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  1. #1
    excess's Avatar
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    Lost 1000 RPM with 15/20 and 81 Reduction Nozzle

    I don't think there is anything wrong with the ski at all the numbers are just a little more drastic than I expected but still think its absolutely normal because the ski isnt making the power for that size ring and impeller. The reason I set it up this way is for my b kit I am going to be putting on some time soon and didn't think the nozzle and impeller would affect me that much until I installed it.

    Revving 8580
    Rotax "Racing" ECU
    Basically a Riva Stage 2 with no supercharger wheel ( stock charger )
    14/19

    After the 15/20 and 81 Ring
    7600 Max RPM

    I understand that the putting it back to 83 would get my rpms back up a little until I put the new supercharger kit in but havent had the time to put it in the water and see exactyl how much I would gain.

    I thought the 15/20 took away say 400 RPMS and 100 for each smaller ringputting me up at about 8000 RPMS but i must be getting the math wrong or simply dont have the power to push it making me lose that much more.


  2. #2

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    To turn that prop and venturi size you will need an external intercooler, RR SC impeller, outside air package, RR recreational ECU (4502), or the full race one, plus adding a 158 degree thrmostat would help a bunch. If you have the above you should turn about 8300 to 8400 RPM. You might have to install a 82 mm venturi to reach 8400.

  3. #3
    excess's Avatar
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    ive got everything there except the supercharger wheel

  4. #4
    excess's Avatar
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    plus from 8580 to 8300 is only 300 RPMS, the impeller and venturi will drop it way below that for sure, so I don't know how you got your numbers.

    Anyone else with some input.

  5. #5
    Moderator Franko's Avatar
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    Yes, that prop and nozzle combo can drop your RPM dramatically. With a stock supercharger, you just don't make the power needed to turn that prop. The smaller nozzle just compounds the problem. 7600 RPM with that set up sounds about right.

    Man, how in the world were you able to turn 8580 RPM with a stock SC impeller? That's the most surprising part of your post to me.

  6. #6
    excess's Avatar
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    I was at 8180 before the ecu so the 400 gained sounded about right to me.

  7. #7
    excess's Avatar
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    with this long weekend ill have time to go to the canal and put the 83 on so I see what difference i get just out of the 15/20

  8. #8
    Moderator Franko's Avatar
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    I'm not doubting you, but I had the same set up with the Vortech SC impeller and never got near even 8400. And if you have the stock valvetrain, you don't want to be running 8580.

  9. #9
    excess. I have what you have 06 boat also. stick withthe 83 for rpms until you bend the prop. all same mods

  10. #10
    Jarrett's Avatar
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    What ECU are you running? I was running 8100 stock and put the RR Racing ECU on and saw very minimal gains. I think 8220 was my highest RPM of the day. How did you get 300 more RPM out of just an ECU install?

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