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  1. #1
    txgp1300r's Avatar
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    is there a maximum rpm for our two strokes

    I hear all the numbers 6000,7000,even 8000, is there a maximum rpm for the 1200/1300 two stroke motors in our ski's.?...and why is it a "maximum" number...4 strokes its the valves and springs and sometimes the rod bolts that are the limiting factor, two strokes????


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    R.I.P. 11's Avatar
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    It's not so much about how high a 2 strokes can rev, it's about how high a jetpump can be efficient.

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    There are many two strokes that can hit RPMs as high as 15,000, Iím not sure what the 1200/1300 physical limits are. However like 11 said, it's the pump that has the limit. It will start losing efficiency I believe around 9000-9500 RMPs or so. That's one reason you see the four-strokes with gearing vs direct drive...

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    Happily Self-Employed WFO's Avatar
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    Quote Originally Posted by RXPkiller
    There are many two strokes that can hit RPMs as high as 15,000, Iím not sure what the 1200/1300 physical limits are. However like 11 said, it's the pump that has the limit. It will start losing efficiency I believe around 9000-9500 RMPs or so. That's one reason you see the four-strokes with gearing vs direct drive...
    Unfortunately you start losing efficiency at a much lower rpm than 9000

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    Hydrotoys's Avatar
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    Yeah, actually closer to 5000 rpms. The curve is unbelievably steep. It goes from like 60% efficient to about 45% efficient in about 2000 rpms. Still, the higher you spin it, the more speed you will get...but the gains are much less than at lower rpms.

    But pumps are relatively safe compared to props, and have unequaled holeshot. If only somebody would design a true variable vien prop. Now THAT would be a work of art!

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    Connecticut CrazyA's Avatar
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    Re: is there a maximum rpm for our two strokes

    Quote Originally Posted by txgp1300r
    I hear all the numbers 6000,7000,even 8000, is there a maximum rpm for the 1200/1300 two stroke motors in our ski's.?...and why is it a "maximum" number...4 strokes its the valves and springs and sometimes the rod bolts that are the limiting factor, two strokes????
    Maximum for reliability? Yes... about 3500 feet per minute of average piston speed. Anything more than that on a watercraft engine starts to get risky. To figure yours, you need to:

    Cm = .166 x L x N

    Cm is mean piston speed in feet per minute.
    L = stroke in inches
    N = RPM

  7. #7

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    My FXHO motor (MR-1) has a mean piston speed of 3800 ft per minute at 10,050rpm. Not too shabby for a pwc !!

    There are new benchmarks for reliable piston speeds now .... The Yamaha R-1 engine in the R-1 streetbike has a mean piston speed of nearly 5000 ft per minute. Yummo !!!!

    Anthony

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    Connecticut CrazyA's Avatar
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    Re: is there a maximum rpm for our two strokes

    Quote Originally Posted by txgp1300r
    maximum rpm for the 1200/1300 two stroke motors in our ski's
    Add in over rev & reduced cooling when unhooked, and higher compression ratios usually associated with 2 stroke race motors, and you don't really want to exceed 3500. (give or take of course)

  9. #9
    just GOT TO DO IT! great lakes gpr 1200's Avatar
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    Re: is there a maximum rpm for our two strokes

    Quote Originally Posted by txgp1300r
    I hear all the numbers 6000,7000,even 8000, is there a maximum rpm for the 1200/1300 two stroke motors in our ski's.?...and why is it a "maximum" number...4 strokes its the valves and springs and sometimes the rod bolts that are the limiting factor, two strokes????
    What is the best top speed RPM for a 1390cc? I am at 7220rpms . where is best? I feel like I have 1/3 the throttle left from top end to wot. can I pitch it up? 1 more degree?

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