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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.



      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

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      • #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.

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        • #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

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          • #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

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            • #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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