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8v92 Johnson & Towers 735 HP Good or No?

Discussion in 'Engines' started by retiredguy, Sep 21, 2021.

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  1. Pascal

    Pascal Senior Member

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    Most DD tend to smoke on a cold start, depending on engine model. Normally it s sign of low compression. Depends on how cold it is too and how quickly it goes away. Most surveyors want to see a cold start, no heater. Unless it’s 30 degrees :)
  2. Capt J

    Capt J Senior Member

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    Generally that HP 8v92 is a 1800-2500 hour engine, Of course it depends on how they were run.
    ArielM likes this.
  3. retiredguy

    retiredguy Member

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    Its all in the color of the smoke.
    White smoke in a cold start up is common and not really of concern if it clears up as operating temps are reached.
    Heavy grey or black smoke is almost certainly a restricted air flow, injection timing issues or bad/leaking injectors, possibly valve adjustment is needed.
    Blue smoke is an oil burning problem and can be from worn engine internals (rings or valve guilds), a leaky seal on a turbo, pvc valve blocked, etc.
  4. retiredguy

    retiredguy Member

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    That seems so strange to me as I come from an environment where Diesels run 8,000 to 12,000 hours with no problem. Of course they didn't have JT pushing the HP so high.
  5. Pascal

    Pascal Senior Member

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    Few high performance engines are going to reach that kind of hours without an inframe.
  6. retiredguy

    retiredguy Member

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    I understand, you should see what happens to engines at tractor pulls. What surprises me is they would use these engines to push large MYs. A bigger engine that doesn't work this hard would last much longer and probably even be easier on fuel.
  7. olderboater

    olderboater Senior Member

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    Space.

    Weight and size to hp.
  8. cleanslate

    cleanslate Senior Member

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    Well my various Detroits that I have experience with are 8v71s, 6v53s and now 471s. The 8v71s would smoke till warm until the were rebuilt. Then no smoke , even when it was a cold start .
    My 6v53s , one would smoke like hell when cold. The other motor would not which was overhauled by previous owner. I had the other motor done, and no more smoke cold start. Both motors just crank a few seconds and the fired right up no smoke.

    My current J & T 471s, port can be a hard start when it drops below say 60 degrees out. It will smoke for 30/40 seconds and clear out.
    Starboard fires right up with no smoke.
    I'm living with the port for now . Oil pressure is good etc. Zero smoke when under way full load at any rpm.

    That's what I have to go by. 92s series is probably different?

    It gets below 50 out then it will take a longer cranking period to fire up. But darn those 6v53s would almost fire up like a fuel injected gas motor. They were great.
  9. SplashFl

    SplashFl Member

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    LOL Replaced 270 hp. Cummins in my 38 Bertram with a pair of 400 hp, 6v53's like 15 years ago. Picked up 6 knots both cruise & on the top plus made engine room "livable." They knew 2 speeds; 1200 fish, and 2600 to and from wherever. Hour meters died so hours were unknown but other then one turbo exchange and one injector replacement any issues were all minor. One smoked and the one that had the "new" turbo didn't so probably time but left it to the next owner. Like another stated, Detroit guys familiar with them have been aging out of the business so although next vessel may have the then standard Detroit's, my offers are based on cost of their replacement. My listing broker told me all those he showed her to loved the boat but not the Detroit's.
  10. Capt J

    Capt J Senior Member

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    The 92 series was built for the Military as the highest HP in the lightest package they could make (at the time). They were designed to be majored every 1,000 hours. The military had mechanics on staff 24/7, so it was no big deal. The 92's were punched out to where they have wet liners and O-rings. Overfueling was/is an issue at trolling speeds with marinizers like J+T putting in the largest injectors they could, etc. etc. etc. 12v92's are worse. I ran a 58' Striker, boat was a 1991 and by 2004 had 8 sets of majors, basically every 500 hours......ran a really clean 54' Hatteras with DDEC 12v92's, it had reciepts for majors by a DD dealer at 975 hours and at 1973 hours, popped a motor on the survey/seatrial at the sale.
  11. boatpoor

    boatpoor Member

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    White smoke is common on the turbocharged 92 series at startup after they get a few hours on them mostly because the turbocharged version is 17:1 compression ratio compared to 19:1 in the naturally aspirated version.( rarely seen in boats)
  12. ArielM

    ArielM Senior Member

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    The 8v92 at 735Hp is a 2000 hour motor if treated right, less if pushed hard imo. I had two and rebuilt both. never had any issues with them till about 1800 hours. I rebuilt both around 2000 hours. A little smoke on start up is ok, but blue smoke is what you want to look for to indicate wear issues.

