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  1. #1
    JFizzleJR's Avatar
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    RPM, Boost, & Prop Questions

    I have a S/C question mostly geared towards people who have experience with the rotax high boost wheel and 2 other questions too. It's advertised at 9.6 lbs of boost at 8100 rpms. Also, with my boost gauge I see that the s/c produced negative boost #'s until about 5000 rpms. So, I'm getting 7650-7700 rpms and 6 lbs of boost. I figured I have a boost leak or the clutch is slipping because of the 3+ lb. loss. I had my supercharger checked and it's slip is perfect. I was going to wait until I found my lost boost before I tweaked the prop. My 3 questions are:
    1) If my ski has no boost leaks, could the difference between the advertised boost and the boost I'm getting (3 lb. difference) be because I'm turning around 7700 instead of 8100?
    2) I've sealed the I/C pipes, the stock I/C B/O is siliconed in place (I'm going to check it again though), and my S/C is working fine. What else can I check that would cause the lack of boost?
    3) How many mm should I tweak this 15/20r 4 blade if I'm seeing 7650 solid? (I want 8100 RPMS)
    Keep in mind I have the 80, 81, 82, and 83mm rings. Right now I have the 81mm in and I'm seeing 7650. Whatever I tweak the prop to I want to still be able to work with the effects of weather by using the ring sizes in order to maintain 8100 for the next few months.


  2. #2
    ou812's Avatar
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    No pro at this but I would say put the 83mm ring & in & see what you get. The 8100 rpm advertised is from using the stock nozzle which is 83mm

  3. #3
    JFizzleJR's Avatar
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    I'm not really concerned at why my RPMS are down, because the prop is
    -2mm already and I need it at the out of the box pitch or less. The advertised # that I'm after is the 9.6 lbs. of boost. I'm way off. I will try the 83mm nozzle though

  4. #4
    Matthew K ReDevilRXP's Avatar
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    Quote Originally Posted by JFizzleJR View Post
    I have a S/C question mostly geared towards people who have experience with the rotax high boost wheel and 2 other questions too. It's advertised at 9.6 lbs of boost at 8100 rpms. Also, with my boost gauge I see that the s/c produced negative boost #'s until about 5000 rpms. So, I'm getting 7650-7700 rpms and 6 lbs of boost. I figured I have a boost leak or the clutch is slipping because of the 3+ lb. loss. I had my supercharger checked and it's slip is perfect. I was going to wait until I found my lost boost before I tweaked the prop. My 3 questions are:
    1) If my ski has no boost leaks, could the difference between the advertised boost and the boost I'm getting (3 lb. difference) be because I'm turning around 7700 instead of 8100?
    YES, the more rpm's the more boost

    2) I've sealed the I/C pipes, the stock I/C B/O is siliconed in place (I'm going to check it again though), and my S/C is working fine. What else can I check that would cause the lack of boost?
    3) How many mm should I tweak this 15/20r 4 blade if I'm seeing 7650 solid? (I want 8100 RPMS)
    Keep in mind I have the 80, 81, 82, and 83mm rings. Right now I have the 81mm in and I'm seeing 7650. Whatever I tweak the prop to I want to still be able to work with the effects of weather by using the ring sizes in order to maintain 8100 for the next few months.

    The engine is a vacume untill about 5 grand.

    Why dont you just put the 83 ring back in ? that will give you around 200 rpms. Id say put the 83 in run it and then tweak the prop then fine tune it with the rings.

    But your set up should be able to turn that prop and some.

    From what Jerry's told me 1mm = 100rpms.

  5. #5
    JFizzleJR's Avatar
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    The thing is the prop isn't the out of the box pitch..
    And if most people swear by the fact the best ring is 81mm and to pitch the prop for 8100 with the 81mm, why would I put the 83mm in? I guess I should've made it more clear; I've still got to re-pitch the prop no matter what ring I put in

    ReDevilRXP, you said more rpms = more boost. I understand that, but do you know the type of ratios that are involved? For example does 100 rpms = 1 lb boost etc..

  6. #6
    shhr's Avatar
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    RPM`s make alot of difference in boost, on my setup at 8000-8100 I see around 11.5 when my RPM`s dropped to 7800-7900 I was seeing 10.5-10.8, so I would check everything then work on the prop to get to 8100 I bet your boost will be where it should be

  7. #7
    JFizzleJR's Avatar
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    Alright, that's what I was hoping to hear. Thanks Scott

  8. #8
    Banned User
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    put the stock nozzle on and see where you fall, its 100x's easy to do that than it is to keep pulling the prop and tweekign it....HECK, you can do the nozzle change on the trailr AT THE RAMP!

  9. #9
    JFizzleJR's Avatar
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    I know but this prop was turning 8400 on a stage III + b-kit setup. It's a little too steep for my Rotax high boost wheel and stock ecu..

  10. #10
    Moderator The Bandit's Avatar
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    Take out the 2mm and you should be in the ballpark. your boost will boost when you are boosting 8100.

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