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  1. #1
    Adam's Avatar
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    GP1200 tunnel mod.

    I have been thinking about how to reduce the volume of the GP tunnel to something like the GPR tunnel for some time now. Finding away to fix something into the top of the tunnel without doing any permanent changes. I have made a piece out of some engineering plastic that has a hole for the driveshaft and will bolt onto the front two mounting bolts for the intake grate. This should reduce the volume of the tunnel and maybe take some load off the pump, hoping to increase speed. I have looked at GPR tunnels and they slope down quite fast from the impeller and GPR have a much shorter drive shaft because of the smaller tunnel. The GPR weighs 673lbs and a GP weighs 525 lbs so all thing being considered the pump is the major difference. I would like to try a 75mm hub pump from a GPR1300 at some stage as it is a better pump , im trying to use what power I have rather than going overboard with 65U engine.
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    Last edited by Adam; 12-30-2010 at 02:09 AM. Reason: More info


  2. #2
    Moderator OsideBill's Avatar
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    Ask member addicted about going with the larger pump, he adapted one to his GP.
    Back in the day the guys racing these went with smaller 140mm skat 12 vein pumps.
    My guess on what this mod will do is decrease hole shot performance, and possibly give you a little bit on top end.
    although I can't see it over stuffing much on top end with that intake grate.

  3. #3
    Adam's Avatar
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    That grate is only for alignment purposes, Worx had a superjet grate that added 2 mph by doing the same thing. I was looking at the tunnel repairs on GPR's and noted the smaller tunnel size. Even the new FZR;s have a smaller tunnel, with the bigger 75mm pump. Not too worried about less holeshot if it holds speed better in the chop.

  4. #4
    Moderator OsideBill's Avatar
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    Im not sure what you are looking at my GPR tunnel has more volume then my GP tunnel does.

  5. #5
    Adam's Avatar
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    Measure the distance from the front of the impeller to the hole thru the hull on the GPR and do the same on the GP tunnel duct.

  6. #6
    Moderator OsideBill's Avatar
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    Compare the width that is the huge difference

  7. #7
    Adam's Avatar
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    The GPR maybe wider but the length of the aluminium duct(GP ) means it takes more water to fill each time it comes unhooked. Thats why I am filling in the top of the tunnel. Thanks BTW about "addicted" , ive just looked at what he did. He did a good job!

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