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has anyone converted from 350 to 8.1/496 ,cost,difficulty parts needed?

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    #16
    thanks to all who responded,my boat is 1989 2556 with omc cobra outdrive 350 engine sorry for not disclosing this in the beginning

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      #17
      Originally posted by funfishinman View Post
      thanks to all who responded,my boat is 1989 2556 with omc cobra outdrive 350 engine sorry for not disclosing this in the beginning
      Your OMC Cobra drive has a limit regarding the horse power in front of it. In other words, it's doubtful that it would hold up to BB power.

      If you want to increase your boat's performance by 25% to 35%, consider purchasing a 1.95:1 Volvo Penta DP-S stern drive and the installation kit.

      Better yet, an AQ series Volvo Penta DP-C or DP-C1 complete system.
      Rick E. Gresham, Oregon
      2850 Bounty Sedan Flybridge model
      Twin 280 HP 5.7's w/ Closed Cooling
      Volvo Penta DuoProp Drives
      Kohler 4 CZ Gen Set

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        #18
        Maybe look for a donor boat, one with a 7.4 and a bravo drive. From there, you could build to a 8.1. here’s an example in my area where boat prices are well above national averages. Pictures of this boat showers it’s well kept so again a higher price. I bet you could buy this boat for 8,000 and sell the trailer for 1,500 so 6,500 for a low hour 7.4 with a bravo drive. I don’t know prices to rebuild to a 8.1 but I’m sure other members do.

        Just a thought.



        Click image for larger version  Name:	48389E0B-998C-4B7D-9A53-82C893BC8E8F.png Views:	0 Size:	1.03 MB ID:	599515
        1982 3270
        Volvo BB 140 A's
        Killbear Marina, Parry Sound, Ontario, Canada

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          #19
          Here is another thing to consider: the boat itself and its designed top speed. This came to me a while back when talking to a marine surveyor. A manufacturer of boats will have a marina architect design a boat for a specific usage, lets say a cruiser. He wants that boat to weigh XXXX and top out at YYYY speed (and cost ZZZZ). They use CAD design and specify the glass/resin/roving, etc to have the strength required for all the above parameters. Overdesigning the boat makes it cost more and would not be competitive.

          So, someone takes out a 350 and sticks in an 8.1 or 496 or what not. Now, you must change the transom assembly, outdrive, maybe even the mounting system, and that's if the stringers being able to support it, etc. Yes, the drivetrain is heavier. Assuming that someone retrofitted everything properly, not you have a boat that's markedly faster. Its stresses to the hull increase as the square of the speed so they are much greater than it was when the boat was new. There now is a better chance of hull failure. Insurance companies will not insure a boat which is overpowered. (That is one where the power package exceeds the manufacturers rating.)

          The above is my $ .02, being a retired engineer.
          Captharv 2001 2452
          "When the draft of your boat exceeds the depth of water, you are aground"

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          • Terry_J
            Terry_J commented
            Editing a comment
            I did a quick google search, You could order this boat new with a 7.4 from Bayliner. I would think he’s OK.

          • builderdude
            builderdude commented
            Editing a comment
            Agree, lots of 2556’s with big blocks and bravo drives.

          #20
          7.4 of that era was 330-340 HP; 8.1 is something like 420..... and heavier. Either way, the drive won't take it.
          Captharv 2001 2452
          "When the draft of your boat exceeds the depth of water, you are aground"

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          • builderdude
            builderdude commented
            Editing a comment
            Correct, the drive will need to be upgraded but the 2556 will take big block power no problem.

          #21
          Here is a link with some relevant reading - its focused primarily on converting from a 454 to 496, but the relevant drawings and such might still be of interest:

          https://www.perfprotech.com/blog/art...54-496-repower

          I think Terry has the right approach, find a donor boat that you can get an outdrive, transom assembly, engine, harness, steering, gauges etc from, big job but people have done it.
          1998 Bayliner 2858
          7.4L MPI / Bravo III
          Harbour Village Marina - Kenmore WA

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          • funfishinman
            funfishinman commented
            Editing a comment
            responses are much appreciated,lots of things to consider thanks again

          #22
          See post #17 again.
          Freshen up the SBC 5.7L or build a SBC 6.3L, and then do the DP-S conversion.
          5.7L = 1.95:1 reduction
          6.3L = 1.78:1 reduction

          No weight issues.
          No over-powering the stern drive.
          ​​​​​​​No sourcing parts for a BBC.
          25% to 35% more thrust to the water.


          .
          Rick E. Gresham, Oregon
          2850 Bounty Sedan Flybridge model
          Twin 280 HP 5.7's w/ Closed Cooling
          Volvo Penta DuoProp Drives
          Kohler 4 CZ Gen Set

          Comment

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