Danny. Read quite a few posts on it, spent some time with Snoman on the phone about it so I cut it. Looking back on it I wish I would have left it alone.

    As far as losing low end torque how noticeable is this? I've been told that Rotax Racing or Mark was trimming the j-pipe to be flush. So I assumed it was beneficial? Maybe not I guess? I'm kind of lost on the issue.


    I'm considering dumping my whole exhaust setup now in favor of the new 2008 midpipe and Riva waterbox.

  4. #4
    dhoad's Avatar
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    I have my J pipe cut short like that and I will tell you my torque/holeshot is just fine. No noticeable difference IMO

  5. #5
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    The difference can be seen on a dyno, but by the seat of the pants...can't tell.

  6. #6

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    Good question and a better response by Danny! You confirmed what I believed. WOW!

  7. #7
    CC Racer kenshin89's Avatar
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    Not sure if it matters, but this is the waterbox that was used with that j-pipe

  8. #8
    HOSS's Avatar
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    I cut mine right at the weld.

  9. #9
    Moderator Snoman's Avatar
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    Rex,

    Just remember on a blown motor your exhaust flow isnt as crutial as a NA motor..You SC is forcing the exhaust out by the pressure created were as NA motor depends on the best availible flow rate..By cutting your J-pipe you will notice NO differance at all..AND the little bit IF any flow differance is NOT gonna hurt you in any way shape or form...Yes we did discuss this and anybody is free to run the flare...Its there waterbox..BUT with a shorty,gutted waterbox it is NOT the way to go..With 8 members on this forum blowing up waterboxes due to flared J-pipes i think the eveidence is in the numbers..This pertains to the Greenbox also...For the people Danny said re routed thier water for cooling....I dont know who or what waterlines they were re-routing..The only line that needs re-routing is the one that goes to the exhaust tip IF your running a greenbox..other than that,you dont need to re-route any water or cooling lines..

  10. #10
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    So the flare I run on my stock waterbox is fine but if I was to gut it and shorten it i should ditch the flare??

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