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Thread: Gear ratio help

  1. #1

    Default Gear ratio help

    Well, my 1981 sea ray sedan bridge has two opposite rotation mercruiser 330s (454s) in it now. They're pretty much done for.i am repowering it with a set of standard (LH) spinning 2003 crusader mpi motors. They'll be more than adequate. My question is my current trans' are velvet drives (72c's) I know I can't run these in the opposite direction. Anyway, even if I could, that's irrelevant to this post. So I'm going to go with hurths. The velvet drives in my boat are 1.52.1. What am I going to need to run with the small blocks??? Here's the skinny on my boat.

    81 sedan bridge.
    1.25" shafts
    12,000 lbs
    Not sure on the props
    Any help would be appreciated. I understand gear ratios in cars real well. But not so much when it comes to marine transmissions. Thank in advance.

  2. #2
    Join Date
    Mar 2009
    Location
    Portland, Oregon,
    Posts
    11,684

    Default Re: Gear ratio help

    Jeff, if I'm reading this correctly, you have gone from BBC engines with a 4.250" bore and 4.000" stroke to SBC engines (I presume 5.7s) with a 4.000" bore and 3.480" stroke. If correct, your engine torque will now be significantly less.

    While I am not able to suggest a final reduction for you, I do know that we cannot prop ourselves out of an incorrect reduction. It just simply doesn't work that way.

    I would suggest speaking with a Hurth or Borg Warner pro and ask for a reduction recommendation.

    As you know, with both new engines being Std LH Rotation, one transmission will need to be "opposite of engine rotation capable".


    One other thought.... if you've not yet purchased these SBCs, the 6.2L (377) or 6.3L (383) version will offer more torque.



    Post back once you've learned what reduction will work!
    Rick ... aka Ricardo..... AQ series Volvo Penta repair
    Portland, Oregon
    28' SDN F/B w/ twin Volvo Penta 5.7L DP drives

    "Intellectual growth should commence at birth and cease only at death." Albert Einstein

    Please... no PMs! Post your questions on the forum.

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