    Parts were all available at the time of rebuild. Johnson and towers are of little help though. They told me in the 90s they had a data base failure and any DD before a certain date they have zero information on.
  13. MYTraveler

    MYTraveler Member

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    Here is my experience based on having bought a boat with twin JT892 TA's at 790 hp. When I bought it, the engines had a little over 4,000 hours since MOH. They ran fine, but created quite a bit of smoke on cold start. I rebuilt the engines and the smoke went away. At the time of the rebuild, the cylinder wear was visually evident (inside cylinder walls compared to inside top of chamber, above ring level, where there is no wear). Once warmed up, the engines ran fine. And upon tear down, there were no problems with any of the bearings, etc. The only problem was cylinder wall and rings. But everything was replaced.
    I overheated both engines, warping the aluminum heads, at about 1700 hours. Until then, they ran fine. I had both rebuilt a second time and put another 1500 on them before selling the boat. Last I heard, my buyer was over 3,000 hours, running fine, no smoke.

    So my conclusions are 1) smoke at cold start is a good indicator of wear and remaining life; 2) the bearings and other internal parts are not particularly failure prone, compared with other mfg engines; 3) high horsepower diesel engines produce a lot of heat when running hard, and the heat transfer capacity of the J&T cooling system is marginal at best. I suspect that the most common reason for needing to rebuild J&T DDs is overheating, and but for overheating they can be good for, perhaps 5,000 hours (when the smoke upon cold start becomes intolerable); and 4) "Pyros", aka EGTs (exhaust gas temperature gauges) are essential to not overheating these engines; and 5) I don't think overheating is a function of engine hours, though it helps to clean the cooling system every few years. In my case overheating was from fishing close to shore, and unwittingly clogging my cooling systems with kelp, then running home at my typical 2100 rpm.

    Personally, I think DDs, in general, have a worse reputation than they deserve (oil leaks are common, they are old technology, etc.) which diminishes market value more than it should. So, if you want the best boat for the best price, DDs may be something to look for.
  14. ArielM

    ArielM Senior Member

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    Have you done compression tests on them at 4,000 hours to see if they were even close to allowable spec? The JT 892ta maxed at 735hp not 790 but in any case, 4000 is far beyond the typical life expectancy of these motors at the higher power band. Maybe previous owner was at hull speed a significant amount of time? 2,000-2500 is probably the general consensus and what I would expect if I bought a boat with them again.
  15. Lepke

    Lepke Member

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    I've been running Detroits for 60 years and rebuilding them for about 53 years. I used the engines both commercially and in yachts I like them because they are old technology. I found them to be the most reliable engine for the ocean. Never had DDECs. Overhaul parts are readily available. I recently completely rebuilt 2 - 1947, 671 naturals and had no issues finding parts. It was their first rebuild and they had run decades and 10s of thousands of hours.
    When you wring out all the HP, turbo DDs do about 3000 hours or less. If you run at 80% or less hp, you can probably double the time between overhauls (and keep the oil clean) . I once made 11,000 hours on a 480 hp 671. Usual was about 7-9000. It's all in how they're run and maintained.
    Parts are still available all over the world.
    Compared to "modern" engines with all the electronics, my Ford diesel PU has been towed 4 times because of failed sensors.
  16. chesapeake46

    chesapeake46 Senior Member

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    If the engines you are looking at are 8V92s and 480 hp, that sounds like a low HP 8V92. I would expect a longer life because the HP suggests Naturals.