Engine Replacement options for DD12v71

Discussion in 'Diesel Engines' started by Brian242, Apr 13, 2021.

  1. Brian242
    Joined: Jun 2020
    Posts: 3
    Likes: 0, Points: 1
    Location: Bahamas

    Brian242 New Member

    I am currently looking at purchasing an old LCM8 that is powered by twin Detroit Diesel 12v71 NA engines. These engines were advertised at 350hp each however, the engines have been parted out so I'm considering just replacing the engines one time.

    I hope to keep the original TwinDisc gear with a 3:1 ratio.

    I'm looking for suggestions on replacement engine options.

    The big consideration for me is fuel efficiency. I would like to keep mechanical diesels as opposed to anything electronically controlled. The boat will be working mostly in a remote area of the Bahamas so I need something rugged so that I don't have to get a computer science degree to work on the engines.

    I have experience with Cummins 6BTA 330s but despite the similar HP rating, I feel they may be too small for this application. Any suggestions would be greatly appreciated!
  2. comfisherman
    Joined: Apr 2009
    Posts: 539
    Likes: 266, Points: 63, Legacy Rep: 10
    Location: Alaska

    comfisherman Senior Member

    I've seen a few repowered over the years. Actually I'm getting old enough I've seen some repowered more than once.

    Usually a 514 in those with a #1 bellhousing and a 14 inch flywheel. The 6125 john deere was popular in the late 90s and early 2000s. Know of one that had twin 3406 cats, and one that had 12v71s then lugger 6125s and just got a pair of john deere 6135s. But they are electronic.

    One repowered with 8.3 liter Cummins but it originally had quad 671s with the goofy combo setup. That particular boat has sold a few times. Last owner said it was plenty of power as long as it wasn't loaded, windy, or in heavy tide.

    The Detroit's are kinda tough to replace, the puked a ton of oil and were noisy. But they had immediate throttle response and lots of torque for what they were.

    Dunno how the Bahamas are with emissions. But I'd be looking for a good used pair or a reman pair of 3406s or mechanical 6125s.
  3. gonzo
    Joined: Aug 2002
    Posts: 16,650
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    Location: Milwaukee, WI

    gonzo Senior Member

    If you want fuel efficiency electronic controls are the answer. If you can live with high emissions, low efficiency and smoke, then mechanical diesel will work for you.

  4. comfisherman
    Joined: Apr 2009
    Posts: 539
    Likes: 266, Points: 63, Legacy Rep: 10
    Location: Alaska

    comfisherman Senior Member

    Eh. It's not as impressive in real world and tier 3 seems to have put us backwards on the fuel burn.

    I've repowered a couple boats from old jimmies to newer 4 stroke electronic engines. The boats that spent a lot of time idling for work saw a pretty big savings on idle time. Reality is when operated in the correct power band the amount of fuel burned per horsepower hr has increased very little over the last few generations, especially tier 3 and 4s getting worse on the lbds of fuel per hp hr.

    If you spend a lot of time idling they can really drop the fuel numbers, we went 8v92 to scania and the idle fuel consumption went down from 3 gph to 1.6. Was pretty happy as that boat will do 5-600 hrs a year idling. At end of the season we appeared to have burned about 500 less gallons of fuel. Sounds like a lot, but fuel is about 3$ gallon. So a solid 1500$ in savings, the main was 77k bare, installed with all the associated expenses of exhaust, hydraulics, and braketry with harnesses and panels it was a bit over 100k. Would have been more but I did all my own work so no hrs $$$ out.

    The electronic engines aren't produced in a vaccum and have environmental impact. Would be interested in seeing that fuel savings balanced out.

    * edit. An older friend of mine has two converted lcm8s one still has jimmies and the other lugger 6125s. They serve double duty hauling freight and fish boat tendering. I asked this question to him. His thoughts were that he would go with whatever brand had the best parts supply in the local area. His luggers are much better engines but as they age parts are becoming increasingly difficult to locate. The 12v71s were so common that parts are still readily available, albeit many of the good Detroit mechanics are either old or dead. In his case he will likely repower with cat as cummins only does dry turbo engines in that size and cat, while expensive locally still has mechanics and parts readily available. *
    Last edited: Apr 14, 2021
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