Announcement

Collapse
No announcement yet.

Changing from a 5.7 Liter OMC to a 7.4 454 Mercruiser

Collapse
X
  • Filter
  • Time
  • Show
Clear All
new posts

    Changing from a 5.7 Liter OMC to a 7.4 454 Mercruiser

    Hi ALL!!!

    Well winter has passed and unfortunately I was one of the unfortunate few who had an engine that didnt survive. Our 1987 Bayliner Avanti 3255 port side engine has a cracked block and I am looking to replace it. Luckily for me I have found two 7.4 Mercruiser 454's with extremely low hours on them that are completely built up (minus exhaust manifolds and risers) for an unbelievable price and 12 month warranty. Plan is to replace both engines in my boat with these but I am of course out of my depth here in knowing whether or not these mercruiser's will actually match up to the OMC out drives that are currently installed.

    Our local Mechanic unfortunately is out of town until next week and I dont want to miss out on these engines if indeed they will fit/match up to our current set up.

    Thanks to all in advance.

    JJ

    1987 Bayliner Avanti 3255
    Twin OMC 5.7L 4V

    #2
    I repowered a 1987 2550 with a merc 454 with bravo I drive, even if they could match up, which I don't think they would, your omc drives would need to be king cobra drives. And I want to say your boat has cobra drives.
    Dave
    2004 Bayliner 305(twin 220hp Bravo II drives)
    2008 Yamaha FX Cruiser
    2012 Yamaha FX Cruiser HO
    http://baylinerownersclub.org/galler...2_itemId=11187

    Comment


      #3
      Since you are installing Mercruiser engines may as well go with Mercruiser Outdrives. I don't think that the OMC's will mate up to the Mercruisers. Golden opportunity here. I would recommend that you go with Bravo II drives at a minimum Bravo III's IMO would be even better.
      Rick Grew

      1981 Carver 3007 Aft Cabin

      2004 Past Commodore
      West River Yacht & Cruising Club
      www.wrycc.com

      Comment


        #4
        The bolt pattern at the flywheel cover between the 7.4 (BBC) and the 5.7 (SBC) is the same, so technically if a 5.7 is bolted up the 7.4 will also bolt up. However as previously mentioned the drives may not do well with the additional torque of the bbc.
        Dave
        Edmonds, WA
        "THE FIX"
        '93 2556 5.7 Bravo II 2.0:1 18 1/4x19 P
        (.030 over-Vortec top end-part closed cooled)
        The rebuild of my 2556 https://www.baylinerownersclub.org/f...76?view=thread
        Misc. projects thread
        https://www.baylinerownersclub.org/f...56-gctid789773

        Comment


          #5
          Good to 350 hp with the Cobras.

          I too recommend bravos 2's.
          Be good, be happy, for tomorrow is promised to no man !

          1994 2452, 5.0l, Alpha gen. 2 drive. Sold ! Sold ! Sold !

          '86 / 19' Citation cuddy, Merc. 3.0L / 140 hp 86' , stringer drive. Sold ! Sold ! Sold !

          Manalapan N.J

          Comment


          • builderdude
            builderdude commented
            Editing a comment
            Good to know chief, I wasn't sure. What about tthe different torque curve, diferent props maybe?

          #6
          I probably wouldn't go 454 these days for a couple reasons. First is there.will be no space in the bilge. Second the weight difference will have a negative effect on your boat. Then there is the expense.of parts swapping. How about engine heigh will they be too tall for your hatch covers.

          I would go 383 or similar if you are looking for power. There are a lot of considerations beyond just a bolt up.


          http://www.boattest.com/oem/497/engines/30000120/3


          http://www.boattest.com/oem/337/engines/30000120/0/


          check out the above links they are a write up on the avanti the next size up from yours. I can tell you that the factory 454 carbs already touch the hatch covers in mine. There is an imprint in the hatch cover insulation where it touches so check your carb to hatch clearance. With 454s you will have no clearance between the engines and changing spark plugs is going to be a real pain in the butt. If you like i can take some measurements from mine. I have a 32 ft with 350's and a 3450 with 454s. As of now my 32 doesn't have a lot of exhaust to exhaust space between the engines.


