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  1. #1
    ksalles's Avatar
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    RXP-X venturi mm relative to rpm HELP

    Ok...I'm at a standstill with my ski. 8200rpm I was 78.0mph with a stock untouched prop in cool low humidity air. Last weekend it was warm and the first time out with the Solas and it ran 8120rpm and basically 78.0mph. Now the air was better and lower humidity today and I ran 78.7mph @ 8200 on the limiter. I like to try and keep things simple and I think changing nozzle diameter could possibly be the ticket to potentially more mph for me. Just a run down of my mods again:

    R&D grate
    4" front intake w/ catch can
    2 degree wedge and VTS extension rod
    stock RXP nozzle
    solas 15/22R
    pump vains worked
    reverse bucket mod
    ride plate holes filled
    (I think thats it)

    Since I've done the Solas and my buddy knife edged the pump vains, the ski shows NO sign of bucking or acting up at top speed any more. Even hitting small wakes etc. and holding it wide open is actually suprisingly calm relative to what it was before the prop and pump vains worked. I'm not sure what helped more, but it definitely feels calm and stable now.

    My question is should I try and run a smaller nozzle to drop rpms and potentially gain more top speed? Before with the way the ski was acting and trying to spit me off at top speed I would have never considered restricting the nozzle any more because of possibly stuffing completely. Basically I'm looking for something to drop rpms and add more speed in the simplest form that I can. I know pitching the prop would basically do the same, but I'm looking to do something simple and easily reversed if need be relative to weather conditions.

    Thanks again for any input and help. This site has been a HUGE help in getting my ski to run what it does with so little.


  2. #2
    h2jim's Avatar
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    I have a venturi that I got from Jerry that has an 81mm ring welded in for sale.

  3. #3
    Mod less, ride more! troyheb's Avatar
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    Contact forum member Duke and have him pitch your prop. My first 15/22r I had I was on the limiter @ 75+ mph. Had Duke pitch it and I ran 77.3 @ 8160-8180 in cool air.

  4. #4
    JT jpt7779's Avatar
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    Cool

    Great performance. I'd send your 15/22 to Erik at SpeedFreeksRacing for his setback mod and repitch! I guarantee you he will deliver what you desire!

  5. #5
    ksalles's Avatar
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    I was hoping since we modded the pump vains a little and changed to the Solas prop, that my pump as a whole was more efficient and less likely to stuff. And as a result, it would allow me to safely run a smaller nozzle diameter to pick up mph and drop the needed rpms. As mentioned, since the Solas and little work to the pump vains, the ski acts totally different and shows no sign of trying to stuff or buck as it did before.

    Whether or not this makes sense, I came up with a quick guesstimate on what my ski should run with the Solas compared to my stock prop. My ski ran 78.0mph w/ the stock prop in PERFECT air. It was at the limiter at 8200.

    So:

    78/8200 = 0.009512195122 mph per rpm (stock prop @ 8200rpm running 78.0mph)
    78/8120 = 0.00960591133 mph per rpm (Solas prop @ 8120 running 78.0mph in the heat)

    So what I figured if I went and ran the ski yesterday in the cool, low humidity air the ski should run to the limiter w/ the Solas prop. IF it did:

    8200 x 0.00960591133 = 78.76mph (8200 limiter x mph per rpm w/ the solas)

    Low and behold, it ran 78.7 on the limiter yesterday. Is that just a fluke, or does my method of thinking and determining prop speed relative to engine rpm make sense?

    We didn't pick up the ski til dark, so the pic is dark as well.

    Click image for larger version. 

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  6. #6
    Having Fun jp1300r's Avatar
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    Quote Originally Posted by ksalles View Post
    I was hoping since we modded the pump vains a little and changed to the Solas prop, that my pump as a whole was more efficient and less likely to stuff. And as a result, it would allow me to safely run a smaller nozzle diameter to pick up mph and drop the needed rpms. As mentioned, since the Solas and little work to the pump vains, the ski acts totally different and shows no sign of trying to stuff or buck as it did before.

    Whether or not this makes sense, I came up with a quick guesstimate on what my ski should run with the Solas compared to my stock prop. My ski ran 78.0mph w/ the stock prop in PERFECT air. It was at the limiter at 8200.

    So:

    78/8200 = 0.009512195122 mph per rpm (stock prop @ 8200rpm running 78.0mph)
    78/8120 = 0.00960591133 mph per rpm (Solas prop @ 8120 running 78.0mph in the heat)

    So what I figured if I went and ran the ski yesterday in the cool, low humidity air the ski should run to the limiter w/ the Solas prop. IF it did:

    8200 x 0.00960591133 = 78.76mph (8200 limiter x mph per rpm w/ the solas)

    Low and behold, it ran 78.7 on the limiter yesterday. Is that just a fluke, or does my method of thinking and determining prop speed relative to engine rpm make sense?

    We didn't pick up the ski til dark, so the pic is dark as well.

    Click image for larger version. 

Name:	787mph.jpg 
Views:	79 
Size:	31.4 KB 
ID:	219548
    Your formula will work to a point and you are going to reach that point very soon.

  7. #7
    West Texas RXP skoepp's Avatar
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    Quote Originally Posted by troyheb View Post
    Contact forum member Duke and have him pitch your prop. My first 15/22r I had I was on the limiter @ 75+ mph. Had Duke pitch it and I ran 77.3 @ 8160-8180 in cool air.
    With OEM or adjustable venturi ?

  8. #8

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    I didn't feel like this was important enough to start a new thread, so I found this one regarding venturi size. I previously posted this on the boat forum, but thought it might be food for thought here.

    I've got a Speedster 150 SCIC, probably running upwards of 270hp, which comes with an 84mm venturi. I had previously tried 83mm and 82mm venturis, but they never helped my performance a bit, I always went back to the 84mm. Well, I had the oddest, but pleasant result recently in trying an 87mm from an RXP-X. These tests were done back to back, same weather and no other changes. With the 84mm, I was running 62.7mph @ 8100-8120rpms. I figured the 87mm would give me better holeshot, but that topend would stay about the same, because my RPMs would go up and create more boost, to compensate for any speed loss. Contrary to my expectations, the 87mm had me at 64.1 @ 8040-8060. I actually dropped RPMs and boost, but gained a big boost in speed. My boat will keep the 87mm on from now on. I just thought it was an interesting result which defies conventional wisdom.

  9. #9
    b_dolla44's Avatar
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    I dont understanding why it would have reacted that way....interesting

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