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8V92 minimum compression

Discussion in 'Technical Discussion' started by Hoopter, Jul 3, 2018.

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  1. Hoopter

    Hoopter New Member

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    What is the minimum compression for a 1978, 17:1 CR, 550 HP, twin turbo 8V92.
    Thanks.
  2. Capt J

    Capt J Senior Member

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    350 psi seems to ring a bell but confirm it. One call to dd dealer should get you the correct answer
  3. Hoopter

    Hoopter New Member

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    Thank you, Capt J.
  4. Capt Ralph

    Capt Ralph Senior Member

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    The important thing; all lungs pop the same or close.
    I like to run the motor quickly at idle cold with the gauge in the injector hole. It takes all day for two passes.
    350 to 400 is very good.
    Why are you asking? exhaust smoke color and history can tell more, faster?
  5. Hoopter

    Hoopter New Member

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    Thanks for your reply, Capt Ralph.
    I'm looking at a 1978 58' Hatteras with these engines in it. Engine oil analysis revealed a 189 ppm level of iron in the starboard engine, as compared to a 36 ppm in the port engine. The owner's tech did a compression test on the starboard engine resulting in the following figures. From front to back, inboard side, 425, 425, 430, 410, Outboard side, 440, 480, 470, 425. The owner's tech says this engine is a 380 psi minimum engine (380 Minimum, 430 New Parts), and that these results are normal for an engine of this age (1600 hours). My tech says this engine is a 450 psi minimum engine (450 Minimum, 500 New Parts), and that there should be n0 more than a 25 psi differential in the cylinder pressures. My tech says this engine needs to be rebuilt.
    Tom
  6. Capt Ralph

    Capt Ralph Senior Member

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    What does the liners look like?
    Bore scope the valves?
    Rings loose and clean?
    Both techs just embarrassed them selves if they didn't think of this?

    Crank case pressures at load?
    Make RPM AND speed during sea trials?

    Was that turning over cold? by starter? or run a few seconds at idle?
    On a 92, the coolant needs to go to the lab also.
    Iron in the oil sounds like a dock queen, lots of sitting. Inside of block condensing.
  7. Capt J

    Capt J Senior Member

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    Call a Dd dealer, specifically the one who marinized it and ask the service manager.

    The pressures being so far apart does worry me. There is almost a 20% differential between the highest and lowest cylinder
  8. Hoopter

    Hoopter New Member

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    Thank you, Capt J.
  9. Hoopter

    Hoopter New Member

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    My tech wanted the cylinder liners scoped so the owner's tech scoped the liners but did not look at the valves, or at least didn't say anything about the valves. The liners had some black spots/areas on them. There was some pitting. And, there was still some cross hatching visible. The owner's tech said the black spots/areas were oil, and the liners showed normal wear for this engine. My tech said the black spots/areas were caused by rust.
    The rings have not been checked.
    Crank case pressure at load has not been checked.
    After the compression test, the racks were adjusted. Afterwards, the engine reached the governor set 2300 rpm in neutral at the dock. The boat has not been taken out since the rack adjustments.
    The compression test was performed by running the engine rather than just using the starter.
    A coolant sample has not been sent to the lab.
    And, you're right, this boat has spent extended periods of time at the dock.
    Thanks,
    Tom
  10. Capt Ralph

    Capt Ralph Senior Member

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    Typical of a fuel wet lung when test like are done. Big 92 injectors dribble like a 15 year old at a kat house. That's why it takes me all day. If all was done in 4 hours, cold dry liners are not going to happen.
    If they did take their time, and the engine was not run a day or two before, the valve seats never cleared and still a waist of time.
    It's hard to get a good reading on old Detroits.
    Now the black spots on the liners is NOT rust. Scuffing where a ring is not in good contact.
    Another reason they should of been run (hard) a day or two before checking. Get those rings clear, under pressure, pushed out away from the piston.
    So you really now, need to pull the air box covers off and ensure the rings are free. If so, the black spots may go away when under load.
    So far, dock queen, lots of idle time, two idiot techs and old 92s.

    Now, for you to think about; Are you going to idle around? Sit months on end? (idle at dock does not do anything but mess it up more).
    Or you going to burn some fuel under load with a big smile and fat check book. If your worried about MPG, walk on if your worried about engine life.
    There are trawlers out there that can do the same and not destroy the engines.
  11. Hoopter

    Hoopter New Member

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    Thanks for that colorful reply, Capt Ralph, I enjoyed it. And, thanks for all your help.
  12. Hoopter

    Hoopter New Member

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    Follow up:

    Obtained a 1978 Detroit Diesel Field Service Data Book which listed the minimum compression for this engine at 450 psi.
  13. Caltexflanc

    Caltexflanc Senior Member

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    Not correct according to the shop manual. It is 380 at 600 rpm for a turbo engine. Average is pegged at 430.
    I would have that test done again, by the way.
    If not, Id run it with those numbers and not lose a minute's sleep.
  14. Capt Ralph

    Capt Ralph Senior Member

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    Turbo engines have just a bit less compression at idle.
    Still, I see no problems with those numbers. Except maybe a wet cylinder from a dribbling injector.
    A visual inspection from the air box covers will be the real indicator of lung issues, ,, after a hard run..
    When you see a lot of shine and no hatch patterns on those high reading lungs, Dribbling injectors or over fueling of the like. Not a big woo, Just pull all and have a good shop pop and match all injectors.
    I've been preaching this for a while, picking up a set of rebuilt injectors with-out pop and match is not good service but so many do it as routine.
    Last edited: Jul 23, 2018
  15. Hoopter

    Hoopter New Member

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    Thanks for the reply, Caltexflanc.
    This is where the confusion comes in. (And that confusion includes me.) In the 1978 and 1979 DD Service Manuals (the two manuals I've seen) the compression figures are listed as 380/430 for this engine. In the 1978 DD Field Service Data Book the figures are listed as 450/500. But the page in the 1978/79 Service Manual that lists the 380/430 figures is dated 1973, and the page in the 1978 DD Data Book that lists the 450/500 figures is dated 1978. I posted the 45o figure as being the correct one because I thought/assumed that between the 1973 data and the 1978 data, the 1978 data would be correct.
    Tom
  16. Capt Ralph

    Capt Ralph Senior Member

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    There were so many variables from the stock DDC to marine and different marianizers.
    The important to remember; Are all the lungs the same in comparison in pressures to each other? Good looking liners?


    Well, for the number techs out there; Figure 17:1 ratio at 92ci to PSI.
  17. Capt Ralph

    Capt Ralph Senior Member

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    Not sure where the 17:1 came from, I thought my 71 TI's were 15:1.
  18. baltimore bob

    baltimore bob Member

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    My Detroit compression test....if they start quicky stone cold and white smoke clears quickly=OK!
  19. Capt Ralph

    Capt Ralph Senior Member

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    Yep..
  20. David Hinkel

    David Hinkel New Member

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    425 Psi @600 rpm

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