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  1. #1
    Vern's Avatar
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    RXP speed/pump ideas

    The GPR has a 155 mm pump. The old pump had a smaller diameter pump hub, and was less efficient. The new one (2005 on up, like on HO) is still 155 mm, but has a larger diameter hub, making it more efficient and faster.

    The GPR uses a 85 – 87 mm venturi (thrust) nozzle, the RXP uses 82 mm and some even do well with 81 mm if rpms are too high. What would happen if we set up a 85 mm RXP nozzle and ‘pitched up’ a 15/20 to bring rpms back to the right level? GPRs are running 15/23 props, sometimes more, with 87 mm nozzles … and are FAST. SOME of that speed is in the pump, impeller and nozzle.

    Generally the bigger HP jets (boats and jet skis) seem to do better with pumps that are bigger diameter, but ALSO with bigger diameter hubs. Makes sense since the impeller tends to ‘throw’ water to the outside, making the inside center of the pump the least efficient, lowest pressure area. Bigger impellers, bigger pumps, and bigger hubs = more pressure and more pressure = more speed.

    The supercharged Kawi 15F setups use 160 mm Skat pumps with (?) hub size and 84 mm nozzles and 10/18 Skat props … on the CC setup skis. I would bet the recreational versions could use much steeper impellers and bigger nozzles.

    If you break down most fast setups, the venturi nozzles kinda fit a formula … they generally equate to dividing the pump diameter by two, then adding a few mm. For example, in the GPRs, divide 155 by 2 = 77.5. Optimal nozzle size seems to be 85 – 87 mm, which equals ½ pump size ( 78 ) + 7 – 9 mm. On the Kawi race 15F SC, it equals ½ pump size + 4 mm, and I bet on the recreational ones it is more like ½ pump size + 5 – 6 mm. The RXP on the other hand is ½ pump size + 4 mm. That tends to yield good takeoff, but less top speed.

    I think some experimenting with a bigger nozzle and bigger impellers MIGHT work well for speed. The funny thing is our RXPs DO have some similarities with the really fast single pipe GPRs … our pumps are similar in dimensions, esp to the newest HO style pump. Yes, the GPR hull is more efficient and faster, but the pumps are similar in style, AND they are pulling pretty close to the same HP too … seriously. The well-ported 1300s with good reeds and more might be within 10 hp of a stock RXP, maybe even equal since our RXPs are overrated from the factory.

    Another interesting pump thought … Kerry now is running 83 mph with a single pipe GPR, Fercho is at 80 mph, and Carl from Island hit 80+ awhile ago too. That is faster than many triple pipe GPRs have run … the interesting part is the different rpms and pump setups between a fast single pipe and a fast triple pipe GPR. The single pipe GPRs are running big impellers around 7100 – 7200 rpm to get 80 mph. The triple pipe GPRs are running less pitch and turning 7800+ rpms … to hit the same 80+ mph. So, the speed the impeller is turned is not all that critical. Instead, it looks like the impeller pitch is adjusted to get the motor to the rpm where it makes peak power. So a smaller impeller at higher rpm will create the same pressure as a bigger impeller at lower rpm. But BOTH setups run 87 mm nozzles usually, even with different HP levels. That is why I wonder about running a much bigger nozzle on the RXP with a steeper pitched impeller. To really test this, we should set up a 85 mm nozzle and pitch up a Solas 15/20 until we get the rpms right and find out if it adds speed. This test might only work with a stock ECU setup, or with the latest upgraded RR ECU.

    Another thing I will throw out there (again) is that the more restrictive 800 grate also gains 1 mph on the GPRs. I still think another thing to test is restricting our Riva intake grates … the faster the machine, the more water will automatically be forced into the pump. Too much water forced into the pump slows the machine down … This has been proven in many different tests, in full size jetboats as well as jet skis. Making the side bars in our grates thicker, especially in the front portion might be another good experiment, especially on the faster modified RXPs. Generally open grates are good for takeoff and eliminating cavitation, and more restrictive grates add top speed.


  2. #2
    Site Admin Green Hulk's Avatar
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    Re: RXP speed/pump ideas

    Quote Originally Posted by Vern
    The GPR uses a 85 – 87 mm venturi (thrust) nozzle, the RXP uses 82 mm and some even do well with 81 mm if rpms are too high. What would happen if we set up a 85 mm RXP nozzle and ‘pitched up’ a 15/20 to bring rpms back to the right level?
    I want to try this!

  3. #3
    Top banana's Avatar
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    You may be on to something there

  4. #4
    Site Admin Green Hulk's Avatar
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    Vern, you are definately onto something.

    Skat has my nozzle and is modding it to accept different sized venturi rings. I will definately test this with a larger diameter venturi.

    Wish i had another Riva grate to mod

  5. #5

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    I really think you may be on the right track. I have always wondered why the Yamaha runs the speeds it does turning far less RPM,s. It has to be in the pump,prop and nozzle.Good luck on your test Jerry let us know the results.

  6. #6
    Site Admin Green Hulk's Avatar
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    anyone have a used Riva grate to sell cheap, or donate to the cause?

  7. #7
    buddydoo's Avatar
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    Just what I posted about about two weeks ago how with the power the rxp has it all in the pump mods.I'm going to doo alot of thinking about that pump and see what I can get machined.

  8. #8
    Site Admin Green Hulk's Avatar
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    Can you machine a duplicate of a Riva grate, but only with thicker bars?

  9. #9

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    I had both the 800 and 1200 intake grates for my gpr and looking at them both side by side the 1200 grate has a steeper ramp where it connects at the pump shoe. This makes it load more water to the top of the impeller. The 800 grate was faster but would spin out of the hole.

  10. #10
    buddydoo's Avatar
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    That grate is casted aluminum so it would have to be welded together to made a custom intake grate.

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