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Detroit Diesels 6v92TA vs 671TIB 1987/90s vintage comparison

Discussion in 'Engines' started by cleanslate, Oct 17, 2019.

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  1. cleanslate, Oct 17, 2019
    #1
    cleanslate

    cleanslate Senior Member

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    Ok , this question is based on my Hypothetical 50' Motor Yacht...lets call it '' M.Y. Hypo'' ! lol.

    Ok if I run 50' M.Y. Hypo at 1500/1800 rpm all day long cruise with either engine combo what is the comparison/differences?

    1- Which engine is better on fuel?

    2- Which engine is quieter of the two? No mufflers of any type.

    3- Which engine is more reliable/good mechanical history record. Motor yacht wise, not sport fish wise.

    Please keep in mind I do not run my boats ''to the pins'' rarely over 2000 rpm.
    Oops except my CC Dory...I've pushed down the throttle so hard , I've bent the pins !

    Thanks, I would really appreciate it.
    Last edited: Oct 17, 2019
  2. Capt J, Oct 17, 2019
    #2
    Capt J

    Capt J Senior Member

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    It all depends on the motoryacht as far as better on fuel...…...fuel burn versus speed

    1. 6-71

    2. who knows, it REALLY depends on setup, turbo's, TA/TI etc...... I ran a 6-71 boat without mufflers in a 41' Viking SF and the noise at slow speeds was unbearable.

    3. 6-71 generally
  3. cleanslate, Oct 17, 2019
    #3
    cleanslate

    cleanslate Senior Member

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    Thanks Capt J , it would be the same " boat" the 671 would be TIBs the 6v92 are TAs
    The 671 would be a lot better on fuel ? Or just a little.

    I have heard those older Vikings with 671s , loud as can be.
    I have to be honest, I don't know if I have mufflers on my 471 Ocean , got a rubber exhaust hose comes of the turbo to a fiberglass tube...maybe glass box..I need to check that out. Any way I would like to think mine are not that loud. It's definitely not unbearable plus my guests sit on the Aft deck while we're running around the bay/ocean with out any complaints .
  4. Capt Ralph, Oct 17, 2019
    #4
    Capt Ralph

    Capt Ralph Senior Member

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    71s are more forgiving. Dry liners. Crankshaft is better supported. Less width. Most parts interchangeable with blocks from the later 30s (except the 4 valve heads).

    Never, Never over heat a 92. Wet liners. Fat & arrogant as my 2ed mother in law.

    In 15 years on YF, I have never wavered, I'm a 71 fan.
  5. cleanslate, Oct 18, 2019
    #5
    cleanslate

    cleanslate Senior Member

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    I know 71s Rule ! But if ya don't run a 92 hard, things can be ''ok'' ? Yes?


    Now I have more DD engine questions;
    My Dad's boat had 8v71Ns and I had 6v53Ns..I think the 53s are wet liners? Anyway, we both had copper double walled exhaust risers where water went around the outside of the inner
    pipe and cooled the exhaust and gases..right? Well his got a hole from with in and water rolled into on of the tops of the piston, went to start the motor in the a.m. coming up from Florida and bent up the internal stuff.
    I never had it happen to me luckily.
    My question is do all ''V'' engines use a double walled liner for exhaust? Or all non turbo models?
    My in line 471ti's do not have double walled exhaust risers, but I believe it's cooled in the elbow after the turbo...?

    Tell me about it, CR.
  6. Capt Ralph, Oct 18, 2019
    #6
    Capt Ralph

    Capt Ralph Senior Member

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    53s were wet liners.

    The exhaust systems and dump cans were mostly up the marinizer or the boat builder (or both).
    I still have my 41 year old copper dump cans on my 12V71TIs.
    I've never liked water on the engine side of the riser hump.
    Your pop just had some bad luck. Ship happens..

    Later riser or dump cans use a wrap around the hot pipe from the turbo past the hump, then water jacket to the water nozzles. This way a internal leak will not drain back into the engine.
    I do prefer this.

    However, I still see new set ups with the water jacket from the turbo, up and over the hump.
    Very pretty and expensive. More expensive when water backs into a lung.
    Last edited: Oct 18, 2019
  7. cleanslate, Oct 18, 2019
    #7
    cleanslate

    cleanslate Senior Member

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    So all in line DD71s do not have risers? Mine are '81 J & Ts. I've got my eyes on a boat with Covington 671s...1985. Not sure what they have. I worry about parts. I see a MR. Cool mfg. for the cylinder type heat exchangers/coolers Covington style used. So J & T, Covington, S&S ...all they did was marinize the motors and soup them up with turbos and oversized injectors? Everything else is Detroit Diesel? I have read some posts where it is hard to find the various copper or cast tubes that carry water to various parts of the motor.....
  8. Capt Ralph, Oct 18, 2019
    #8
    Capt Ralph

    Capt Ralph Senior Member

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    I did not say that.
    It does depends on the boats water line vs exhaust height.
    The copper and steel tubing I have replaced with Fiberglass tube and hose.
    All else is serviceable.
  9. cleanslate, Oct 19, 2019
    #9
    cleanslate

    cleanslate Senior Member

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    Ok, after 25 years, I think I have finally got it! Lol Thank You Capt Ralph and to the many others on YF.
  10. PacBlue, Oct 19, 2019
    #10
    PacBlue

    PacBlue Senior Member

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    With the conservative way you run your boat your are probably better of with a non turbocharged Detroit especially if you are spending a lot of time at 1500 rpm’s . Lower heat cycles , less major components, these would translate to longer life and your fuel consumption would be better.

    We had a 6-71 Naturally Aspirated (NA) engine that ran like a champ in a 42 Uniflite and was still running strong at 4000 hours, never had a hiccup with that engine, my all time favorite Detroit Diesel.
  11. Capt J, Oct 19, 2019
    #11
    Capt J

    Capt J Senior Member

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    I don't think he has a choice in the HYPO 50' MY he is looking at and the only engine options are the 2 he stated. I agree that the naturals would be the best choice for his usage, and a great engine, however, every natural DD I've ran was noisier than hell itself without the turbo's to quiet the exhaust noise down a bit.
  12. cleanslate, Oct 20, 2019
    #12
    cleanslate

    cleanslate Senior Member

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    Yes..you are correct Sir..I'm at the mercy of what already comes with the older boats which I can only afford and have to live with , in that particular boat. I would love to find good old 671s or 8v71s both with mufflers of course, in a boat that meets my needs.
  13. Lepke, Oct 21, 2019
    #13
    Lepke

    Lepke Member

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    I've rebuilt many DDs for myself and others. Mostly commercial and usually 71s of all types. A 92 doesn't go as long between rebuilds as a 71. Turbo or natural, it's still a shorter life. A 671TIB (I think) is rated at 2500 so at 1800 will have a longer life because of cooler EGTs. Older naturals were rated 1800 at max continuous. At that rpm, the hp used may be close. I wish new 71s were still available.
  14. Transmaster, Mar 6, 2020
    #14
    Transmaster

    Transmaster New Member

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    Boss had a 50 ftguthrie he took Cummins out and put 871’s in . No room for mufflers. Luckily at speed it out ran noise but trolling we used ear plugs . Couldn’t catch a fish until we dumped 1/2 spools of line