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QSD 4.2L max rpm-gctid828121

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    QSD 4.2L max rpm-gctid828121

    After removal/re-installation of my boat's engine and also replacing most of its transmission, including repair (not replace) of the dual props I now have a much better performance from my boat/engine. Obviously I am not complaining but I am curious.

    Previously, max-rpm (WOT) was 3700 and speed @ max rpm was about 33 mph. The engine rpm needed to be above 3600 (around 3650) before it would reach planing speed. When we re-installed the engine (no work was done on the engine, just the coupler was replaced) we spent a good deal of time aligning the coupler and the drive using the Mercury alignment tool (really a barbed shaft so you can feel how easily it goes in/out all the way into/out of the coupler) to make sure we had as good a fit as possible. The props were straightened and chrome plated. I now have WOT rpm at 3850. Boat will reach plane at 35xx rpm and max-speed is about 38 mph. My most economic cruise is at about 3470 rpm, 24 mph, burning 1.9 lt/mi.

    I am happy but want to make sure that everything is OK. I thought the engine was rated at 3800 rpm at WOT. Whereas I do NOT go above 3670 rpm at all, is it OK that I have capacity to exceed that? Am I just lucky to have a seemingly well tuned/balanced set up?

    Retired, computer expert / executive
    Bayliner 285 Cruiser / Mercruiser QSD 4.2L 320 HP Diesel
    Live in the Bay Area, CA, USA, boat in Turkey
    D-Marin @ Turgutreis in Bodrum/Turkey
    [email protected]
    [email protected]

    Yes the max rpm is 3800. This should be controlled via the computer. Do you have an analog tach or digital? Maybe the tach is off?

    The higher rpms could be due to the props being tuned, and possibly having the pitch changed slightly when they were repaired.

    It sounds like you are propped very close to optimal.
    1987 3818 Hino 175
    "Knotty Girl"
    Prince Rupert B.C.