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6-71 or 6-92?

Discussion in 'Post Yacht' started by On Liberty, Dec 9, 2014.

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  1. On Liberty, Dec 9, 2014
    #1
    On Liberty

    On Liberty New Member

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    Hello,

    I have been researching purchasing a sport fish and have settled on a 46 Post for obvious reasons.

    I have narrowed my search down to a '86 with 6-71's and a '91 with 6-92's. Both boats have roughly 1300 hours on the mains.

    I would appreciate any input other owners may have with regard to which power option I should favor.

    Reliability, ease of maintenance, known problems, etc.

    I am a hands on "do it myself if I can" boater and want to know what I am up against.

    Thanks!!!
  2. captholli, Dec 9, 2014
    #2
    captholli

    captholli Senior Member

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    Hi,
    Not an owner but have been around both quite a bit. We know that the 92 series is turbo and after cooled but we don't know if the 71 your looking at is a naturally aspirated or a turbo unit & that's where all the difference in the world starts. It depends on what you want to do with this boat. Give us a little more info and I'm positive you won't be disappointed with the knowledge of a large number of members on this forum.
  3. On Liberty, Dec 9, 2014
    #3
    On Liberty

    On Liberty New Member

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  4. On Liberty, Dec 9, 2014
    #4
    On Liberty

    On Liberty New Member

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    it says 6-71 TI, so turbo. Thanks for the reply. I am new to the diesel game. And you are correct. This is a great forum!
  5. RER, Dec 9, 2014
    #5
    RER

    RER Senior Member

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    From my experience the 6-92 @550hp has a service life of about 3000 - 3500 hours if it's treated well - one of the shortest lives of any Detroit Diesel.

    The 6-71 depends on HP. The 485hp TIB version is going to have about the same service life as the 6-92. Longer life from the 450hp version of the 6-71 and the 425hp TI or 410hp TI should give you 4000 - 4500 hours or more if treated well.

    The 92's will give you more speed and burn more fuel ...and besides motors, you should factor in what, if anything, a 5 years newer model does for you.
    Last edited: Dec 9, 2014
  6. On Liberty, Dec 9, 2014
    #6
    On Liberty

    On Liberty New Member

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    Thanks for the reply! I am set on the 46 post, just want to get the one that will suit my family for many, many summers ahead. I am hoping to gain as much insight from the experience of others before scheduling the mechanic/surveyor for a full power trail. Thanks!!!!
  7. Capt Ralph, Dec 9, 2014
    #7
    Capt Ralph

    Capt Ralph Senior Member

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    92s are not forgiving. If overheated slightly, it is usually toasted. Wet liners, thin heads and don't like a lot of low rpm hours. At higher rpm (hp) they have a short life. They make a lot of HP, go fast, suck fuel and look good.
    71s are forgiving. Can take a lot more abuse. Dry liners, heavy heads, parts are cheaper. Some parts are still interchangeable since the '40s. They can make a lot of hp also, maybe lacking on a lil torque (cubic inches/liters).
    I have 12v71s and can run against 8v92s in the 650 to 900 hp tune (when I feel rich and famous). Both will use the same fuel per HP. My 71s will run longer.
    There is not much else to brag on here on the Detroit 2 strokes. The HP to weight sux. If anybody had the money and loved the boat, a repower with much lighter blocks would be smarter. I'm still waiting for the FL lotto to come in.
    For me in a sport fish or sport cruiser with detroits, go with the 71s. The weight reduction will help on not having those extra ponies the 92s give.

    71s rule
  8. P46-Curaçao, Dec 9, 2014
    #8
    P46-Curaçao

    P46-Curaçao Senior Member

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    VI found this: http://www.yachtsurvey.com/comparing_diesel_types.htm
    and it is quite informative, but I would prefer an experienced owners deck plate wisdom.

