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  1. #1
    GGG's Avatar
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    Tunnel pressure relief idea

    Opinions are welcome. Here's what I did. Pictures are blurry because my galaxy s5 lost its ability to focus.
    Click image for larger version. 

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  2. #2
    Prism's Avatar
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    That method has been used by others here before (Kerry H) and worked well from what I understand.

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    GGG's Avatar
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    I'll have to look for it. Thanks for the info

  4. #4
    Cool!

  5. #5
    Happily Self-Employed WFO's Avatar
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    holes are waaaay too big

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  7. #6
    GGG's Avatar
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    What's wrong with too big? Less pressure built up

  8. #7
    RXP 260 X Short Course Veteran RXP244's Avatar
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    Quote Originally Posted by WFO View Post
    holes are waaaay too big
    I might tend to agree. I used 11mm drill and lined the holes with brass tubes with ID of 10mm. And the reason I lined it was because the shoe is hollow and my rubber inserts kept moving and blocking the holes. So I just put tubes through from top to bottom and finished them off smooth at each end. Works great.

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  10. #8
    GGG's Avatar
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    I welded aluminum tubes in there. I know that some people say you lose speed if you don't have enough pressure but I'm more concerned with never ever coming close to bucking. I'm building this GPRSHO 100% stock for my girlfriend. Plus I could always put restrictions in there if I really had to

    Are there any other down sides to having the hole too big?

  11. #9
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    too big will cause a loss of speed in smooth water. BUT the faster u go the bigger they made need to be. I like to put them on a slight angle so that they tend to be self regulating ie the faster u go the more they relive. Vacuum effect of water rushing past the hole to pull more out. They can also allow more water to reverse and come into the hole under vacuum ie takeoff.

  12. #10
    Happily Self-Employed WFO's Avatar
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    too many holes or too big and the jet pump loses efficiency

    It also depends on which intake grate you are running and the amount of angle on the plate/hull

    it has been tested years ago, and even 1/2" holes were too big... Hibdon did the initial testing

    on stock or low HP 4 stroke GPR conversions I usually put (2) 1/4" holes in the shoe with no valves (up to 84ish mph)

    big HP GPR conversions, (2) 1/4" holes with relief valves (85+ mph)

    oh and BTW, ive been over 83 on a 2 stroke GPR with no relief and was fine, but you must remember to be careful when coming off the throttle

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