Marinising automotive diesel, what about torque and rpm

Discussion in 'DIY Marinizing' started by Flork, Jan 22, 2020.

  1. Flork
    Joined: Jan 2020
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    Location: Spain

    Flork New Member

    hello, i think about marinising 3.0 mercedes engine, but i have one question, lets say mercruiser 4.3l have max 5000 rpm wot, and generate max torque somewhere around 4500, if i put diesel engine on its place it will generate max torque around 2300rpm, this will be to slow i think to plane a 8m searay on similar rpm to mercruiser it will be have almost no power, so it have any point to do this? ok someone can tell to change propeller, but i dont know even if that exist with that much bigger pitch, even if exist, that doesn't generate destroying force for upper gearbox?
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    What is the max RPM for the Mercedes engine ? Putting aside whatever difficulties are involved with marinizing it, there may not be a big difference that can't be adjusted for with the prop selection. I would not dwell on the max torque point as a guide. As to whether the drive can handle it, consult the maker's specifications.
     
  3. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    To start with, the 4.3 is rated at over 200HP. The Mercedes turbo at 188HP so they are not far apart. However, installing a turbo in a marine setting is not completely straightforward. The main problem is moisture. Each time you stop the engine, the exhaust gets full of steam if you have a standard wet exhaust. You would have to change the gears, which for an Alpha 1 can go as much as 1.46:1. However, in diesel applications the Bravo is usually used, because of the increased torque.
     
  4. Flork
    Joined: Jan 2020
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    Location: Spain

    Flork New Member

    ok but anyone have real world experience and can tell some details? my biggest worry is boat dont go plane, since prop and upper gearbox are tuned for gasoline engine, where torque and power are more linear. if boat plane at 3800-4000 rpm, that diesel at this rpm range have almost no power, economy also will go bad, best for diesel engine will be if the boat will plane at about 2500rpm, but its about 1500rpm less, and i dont know if i can adjust that by only prop changing. also i dont wanna doing any modification to upper gearbox, since i know how expensive is gears, so i want use stock gearbox
     
  5. Mr Efficiency
    Joined: Oct 2010
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    Location: Australia

    Mr Efficiency Senior Member

    Planing boats with diesels run them high in the rev range, not half peak revs, I think you are on the wrong track worrying about torque.
     

  6. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    In general diesel engines have a more linear torque curve than gas engines.
     
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