          I'm not trying to be negative but realistic. I'm always one for more power. If I thought I thought it would work I would do it but I think you can get just as much power 383 it with much less weight and better fuel economy. However i do understand the fact that you found twins that you.possibly can use. Just make sure they are going to fit.
          1989 Avanti 3450 Sunbridge
          twin 454's
          MV Mar-Y-Sol
          1979 Bayliner Conquest 3150 hardtop ocean express.
          Twin chevy 350's inboard
          Ben- Jamin
          spokane Washington

          Comment


            #7
            Yatchman makes good point about loosing space and adding weight. 383's should be an awsome choice if you have to go with new power. Have you thought about a possible JB weld repair on the crack? I've never done this myself but others have saved engines doing this, might be worth a try if your engine was a strong runner previously.
            Dave
            Edmonds, WA
            "THE FIX"
            '93 2556 5.7 Bravo II 2.0:1 18 1/4x19 P
            (.030 over-Vortec top end-part closed cooled)
            The rebuild of my 2556 https://www.baylinerownersclub.org/f...76?view=thread
            Misc. projects thread
            https://www.baylinerownersclub.org/f...56-gctid789773

            Comment


              #8
              Depending on where the crack is, the JB Weld idea is a good one. Properly done it will last as long as the engine. If the crack is external and not between cylinders or the heads you stand a good chance here. Often the crack is on the side of the block. May be pushed out a bit even. There is no real pressure here and if that is all a JB Weld repair costs next to nothing and is worth exploring.
              Doug
              Started boating 1955
              Number of boats owned 32
              Bayliners
              2655
              2755
              2850
              3870 presently owned
              Favorite boat. Toss up. 46' Chris Craft, 3870 Bayliner

              Comment


                #9
                If you do go with the 454's one other thing to check is the engine room vents. Engines need fresh air and the larger the engine, the more it needs. Often there is just enough vents for the engine installed. No big thing but it could cause a performance issue if there isn't enough.
                Started boating 1955
                Number of boats owned 32
                Bayliners
                2655
                2755
                2850
                3870 presently owned
                Favorite boat. Toss up. 46' Chris Craft, 3870 Bayliner

                Comment


                  #10
                  OMC drives are used for engines from 5.7 liters to 8.0 liters. The damper coupling at them (at 350 and at 454) is also the same. So replacement is possible.
                  http://www.boats.net/parts/detail/omc/O-0984290.html
                  http://www.boats.net/parts/search/OM...NTS/parts.html

                  Comment


                    #11
                    OMC made 2 Cobra drives. The Cobra and King Cobra. The King was for the larger engines and because it wasn't produced in great numbers, some parts are hard to find and are expensive. The regular Cobra drive is easy to maintain. I question the ability of it to withstand the torque of the larger engines. If it could, there would have been no reason to have the King.
                    If you do change, cautious use of the throttle getting up to speed would be my suggestion.
                    Doug
                    Started boating 1955
                    Number of boats owned 32
                    Bayliners
                    2655
                    2755
                    2850
                    3870 presently owned
                    Favorite boat. Toss up. 46' Chris Craft, 3870 Bayliner

                    Comment


                      #12
                      Small block Chevy 5.7 & Ford 5.8 were the biggest engine OMC mated to a regular Cobra drive.
                      Earlier big block 460 was coupled to a dog clutched King Cobra AKA baby king,
                      later on coupled to a cone clutched hump back King Cobra... must be a good reason for them to doing it!
                      Joon, Kathy, Jaden & Tristan
                      Uniflite 42 AC, DD 671N
                      93 3058 sold
                      92 2855 (day boat)
                      91 Fourwinns 205 (lake boat)
                      Longbranch WA
                      Life is Good

                      Comment


                        #13
                        Thank you all for the great responses and huge amount of intellect here!

                        First, I want to note that my desire in looking at the 454's was due to their local availability only, not based on an upgrade in power. NOT that that's a bad thing in any way because, lets be honest..... ITS MORE POWER!!!! But in all honesty, they are at a great price and it would be a perfect opportunity to upgrade what is currently on the boat. Having said that though, I believe the overall aspects ($$$) of what may be required to fully take on this type of swap significantly outweigh my wants/needs and wifes desire that I not sink a ton of $$$$$$ into said boat! HA!

                        Back on the hunt for a 5.7 L OMC!!!!!

                        As always, my sincere thanks to all for the added advice and continued guidance!!!!
                        1987 Bayliner Avanti 3255
                        Twin OMC 5.7L 4V

                        Comment


                          #14
                          Check here. Nothing but good things said about this company. http://www.michiganmotorz.com/
                          Started boating 1955
                          Number of boats owned 32
                          Bayliners
                          2655
                          2755
                          2850
                          3870 presently owned
                          Favorite boat. Toss up. 46' Chris Craft, 3870 Bayliner

                          Comment


                            #15
                            Both short and long block 5.7's are readily availible and you won't have to fuss around converting anything. If your current engine was a good runner you may save a bit going short block method.
                            Dave
                            Edmonds, WA
                            "THE FIX"
                            '93 2556 5.7 Bravo II 2.0:1 18 1/4x19 P
                            (.030 over-Vortec top end-part closed cooled)
                            The rebuild of my 2556 https://www.baylinerownersclub.org/f...76?view=thread
                            Misc. projects thread
                            https://www.baylinerownersclub.org/f...56-gctid789773

                            Comment

                            Working...
                            X