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  1. #1

    144mm Yamaha pump Pitch vs Nozzle diameter question.

    Hey guys. I built a Zodiac Projet (see picture) for my son. I put a 760 motor with a 144mm gp760 pump in it. I used an 83.3mm reduction nozzle (stock 701 venture) with an Impros 10/16 Hooker prop.
    The machine is a rocket and tops out at 46mph on GPS! You would think leave it alone right? Well what bothers me is the max rpm it will pull is 6750. I think the 64x cdi module will allow 7100 rpm (could be wrong).
    I was wondering if the nozzle size (up or down) could give me those lost rpms. Should I try larger or smaller to get the rpms up?
    I also have some bottom hull work to do. The wet surfaces are less than spectacular for speed.
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  2. #2
    There is an 84mm reduction nozzle on ebay and I was thinking it may free up those rpms vs my 83.3mm. Is that too little? Or do I want smaller?

  3. #3

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    Why don't you try boring out the nozzle you have 1st.

  4. #4
    I dont know if a larger opening is going forward or backward. Also I would rather have different ones to swap back if it dont work out.

  5. #5
    Yamaha artisan Cutlass's Avatar
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    A larger nozzle gives you more volume but less pressure (or water flow velocity) which will result in a better holeshot but reduced top speed. You can accomplish larger or smaller nozzle size(volume vs. velocity) with a pump cone change, also.

  6. #6
    I learned that theory with the garden hose. I need an answer to the RPM question.
    Quote Originally Posted by Cutlass View Post
    A larger nozzle gives you more volume but less pressure (or water flow velocity) which will result in a better holeshot but reduced top speed. You can accomplish larger or smaller nozzle size(volume vs. velocity) with a pump cone change, also.

  7. #7
    Yamaha artisan Cutlass's Avatar
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    really. you learned how nozzle size affects top speed vs holeshot with a garden hose. What are you trying to accomplish with with more RPMs? Because right now your rpms are about where they should be for a 760.

  8. #8
    I like Boobs. wetwolf's Avatar
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    Put a pipe on it, you won't get to 7 grand with the stock motor and pipe combo. Increasing the nozzle size MIGHT get you some extra RPM, but only if the motor is capable of it.

  9. #9
    Its not the theory of higher revs I am looking for. Its the use of 100% of the engines power delivery. Are you saying that you know from experience that a stock 760 engine is peaked at 6750 rpm? I recall reading plenty of posts that say differently. Of course most machines are not stock. However I had not yet heard of 6750 or even 6800 being considered the peak.
    I do appreciate the help guys. For what its worth and for the benefit of my education lets make believe the engine peaks at 7000 rpm. Based on nozzle size (not prop pitch) what would I want to do to find that additional 250 rpm?

  10. #10
    I like Boobs. wetwolf's Avatar
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    bore the nozzle out 3mm.

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