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Diesel engine rpm

Discussion in 'Engines' started by ArielM, Aug 29, 2010.

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  1. ArielM, Aug 29, 2010
    #1
    ArielM

    ArielM Senior Member

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    So I have couple Detroit 6-71 engines in my boat and each tachometer (I have four of them. 2 helms) shows a different rpm at full throttle. I have tried adjusting the tachs at cruise speed but once I get to full throttle they are each different. I need to first figure out which gauge is accurate then I have figure out why the gauges don't work. I have heard of hand held tachometer which I intend to purchase but I am not sure exactly where to place it for my reading. I could put it in the center bolt on the balancer at the front of the engine. Would this work? The reason I question this is because any tach sending unit must be attached to some sprocket or gear which has a diameter. I am afraid that diameter would change the rpm reading some what. Any thoughts? Any point on where to look for the problem as to why the gauges read different? It is a 1980 43 foot viking. Tach gauges have a black and grey wire hooked to them. Any input would be great.
  2. Silver Lining, Aug 29, 2010
    #2
    Silver Lining

    Silver Lining Member

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    The easiest way and perhaps most accurate way is to get a nice digital laser tach and put the shiny trigger stickers on each harmonic balancer and calibrate your gauges. I have done this on my boat and check the calibrations each year. Mine have remained remarkably stable. I just add the 100 RPMs to my upper helm starboard motor for example. Be aware of which tach or motor the synchronizer is triggering from.

    You could of course replace the pickups and gauges, but you will still probably have to verify the accuracy.

    -Rick
  3. ArielM, Aug 30, 2010
    #3
    ArielM

    ArielM Senior Member

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    Thanks that is what I was thinking but where do I measure from on the harmonic balancer? Like the balancer is let say 8" wide. The farther fromthe center I put it the lower the rpm will read as opposed to if I measured from the very center. Do you know what I mean? Also at what rpm do I calibrate the gauges? At cruise rpm at wot ?
  4. K1W1, Aug 30, 2010
    #4
    K1W1

    K1W1 Senior Member

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    Hi,

    This is not how it works.

    If you put the tape on the very outer edge of the Balancer you will get an accurate representation of the RPM. If the tape was in the very centre you would likely get a reading of Zero as it is the number of times a minute that the tape passes the light that gives the RPM (Revolutions per Minute)

    The outer edge of the balancer will travelling at a greater velocity than one closer to the centre but it will still be at the same RPM.

    A lot of engines have the speed sensors in the flywheel housing, this is about as far from the centre of the Crankshaft as you can get.

    If your car normally has 15" Tyres on it and you fit 17" Tyres to it, your speedo will read higher than the actual speed as the wheels are driving it, if you had fitted 13" Tyres your speedo will read lower. This is because of the different circumference between the different sizes.

    If you drove your car at 4000 rpms (going by the in car Tacho) in top gear the wheels will be turning the same number of times per minute regardless of the overall circumference.
  5. ArielM, Sep 1, 2010
    #5
    ArielM

    ArielM Senior Member

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    So i bought a laser tachometer and took the engine to full throttle under load and got a max of 2290 RPM. Bottom is clean, fuel tanks at half. Now this is a Detroit Diesel 6-71N 310hp JT engine which has a rated max loaded RPM of 2500 and a max unloaded RPM of 2650. Air inlets are clear and the fuel filters appear to be clean. They are about a year old but there is no water or sludge in the water/fuel separator and they seem clean. At what point do you start looking to see if the variable speed governor, injector racks etc need tune? The boat is a 1980 viking 43 aft cabin and im sure the owner has never done a proper tune up before. Could this be a reason i am not getting the extra 210 RPM? Anything to look for before i start getting into the engine for tune up?

    I know i should be checking to see if i get 2650 RPM when the engine is NOT loaded but i am very apprehensive about putting that kind of RPM on an engine in neutral. Any other way around this?
  6. K1W1, Sep 1, 2010
    #6
    K1W1

    K1W1 Senior Member

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    Hi,

    If the engine is warmed up properly running it at WOT in Neutral to see what it gets to will not do it any damage, if it does then the revving in Neutral is not the root cause of the problem, there are bigger issues.

