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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. retiredguy

    retiredguy Member

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    Help a guy out here.

    I am looking at a MY with two of these engines and several people have said to walk away from any MY with them. I am told these are 480 H.P. Engines when built by DD and then J&T push them to the limits in boost and fuel injectors. The argument is when you push an engine that hard they have a short life span between major rebuilds. I am also told they are fuel hogs in order to get that power from them.

    The ones I am looking at have about 2700 hours on them since the last major and I've been told by more than one person they are running on borrowed time and I am looking at a $40K to $50K bill in the very near future. I won't be spending that as I will do the work myself (I am a mechanical engineer and can read the DD manuals) but still, its a lot of work and expense. I have a sea trial scheduled next week with a SAM surveyor.

    So, are these really "just don't buy them" engines or is there a lot of smoke being blown up my backside?
  2. BRyachts

    BRyachts Member

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    Just MY opinion, Your Mileage May Vary

    Run Forrest Run!!!
  3. motoryachtlover

    motoryachtlover Senior Member

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    If I am not mistaken BR is an engineer. I am not but am a former Detroit owner. Since BR didn’t say why he doesn’t like them I will tell you the pros and cons as I know them. I would assume they need a rebuild. You said an MY so will you be idling along mostly or running at cruise speed. If running at cruise mostly then you probably need to figure a rebuild at 2500 hours or so. If running at hull speed I think you will have much longer time between overhauls. The pros of Detroits especially for you who can work on them is that they are relatively simple and don’t have a computer that can be taken out with a lightning strike (unless DDECs). If they are DDECs I would walk away. They will tolerate things that a modern engine won’t and get you home. The cons. Old technology, heavy, inefficient, parts availability is an issue and available techs that are knowledgeable about them are aging out. All of this being said I don’t want another boat with them because of product support is aging. I hope I have helped. Others with more experience and knowledge will chime in.
  4. Pascal

    Pascal Senior Member

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    It depends on how they ve been run which with mechanical engines is going to be hard to know. 8v92 at 725hp are right at 1ho/cid which many consider to be at or just over the reliability ratio.

    what size boat? If in a heavy boat they re likely to have been pushed harder.

    two important test before even getting into a survey are ... how much smoke on a cold start, how quickly do they settle to a smooth idle without hunting and do they turn rated RPM flat out.

    You may want to talk to the surveyor and discuss the possibility of aborting the survey if they smoke too much and hunt too long on a cold start and don’t turn the rated 2300rpm. If that a the case no need for a full survey and haul out

    ideally you need a DD specialist to do a mechanical survey incl a borescope inspection thru the air box covers That will tell you the amount of wear.
  5. DOCKMASTER

    DOCKMASTER Senior Member

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    What year are the engines? Are they DDEC? What version of DDEC? These are not inheritently bad engines. It depends on many factors as others have said. How have they been run? Are they propped right so as to make full RPM and not be overloaded? Have they been properly maintained? I think you would find average hours on these before MOH right around 3000 hours. And some will go thousands of hours more, some less. You can still get most parts for the engine internals (piston kits, heads, rods, rings, etc). But DDEC components are no longer made any can be difficult to find. It can also be difficult to fine someone to troubleshoot DDEC issues if you experience them. Some of the external parts J&T made are not available either like the heat exchangers depending on what you have. There are lots of these engines around so used parts are usually available for lots of items.
  6. retiredguy

    retiredguy Member

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    Engines are 1991, boat is 68' at 83,000 lbs. give or take....

