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4,3 Mercruiser under propped ( I think)

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  • 4,3 Mercruiser under propped ( I think)

    I have a 1989 Thundrcraft Magnum 220 with a 4.3 Mercruiser Alpha 1 drive.
    It is a heavy boat at 4000 LBs
    When I purchased the boat it had a 14 D 19 P 3 blade Piranha (replaceable blade)prop.
    The boat never achieved WOT with that prop. and with a load would not even plane.
    I changed to the spare prop that came with the vessel which was a 14 D 14 P.
    It now rides and handles much better and cruises at a speed of about 25 MPH. at about 3200 RPM
    When I research all that I could most recommendations come back at a 19 P. prop
    Is a 14 P prop seriously under propped for that boat motor & O/D?
    The motor is solid, starts good and runs good except when approaching WOT.
    When hitting about 3500 RPM it seems to lag and almost seems to stall.
    I have never pushed it above that point.


    Just wanting some "reasonable" feedback from those in the know.

  • #2
    Ayuh,..... You need a prop that allows the motor to turn 4400 to 4800 rpms at Wot, with a light load, 'n properly trimmed,.....

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    • #3
      Welcome aboard

      lugging a motor leads to killing the motor

      with the throttle mashed against the stops and the boat loaded as normal, and on plane..... what is your RPM (with a verified tach)

      if you cant make the minimum recommended RPM for WOT, look at the low WOT check list in the stickies.
      Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

      1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - https://forums.iboats.com/forum/owner...988-rogue-2420

      Past Boats
      1970 Wooster Hellion - Merc 9.8
      2002 SeaRay 190BR - 5.0 - A1G2 - "Cheeseheads in Paradise"
      1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

      What’s behind you doesn’t matter.Enzo Ferrari

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      • #4
        A lot of Boat for a small engine. First of all, is the engine in good Mechanical Condition? Good Compression? Timing set properly? Timing advancing? Throttle linkage adjusted so the Carb is opening fully? 2 or 4 barrel carb? If 4 bbl, are the Secondaries opening?(Unlikely to be fully open as the 4.3 is a small engine)

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        • #5
          Without doubt if it really is a 14”p...it’s far too small a pitch. It doesn’t quite make sense to me though if it’s seeing that speed at that rpm though.
          I would definitely be ditching both those props. That replaceable blade prop is nothing more than a gizmo and is never going to be as efficient as a good prop.
          I would say a 17-19” would be ideal for that set up. There are plenty of boats over 4000 lbs with 4.3 engines running 19” props in the rpm range with a 4 barrel carb. The bayliner 2255 is one really common example. I had one myself with a 19” laser and 19” high 5. Might have been better with a 17” but I mainly had it propped for cruising with only a couple on board.
          I’ve only ever seen one 4.3 magnum in the flesh, but I’m sure they have more power than the normal 4.3 4 barrel. This further justifies the case perhaps

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          • #6
            Hi

            i briefly looked into this, in terms of your engine. I’m pretty sure I’m nearly correct in your engines case. The 4.3 magnum....is perhaps more commonly known as the 262 magnum here...at least where I am. It’s a 4.3 efi engine.
            If im right. I’ve onky seen one of them before and it’s a tbi early 4.3 fuel injection. Not sure that it’s 220 hp like the mpi, but it’s defo more powerful than the carb and I think 205/210hp anyway.

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            • #7
              Originally posted by QBhoy View Post
              Hi

              i briefly looked into this, in terms of your engine. I’m pretty sure I’m nearly correct in your engines case. The 4.3 magnum....is perhaps more commonly known as the 262 magnum here...at least where I am. It’s a 4.3 efi engine.
              If im right. I’ve onky seen one of them before and it’s a tbi early 4.3 fuel injection. Not sure that it’s 220 hp like the mpi, but it’s defo more powerful than the carb and I think 205/210hp anyway.
              More HP but by shifting the HP curve higher, Low End Torque is reduced, and the Hole Shot suffers, but the Top Speed is a bit higher.

              Given a Choice between, say, a 260 HP 5 litre with MPI, or a 260 HP 2bbl carbed 5.7, i will chose the carbed 5.7 in a Heartbeat. Much better Holeshot and same top speed. On the VPs the 2bbl carb used an Adapter to fit on a very nice 4bbl Intake. It does take much effort to add a 4bbl and gain another 25- 30 hp, while still maintaining the Low End Torque.

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              • #8
                Originally posted by jimmbo View Post

                More HP but by shifting the HP curve higher, Low End Torque is reduced, and the Hole Shot suffers, but the Top Speed is a bit higher.

                Given a Choice between, say, a 260 HP 5 litre with MPI, or a 260 HP 2bbl carbed 5.7, i will chose the carbed 5.7 in a Heartbeat. Much better Holeshot and same top speed. On the VPs the 2bbl carb used an Adapter to fit on a very nice 4bbl Intake. It does take much effort to add a 4bbl and gain another 25- 30 hp, while still maintaining the Low End Torque.
                Agree with what you say about the power. But the choice of a 5.7 carb or a 5.0 mpi...the mpi every time for me. For many reasons.

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                • #9
                  The 5.7 will eat the 5, in torque.

                  My 5.7 accelerating. This is with the RAKER, the Enertia is quicker
                  https://www.youtube.com/watch?v=VepbXm5RFDI

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                  • #10
                    Nobody should ever choose a 5.0 over a 5.7.

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                    • #11
                      Originally posted by flyingscott View Post
                      Nobody should ever choose a 5.0 over a 5.7.
                      I would certainly have a 5.0 mpi over a 5.7 carb. As long as I didn’t need to pay for it from scratch. Twice the price. Saying that. Mine isn’t running well just now. When it’s running well. There is no doubt though.

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                      • #12
                        Originally posted by jimmbo View Post
                        The 5.7 will eat the 5, in torque.

                        My 5.7 accelerating. This is with the RAKER, the Enertia is quicker
                        https://www.youtube.com/watch?v=VepbXm5RFDI
                        Did you change out the 4.3 for the 5.7 ? It sure accelerates well. Better than mine but I think my boat will have about 300/400 kgs on the glastron

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                        • #13
                          It came from the Factory with the 5.7. From information I have gotten out of Glaston, it is the only one with the 5.7. The engine options for that hull were the 3 litre, the 4.3, 2 versions of the 5 litre(carb and FI). The 5 litre Fi had the same HP(260) as the 5.7(2bbl carb), but I ran against the FI 5 litre in the same boat, big difference, when I put the 4bbl on(1 hour swap) I get another 500 - 600 rpm, requiring more pitch on the prop

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                          • #14
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                            Originally posted by jimmbo View Post
                            It came from the Factory with the 5.7. From information I have gotten out of Glaston, it is the only one with the 5.7. The engine options for that hull were the 3 litre, the 4.3, 2 versions of the 5 litre(carb and FI). The 5 litre Fi had the same HP(260) as the 5.7(2bbl carb), but I ran against the FI 5 litre in the same boat, big difference, when I put the 4bbl on(1 hour swap) I get another 500 - 600 rpm, requiring more pitch on the prop
                            Great stuff. Was the 5.0 fi a efi or mpi ? Big difference as you know. You would think the top speed would be equal, all other things being equal, like prop etc. I agree the 5.7 carb will defo be stronger out the hole though.

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