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  1. #1
    hill160881's Avatar
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    Please explain this low RPM 2stroke thing?

    So I am a chain saw and dirt bike guy and am used to a 2 stroke making at least 10k rpms usually more like 12-16k. Now I know that these watercraft engines are designed for torque over hp but I just don't understand why I don't see and skis that spin over 9k? My saws operate between 10k-14k rpm.

    So what's up? Is there simply not enough low end power to get out of the water or just to much resistance all the way around? Hell many 4 strokes are running higher rpms.


  2. #2
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    it's simple physics..this will give you some ideas to ponder

    http://www.hipermath.com/engines/max_rpm

  3. #3
    hill160881's Avatar
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    Most of that is accurate but #2 is totally wrong until you get to much larger engines than we use in watercraft. My friends drag boat with a 702(I think) does 11k rpms and 126mph. It has more to do with the design and circumference of the main bearings not displacement . The oil bearing is the main limiting factor in final top rpm. Which is why some engines that spin up higher use a roller bearing. Same holds true with turbos, ball bearing turbos spin up faster and turn at higher rpms than thrust and oil bearing turbos.

    My my point is that most two strokes run at a higher rpm. Chain saws dirt bikes..... Why not watercraft. Is it the constant load on the engine? The resistance of the water?

  4. #4
    This is how I run a jetski shop in the desert nmpeter's Avatar
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    the adjustable rev limiter found in some aftermarket cdi's allow for engine rpm up to 10k ( of course with dire warning about engine life)

    the issue could be the flow dynamics of the jetpumps, or the actual issue is that there was little to be gained by allowing stock models to turn more then xxxx rpm based on engine life/performance and fuel consumption.

    rotating mass is far lower on chainsaws and motorcycles of the two stroke ilk. Ducati motorcycles with their gear driven camshafts routinely turn 10krpm in a v-twin configuration, so it really points to a design/use issue.

    What is the max rpm on the new spark?..wouldn't be shocked if that little snowmobile mill could turn 9k+

  5. #5

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    Quote Originally Posted by hill160881 View Post
    Most of that is accurate but #2 is totally wrong until you get to much larger engines than we use in watercraft. My friends drag boat with a 702(I think) does 11k rpms and 126mph. It has more to do with the design and circumference of the main bearings not displacement . The oil bearing is the main limiting factor in final top rpm. Which is why some engines that spin up higher use a roller bearing. Same holds true with turbos, ball bearing turbos spin up faster and turn at higher rpms than thrust and oil bearing turbos.

    My my point is that most two strokes run at a higher rpm. Chain saws dirt bikes..... Why not watercraft. Is it the constant load on the engine? The resistance of the water?
    The efficiency of the jet pump degrades with increasing engine RPM. This doesn't mean that the boat won't go faster if the pump spins faster but it will take a larger amount of fuel (on a lb/hr/hp basis) to get there. The manufacturer's select the "sweet spot" for fuel consumption targets. At least this is what I've always thought. An additional challenge that a engine designer faces with a PWC engine is that there is no transmission to change the required torque/vehicle speed relationship. For this reason the required engine torque is much different than that in other applications.

  6. #6
    hill160881's Avatar
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    This makes sense. Water does not compress and a hose can only flow so much water without efficiency issues and the lack of a transmission does not allow for the higher rpms like you can get in other two strokes.

  7. #7
    BigHaag's Avatar
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    Yes pump efficiencies are the reason for limiting engine rpm. Through testing, the manufacturers (plus racers, aftermarket companies, etc) have found the watercraft jet pumps to start becoming less efficient as rpms increase (over 9000 approx). I doubt fuel consumption was a main reason for reducing rpm limits especially when carbureted 2 strokes was the main engine technology for PWC. That is the least fuel efficient engine technology.

  8. #8
    leejax01's Avatar
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    You're talking about a smaller cc, single cylinder engine vs a larger cc, 2+ cylinder engine. The Yamaha MR1 4 stroke engine revs 10k, but needs a reduction and isn't terribly fast or torquey.

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