    I will know more when I tour the boat next week.
  7. retiredguy

    retiredguy Member

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    LOL...
  8. DOCKMASTER

    DOCKMASTER Senior Member

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    The DDEC's came out around 1986 (ish) era. I'm not sure what year J&T started with them but almost certainly those 1991's are going to be DDEC. Likely the DDEC version II I believe. Might be the III series but I don't think those were until the last years in like 93-95. Don't quote me on any of these years but I think they are close.
  9. olderboater

    olderboater Senior Member

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    You need more than your SAM surveyor. You need a mechanical survey by an expert on these engines.
  10. retiredguy

    retiredguy Member

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    I was curious and found a place that sells rebuilt ecm (DDEC) units for about $1000 should it need one. Might almost be worth having one as a spare *if* I buy the boat and rebuild the engines.
    bluebyu likes this.
  11. retiredguy

    retiredguy Member

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    Or not. By that I mean it doesn't really matter at this point because in my mind a MOH is going to be needed regardless due to the hours on them. I will simply reflect that in my offer when the time comes.

    I know I started out wondering if I should walk away but I now think that if done right this "weakness" can be made to work in my favor in the total cost of the boat.
  12. RER

    RER Senior Member

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    There’s no free lunch for a boat owner. You can buy an older inexpensive DD powered boat and then spend whatever it takes to keep them running, or you can spend more $$$ on a newer boat and newer motor technology with an expectation of better reliability.

    Newer motors fail sometimes too, though. So there’s always a risk unless you are buying under warranty as in the case of new motors.

    It depends on what fits your lifestyle, budget, and intended use. But either way you’re going to pay for it.
    Last edited: Sep 21, 2021
    SplashFl likes this.
  13. olderboater

    olderboater Senior Member

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    Doesn't matter to know the condition of the engines....interesting. Sounds like you'd made up your mind before asking here. Good luck.
  14. retiredguy

    retiredguy Member

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    Nope, not at all and the input given has moved me along in the process and thinking about total cost. In fact I may use this thread during negotiations. As to condition tell me, if the engines are over their expected usable hours and need a MOH on that basis alone, does the actual rings, valves, bearings tolerance wear really matter?
  15. retiredguy

    retiredguy Member

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    Mostly I would agree but,,, if a good negotiation leads to a great purchase there are always deals to be had. I *may* be able to make that happen here. Time will tell.
  16. Pascal

    Pascal Senior Member

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    confused about how you re going about this. You initially said you are surveying the boat next week and now you say you have yet to make an offer.

    that s not how it works normally.
  17. retiredguy

    retiredguy Member

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    I've been working with a broker I have known for a couple decades (buying and selling blow boats) and he is flying me in to go over this boat. He knows I *am* going to make an offer. I have made that commitment, the real question is the amount.

    You are right in one regard, I rarely do things the usual way. lol
  18. cleanslate

    cleanslate Senior Member

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    I have old J & T DD 471s ramped up by J & T in my Ocean Yachts, MOTOR yacht not sport fish which tend to get pushed to the limits for long periods of time.
    They run great. Hours are unknow. Meters not working when I bought the boat broken at 1575hrs. Still not working , but I keep a log on run time . I've put in 830 hrs so far on them, and still no major problems. Did have to replace exhaust elbows, turbos, water pumps as they rusted out.
    If they look clean, fire up when cold with minimal smoke and you sea trail the heck out of them before purchase as I did you should be in good shape. Parts are still around. J & T is still around and can get them for you.
    71s are the better motors than the 92s overall, but if in good shape and the were properly used and maintained it's worth a shot IMO. As long as it's a MY not a sportfish.
  19. retiredguy

    retiredguy Member

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    Thanks for the feedback, it really is appreciated. I am by no means into running fast, a comfortable cruise, a day of good weather, friends, kids and grand kids along and the admiral in a good mood... My kind of boating!
  20. DOCKMASTER

    DOCKMASTER Senior Member

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    I am curious about folks comments about smoke at cold start and that's indicator of some issues? What issue are you suggesting more smoke at cold start indicates? When I had my 12v92's if I didn't have the block heaters on for a day prior to firing up it would look like the marina was on fire :). They would do this even right after a MOH with everything replaced. Just have the heaters on and all is good. There have been lots of threads on this and people spend thousands on injectors and special air filters and other gimmicks to no avail. Block heaters are the best thing you can and should use.
    T.T. likes this.