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  1. #1

    4 Stroke Transfer Gears

    I have a question, I was thinking about dropping twin FX HO motors into my Exciter, when I noticed something in the Yamaha drawings.

    The pump isn't directly driven by the crankshaft in 4 strokes, it's got a set of transfer gears, #'s 11 & 19 in the attached illustraion, which by my count are 18/25, making for a .72 overdrive of the output shaft.

    ie. The engine might be spinning 10,000RPM, but the pump is only actually spinning 7200RPM.

    Wouldn't more RPM = more MPH in the top end? And/or the ability to run a taller prop & still have the torque to spin it?
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  2. #2
    cheatin' piston popper addicted's Avatar
    Join Date
    Jul 2006
    north jersey
    yes, the yami 4 strikes need a reduction because they are street bike based motors and like to rev to the moon. It's pretty commonly accepted that these kind of jet pumps loose efficencey above 8000 rpm or so.

  3. #3
    Hydrotoys's Avatar
    Join Date
    Jul 2005
    Mesa, AZ
    Yami undersized that engine by about 400cc's. As Mike said, they had to compensate the streetbike engine, by spinning it up in it's powerband.

    No other fourstroke engine in a ski does this.

  4. #4
    fastkaz's Avatar
    Join Date
    Jan 2007
    Cayman Islands/Orlando Fl
    is there a way to get the full 10,000 rpm directly to the pump changing the gears?

  5. #5

    Join Date
    Sep 2007
    Lexington, Ky
    You dont want the full 10k, but you would benifit if you could up the actual engine output rpm's, maybe run the shaft at 85-8600 ideally. That motor has GOT to be good for 11000 at LEAST.. The $100k question is how many parts are exact R1 swap overs? From what I have observed 8000-9000 is the sweet spot for top rpm on these pumps.. (but take that with a grain of salt, Im old school engine/boat guy, but new to these PWC jets). Yeah, thats the the thing that really kills me about these motors.. they didn't even take the time to make a proper cam, so the motor would make power in the appropriate power band usable for a jet system, if it was even possible utilizing an engine built for high rom power operation.. That being said, I haven't done the calculations, but that drive system does increase gross power output of the engine throughout the entire rpm range, since it is a down gearing.

    Knowing Yamaha though, and seeing that a short block comes with the gearing, I would assume the 140hp output is at the down reduction box output shaft. I have been wanting to get one of these, but missed the auction for that shortblock. And get an R1 and start swapping parts around.. to compare it to an r1 engine setup... Because R1's can make 200hp with a little work albeit at 14k without forced induction, maybe using that short block, or internals, with the gear reduction, and your looking at a (just off the top of my head, 250hp 8500-9000max rpm revving monster.. this is if alot of the parts are crossovers... Anyway... just an idea over head, If I recall the R1 engine makes ~189hp at 14k ??? you'd just have to re-cam it for maybe 1200 rpm... anyway, enough ramblings.. the Yamaha gear reduction pisses me off, its a cop out, easy fix, instead of properly adopting the engine for a marine application, period... (stepping off soap box )
    Last edited by boostaholic; 12-24-2007 at 04:39 PM.

  6. #6
    volltrex's Avatar
    Join Date
    Dec 2007
    Jersey CI
    boostaholic, unfortunately you will find that the MR1 has very few common parts with the R1!! The block is different as there is no gearbox, the crankshaft is different due to the R1 having to turn a transverse gearbox, different head due to exhaust water jacket for cooling, different bore etc etc, the list goes on so no cheap parts from the breakers yard.

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