[Question] GSXR 750 motor installed in Seadoo jetboat - Page 2
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GSXR 750 motor installed in Seadoo jetboat

  1. #15
    Hoodys and swim trunks Elkidminoltu's Avatar
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    Quote Originally Posted by Carnivalride View Post
    I see 17 to 20 being a 85%
    I see 30 to 19 being a 1.58%
    so overall would be a 1.34% over drive

    so if you change the 17 for an 18 you be at 1.42% or for a 19 would be 1.50%
    AGK - Gear Ratio Calculator - Affordable Go Karts
    Jackshaft Ratio Calculator

    Final ratio of 0.745 at the impeller shaft. Engine RPM for the Calculator would be transmission shaft speed at 5000/1.18 gear ratio = 4234 RPM

    Engine RPM = 5000

    Trans output shaft RPM (from first link):
    1st: 1795
    2nd: 2437
    3rd: 2974
    4th: 3448
    5th: 3834
    6th: 4234

    Now divide by the final drive ratio in 6th gear: 4234/0.745 = 5687 impeller shaft RPM

    I need a stiff drink

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    Quote Originally Posted by rschap1 View Post
    Be careful with that hot exhaust inside an enclosed compartment too.
    Thanks for the warning.....I thought of that as well so we installed an aux fan in the decklid with a "hood scoop" to allow hot air to be exhausted.

  4. #17
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    Guys i really appreciate the assistance!! BUT, this is waaaay over my head. Hard for me to grasp it, numbers just aren't my thing. Let me ask a few questions...

    1. Is the current sprocket setup a good one for this application?

    2. My current impeller is a 13/19 stock seadoo impeller and it didn't perform well. Should i go bigger or smaller?

    3. What is the thought process with this gearing, teeth and drive selection? Does it appear that the builder thought this out and put in place for a purpose OR did he throw sprockets that were laying around at it?

    Define didn't perform well.......No reverse thrust unless I RPM'd motor to 7k. Forward thrust at 4k. I started off in 4th gear so I could rev high without overworking the motor.

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  6. #18
    Hoodys and swim trunks Elkidminoltu's Avatar
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    1. Is the current sprocket setup a good one for this application? By the looks of the RPM output, it could work. You need to run it in sixth the whole time to do anything at all. Also...did you verify you were actually in 6th? How?

    2. My current impeller is a 13/19 stock seadoo impeller and it didn't perform well. Should i go bigger or smaller? What other options are there?

    3. What is the thought process with this gearing, teeth and drive selection? Does it appear that the builder thought this out and put in place for a purpose OR did he throw sprockets that were laying around at it? Again, based on having to run in 6th all the time to make any RPM at the shaft, I'd say no. He threw in whatever he had. Can you ask the builder?

    Define didn't perform well.......No reverse thrust unless I RPM'd motor to 7k. Forward thrust at 4k. I started off in 4th gear so I could rev high without overworking the motor. So that's part of your problem. You need to leave it in one gear, sixth, and test it. That's the closest to a 1:1 ratio you're gonna get and test performance then. Anything under 6th is a waste of your time. The tach is not going to help you unless you're in 6th gear. Everytime you shift down a gear, the impeller is spinning slower and therefore less thrust, no matter what the tach says, it's all about that impeller RPM which is affected by the transmission.

  7. #19
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    Now we're moving mountains~!!!! I know 6th gear by 1 down and 5 up shift pattern plus the lights on the dash indicate which gear it's in.


    Options for impellers are endless. Glen over at SkatTrak is assisting me with this.


    Yes, I agree the more RPM at impeller the faster I should go, so 6th gear is the best choice!
    Last edited by CollinCountyCab; 04-16-2015 at 02:20 PM.

  8. #20
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    In a stock seadoo jetboat even at 2k RPM your moving forward. With my current setup I'm not moving at 2k....i need 4k to have forward thrust.

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    Question...If i wanted to go to the extreme low end pitch of a impeller what would it be? If I was only worried about low end torque....forget about top speed. I'm thinking the lowest pitch possible to see if it will have instant thrust at just above idle. What would that number be in impeller....ie 9/14? thanks

  10. #22
    Senior Member DuaneHTP's Avatar
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    To get quick acceleration, try a higher pitch impeller.

    One thing that comes to mind here that hasn't been mentioned is; How much weight did the new motor add to the jet ski. Maybe your weight/buoyancy ratio is way the hell off.

    If the weight ratio is the same, the ski should go the same speed with the new motor as it did with the old motor. If the new motor is way heavier, that may be what you are fighting.
    Last edited by DuaneHTP; 04-18-2015 at 10:11 AM.

  11. #23
    Or Seth, either one Budweiser's Avatar
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    If you floor it, does it hit the rev limiter or pull to an rpm and settle. If it settles at an rpm, what is that rpm? If you haven't tried it, gonna have to. 6th gear ALL THE TIME. Might even consider disconnecting the shifter, leaving it in 6th.

    Without knowing that, I agree with the above. More pitch. Definitely not less.

  12. #24
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    Hmmmmm.....weight? Never thought of it. Will have to look into it but I cant see the new motor being more than 200lbs more if that. So I need a higher pitch impeller. I though higher pitch was for topend. What do i know.....thanks!!!

    I'm currently flushing out the cooling system and swapping out an old hose for new one.

  13. #25
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    Default Update....

    i wanted to flush the "closed cooling system" but I couldnt get the heat exchanger cap off. Couple hits with the hammer..BAM it came off allright. Broke off! I ordered a new weld on filler neck made of aluminum. Took it to a local aluminum welder and we are back in business. Put it all back together...filled it..bled the air out of it. Like brand new. Just got back from the lake and had a blast. Almost makes me want to figure out the right impeller and keep it!! BUT>>> I really want a over transom "monster"

    Once again...thanks for all the help!

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