    Great article, never seen it before, but as far as I can see, you’ve got your answers!
    I love my 6-71 TI's, easy to access via the salon floor (81 Post 46), easy to maintain too!
    Parts no problem at all, and relatively cheap...
  9. On Liberty, Dec 9, 2014
    #9
    On Liberty

    On Liberty New Member

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    You make a good argument for the 71's. We plan on family cruising so the sex appeal of big ponies is not a deciding factor for us and neither is a power plant that is less than "forgiving". Cheap(relative) and abundant parts is a plus! As is the weight reduction. Hadn't considered that. Thanks for the advice & good luck with the lottery!!!!!
  10. On Liberty, Dec 9, 2014
    #10
    On Liberty

    On Liberty New Member

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    Thanks P-46! I think I read your post on replacing the heads and holding tanks? Hope it went well.
  11. P46-Curaçao, Dec 9, 2014
    #11
    P46-Curaçao

    P46-Curaçao Senior Member

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    On Liberty, yes all set and done, but I would like to have a larger waste tank, but in my '1981 model' suitable space is hard to find...
  12. mwwhit1, Dec 10, 2014
    #12
    mwwhit1

    mwwhit1 Senior Member

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    Boy that avatar picture looks very familiar... :) I hope you can post the same one next summer from your new 46!! Good luck

    All the engine model advice is right on. Another factor is what cruise speed you will be happy with. Not knowing these two boats exactly, I would say the 671's will give you an 18-20 kt cruise and the 692's 20-22 kt cruise.
  13. On Liberty, Dec 10, 2014
    #13
    On Liberty

    On Liberty New Member

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    Mwwhit1, yup... I am stone cold busted for the avatar! haha. My guess is you took the pic? I thought the grey question mark thing the forum issued me was lame, but didn't want to post a avatar of someone else's Post. So I googled 46 post and this image seemed fitting as it is what I want to see over my shoulder soon. As soon as we realize our dream, I promise I will update with a On Liberty original!!!! Until then, I guess I will remain a poser... thanks for validating the info provided. What a helpful group of boaters you have gathered here!!!!
  14. P46-Curaçao, Dec 11, 2014
    #14
    P46-Curaçao

    P46-Curaçao Senior Member

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  15. Capt Ralph, Dec 11, 2014
    #15
    Capt Ralph

    Capt Ralph Senior Member

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    Yep, We like to watch (boat projects too).
  16. Capt J, Dec 12, 2014
    #16
    Capt J

    Capt J Senior Member

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    I've seen where two have been installed with a crossover 1.5" pipe at the bottom of the tank.....or if one tank is higher from the bottom of the higher tank to the top of the lower tank. You want the lower tank completely sealed. It is pretty preferable to have a straight run with the hose.
  17. captholli, Dec 12, 2014
    #17
    captholli

    captholli Senior Member

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    I'm trying to visualize this J, top tank is vented, bottom tank is completely sealed. If the bottom tank isn't vented how does it's volume fill ? Not being a smart a** just curious. Is there a return vent line off the top of this sealed bottom tank going back to the top tank?
    Sorry if we're off topic.
  18. On Liberty, Dec 12, 2014
    #18
    On Liberty

    On Liberty New Member

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    as long as we are talking 46 Post, we are on topic! :)
  19. Capt J, Dec 13, 2014
    #19
    Capt J

    Capt J Senior Member

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    Yes, you vent the top of the lower tank to the top of the top tank....then vent the top tank out the side of the boat......Hatteras does it a lot with fresh water tanks. The 64' MY was that way. Many manufacturers do it with fuel tanks. I've seen several do it with holding tanks. Basically the higher tank gravity feeds to the lower tank, you'd have your macerator pump pickup at the bottom of the lower tank. The higher tank doesn't even need to be above the height of the lower tank, just the bottom of it has to be 6" or so above the bottom of the lower tank. As the macerator pump, pumps out the bottom tank, the top tank gravity feeds the lower tank........like one of those 5 gallon water coolers you see in offices with the upside down bottle.
  20. P46-Curaçao, Dec 13, 2014
    #20
    P46-Curaçao

    P46-Curaçao Senior Member

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    But only the fluids will flow to the lower tank.
    What happens after a while, with the non fluids, like paper ect?
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