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what's the best outdrive gear ratio

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  • what's the best outdrive gear ratio

    I'm planning to replace the omc outdrives on my 265 sedan bridge searay with the sei outdrives and conversion kits but i don't know what gear ratio i should order. How can i find out what i have and most important what would be best? I currently turn 15/15.5 al props and turn about 4400rpms. I have to run at 3800 rpms which gets me about 27mph or it starts plowing. I would like to get the rpms down but still get the 27mph. Is this possible?


  • #2
    Re: what's the best outdrive gear ratio

    What engines do you have? and what horsepower?

    You should be able to calc the ratio on one of your current drives once it is pulled by rotating the input shaft and counting the number of turns it takes to turn the prop shaft one revolution.
    1991 HydroSwift 2200 Cuddy Volvo Penta 570/DP
    1980 Glastron SSV167 90HP Mariner

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    • #3
      Re: what's the best outdrive gear ratio

      Originally posted by vdomt View Post
      I'm planning to replace the omc outdrives on my 265 sedan bridge searay with the sei outdrives and conversion kits but i don't know what gear ratio i should order. How can i find out what i have and most important what would be best? I currently turn 15/15.5 al props and turn about 4400rpms. I have to run at 3800 rpms which gets me about 27mph or it starts plowing. I would like to get the rpms down but still get the 27mph. Is this possible?
      Turning that RPM sounds like you have big block 454's, if this is correct then your probably doing the best you can. Wide Open Throttle (WOT) should be limited to no more then 4600RPM. The WOT can go above that but is not recommend because things can start comming apart. You can reduce your prop pitch but that will bring you above 3800. Reducing the engine RPM will only put more load on the prop and outdrive, it won't save fuel but it will increase the time it takes you to come up on plane.

      The OMC's gear ratio chart can be found here http://www.sterndrive.info/chart.html Doing some quick calculations on the King Cobra the ratios look to be 2.0 to 2.4. I'm going to guess you have the 2.0 but it's just a guess. Hope this helps.
      94 Formula 27PC w/ 8.2L EFI 0F439422 B3-0L411397
      95 Rinker 232 w/ 7.4L Carb 0F425011 B1-0F486471
      07 SeaDoo GTX
      Raw Water Pump Manitenance
      Hang in there iboats web site will get better

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      • #4
        Re: what's the best outdrive gear ratio

        If the boat is plowing, trim the drive up some, don't change gear ratios.

        You don't calculate the gear ratios for boats, it's already done. The manufacturers calculate which drive goes with which engine.
        But, until we know the year, and engine type and drive type, it's impossible to guess which gear ratio you should have.
        Don S.

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        Please, no PM's (Private Messages) regarding boat/engine problems.
        That is what the forums are for.
        Only forum/moderator issues will be answered in PM's.

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        • #5
          Re: what's the best outdrive gear ratio

          Morning Don S. I noticed in the original post that he planned to replace the drives with SEI's. I'm guessing he want's to know what his ratio is now for a baseline.

          Best Regards, Bob
          94 Formula 27PC w/ 8.2L EFI 0F439422 B3-0L411397
          95 Rinker 232 w/ 7.4L Carb 0F425011 B1-0F486471
          07 SeaDoo GTX
          Raw Water Pump Manitenance
          Hang in there iboats web site will get better

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          • #6
            Re: what's the best outdrive gear ratio

            That may be what he wants to do, but if he doesn't have a Cobra, then it won't work. If he is running twin stringers, there is no conversion.
            Don S.

            sigpic

            Please, no PM's (Private Messages) regarding boat/engine problems.
            That is what the forums are for.
            Only forum/moderator issues will be answered in PM's.

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            • #7
              Re: what's the best outdrive gear ratio

              Thanks for the inputs sorry i forgot the engines. They are 1987 4.3 v6s rated at 175 hp each.

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