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    Low WOT RPM

    The factory sticker on my engine (Mercruiser 7.4 MPI, big block GM generation VI) says the WOT RPM should be 4000 - 4400.

    With my new propeller I have no problems reaching 4200 RPM when loaded, and 4400 when somewhat empty.

    So I am probably OK in terms of propeller pitch vs outdrive gear ratio.

    My question is, why my WOT RPM are specified lower than for other similar engines?

    I have seen several 7.4 engines with 4200 - 4600 spec, and higher.

    Why my engine is considered safe to work under full load at a lower RPM?

    Is there a downside to it, like lower compression ratio or worse fuel economy?

    Just curious.
    Alex
    1997 2859, 7.4 MPI, Bravo II
    1993 2452, 5.7 Bravo II - SOLD

    #2
    The reason for the rpm difference is buried in the engineering. I'm surprised it's that low, but in my experience when I exceeded the max rpms prior to rev limiters, the valves would float. The valve springs couldn't close the valves quickly enough.
    P/C Pete
    Edmonds Yacht Club (Commodore 1993)
    1988 3818 "GLAUBEN”
    Hino EH700 175 Onan MDKD Genset
    MMSI 367770440

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      #3
      I found online a list of WOT RPM ranges for most marine engines including outboards.

      https://www19.corecommerce.com/~thep...PM%20Chart.pdf

      It looks like mine is at the very bottom of the list, the only engine that has the same RPM spec is 1994-1997 Volvo Penta FL with Ford 5.8 (351).
      Alex
      1997 2859, 7.4 MPI, Bravo II
      1993 2452, 5.7 Bravo II - SOLD

      Comment


        #4
        A couple reasons.

        Most BBC marine builds use a cast crank with 2 bolt mains. These have big heavy internals and tend to sometimes come apart at high rpms.

        The other reason, is that the 454 makes gobs of torque at a low rpm. There is no need to rev it past 4200-4500 rpm. Really anything past 3500 and the torque really starts to fall off. Basically the extra rpms are wasting fuel and wearing the internals prematurely.

        I had a heavy old Reinell with a 454, I found that 3000-3500 was the sweet spot for the best economy, and life of the engine.

        You may want to consider a bit more prop to get the rpms down to where that big block is making the most torque.
        Joel
        1987 3818 Hino 175
        "Knotty Girl"
        Prince Rupert B.C.

        Comment


          #5
          That makes good sense, I believe my engine has a 4-bolt mains and forged crank.

          And I am very happy with my current prop (Bravo 2 stainless 4 blade 20 pitch), most of the time I am cruising at 3200 RPM that moves the boat at about 23 knots.

          I was just worried that I am still overloading the engine in spite of the ability to reach the specified WOT RPM.
          Alex
          1997 2859, 7.4 MPI, Bravo II
          1993 2452, 5.7 Bravo II - SOLD

          Comment


            #6
            You must at wot be able to reach recommended wot rpm " range " i used to run wot for only a few minutes each season ...

            First launch of the season on the way back to the dock, about a mile or 2 out and only for a few minutes.

            l last launch of the season, on the way back to the dock.

            The engine will tell you if something is amiss.

            The rest of the time was crusin ... 2 / 3 throttle or less, leaving the rest for a emergency such as outrunning a thunderstorm, or getting out of the way of a Navy vessel, or barge.

            3000-3500 rpm 20-25 mph. My back just couldn't take the pounding.

            Good question !
            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


              #7
              Really sounds like you are sitting in the right rpms. A fuel flow readout would be another way to verify your sweet spot. My sbc loved 3700rpm over 3400 all day for fuel.

              Also i would never want to run wot for more than a few seconds, on a bbc id poo my pants.

              Yes you have forged internals and a roller block. Lucky you.
              1993 formula pc 31 twin 454 bravo 2
              1989 2655 cierra 5.7 omc cobra
              2014 "searay" tandom trailer

              Anchor bay clinton river
              Michigan

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