Choosing alpha gear ratio for a diesel conversion

Discussion in 'Sterndrives' started by PickleRick, Dec 15, 2017.

  1. PickleRick
    Joined: Dec 2017
    Posts: 20
    Likes: 0, Points: 1
    Location: Greenville

    PickleRick Junior Member

    Hey guys. Im yanking a 350 for a 2.0 vw tdi.

    Im using the alpha outdrive.

    I currently have the 1.47 gear ratio leg

    Boat weighs 5400 lbs
    26 ft loa
    21 ft low
    8 ft beam
    Draft about 2 ft 10
    Modified v hull

    Motor makes 170 hp at 3800 rpms
    Making 280 ft lbs torque there

    Makes 310 ft lbs torque 1900 to 2400 rpms

    Would you recomend keeping the 1.47 leg or use a different ratio?

    The motor can easily be tuned for more hp but i am worried at that point about eating outdrives. I can do 250 hp without really pushing the engine internals but then torque is some 400 ft lbs.

    Goal speed is comfortably as close to 3000 rpms as possible at 20 mph. We are sailors who want a easy boat to trailer with us on vacations. I have no desire to see what speed the 4000 rpms redline will get me. We just want range and fuel economy.

    The 350 will do 20 mph around 3200 rpms.

    I know someone will recommend a bravo. Once bugs are worked out ill start looking for hurricane damaged boats to get a volvo or bravo drive off of. Thanks guys!
     
  2. CDK
    Joined: Aug 2007
    Posts: 3,324
    Likes: 145, Points: 63, Legacy Rep: 1819
    Location: Adriatic sea

    CDK retired engineer

    Using the same drive, your VW engine will also need 3200 rpm for 20 mph. Maybe a little less because the VW engine is lighter.
    At that speed the engine will generate approx. 140 hp, an amount the engine can easily handle.
     
  3. PickleRick
    Joined: Dec 2017
    Posts: 20
    Likes: 0, Points: 1
    Location: Greenville

    PickleRick Junior Member

    Thank you so much!

    Would you recommend a different prop or would you use the same prop?
     
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