    Increase the revs smoothly and bring them down smoothly.

    The Fuel Rack position will be a lot less at WOT in Neutral than it would have been at 2290 underway so there will be a lot less fire and brimstone in the system to start with.

    Do both engines get to the same revs?

    It might be that your props are a bit too big if this is the case or over the years your boat has got heavy with all sorts of un needed goodies that you could do well without.

    A Detroit Diesel 2 Stroke is a wonderful thing to do a top tune on, you can really see a difference when tuning one of theses engines vs doing a similar service on a 4 stroke engine.

    I haven't done one for more than 25 yrs myself but used to like the 6 71's a lot more than the 8V's mainly owing to accessibility of the top ends in the applications we had them.
  7. jhall767, Sep 1, 2010
    #7
    jhall767

    jhall767 Senior Member

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    There is very little stress on those engines in neutral. Running diesel engines at WOT in neutral is standard procedure. It's necessary to set the governor. Warm each engine up fully before doing the procedure. You might want to start by ordering a set of shop manuals from J&T. I think they were about $110 last time I ordered them.

    Have you checked the shutdown dampers to make sure they are functioning?

    Do you have any service records on the engines?
  8. ArielM, Sep 1, 2010
    #8
    ArielM

    ArielM Senior Member

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    Thanks for the quick replies guys. It could very well be the props oversized which is why i think i must do the WOT in neutral to verify RPM. If i am getting the rated 2650 RPM in neutral then i know it has to do with the props. But IF i dont get the 2650 the engine itself has some kind of issue. Is there anything to look for before i open the valve covers and take the cap off the governor to do a tune up? As mentioned i dont think in the 30 years this boat has been around anyone has done a tune up on these engines. Is common for the engines to get out of tune on their own? I have a full Detroit Diesel manual for the engines with step by step instructions. all input is appreciated.

    Emergency shut offs work correctly in case i have a run away.
  9. Capt J, Sep 1, 2010
    #9
    Capt J

    Capt J Senior Member

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    Ok, change the fuel filters both primary and secondary so you know there is not a restriction there. Run the engines up in neutral and see if they make proper rpm. NEXT, remove the air filters (make sure nothing could get sucked in there) and run the boat and see what RPM's it makes. I have seen even clean looking old airsep filters close down at cruise and restrict airflow a few different times. Make sure the bottom and props are clean before doing this......

    after you've done all this. It may also be in the turbo's and tune-up. After you've done all of this it might be time to put gauges on and make sure the turbo's are putting out the proper amount of boost, then do a tune-up.....see where you're at....... and then the props last. You don't want to take pitch out of the props when you're actually loosing HP somewhere.

    I had one owner with the same problem and about 5k hours on the 6v71's and it turned out to be in the fuel injectors. When he had them rebuilt, he went another 7k hours and the motors ran great and the power was back and then he sold them with 12k hours on them running good.
  10. ArielM, Sep 1, 2010
    #10
    ArielM

    ArielM Senior Member

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    well i dont have air seps on this engine, i have air silencers. I could run the boat with the silencer off but i dont know if thats it. I dont have turbos either, just a blower as it is naturally aspirated detroit. engine has 1800 hours on them.
  11. Capt J, Sep 1, 2010
    #11
    Capt J

    Capt J Senior Member

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    Yeah, sorry I forgot you have naturals. Well make sure you have new fuel filters and a clean bottom, and see what you're getting. I'd go the route of having tune-ups done before cutting the props and see what that nets you.
  12. saltysenior, Sep 1, 2010
    #12
    saltysenior

    saltysenior Senior Member

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    are you testing with plenty of hatches open???
  13. jhall767, Sep 1, 2010
    #13
    jhall767

    jhall767 Senior Member

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    Technically an 'N' Detroit stands for normal not natural. Natural diesel engines rely on atmospheric pressure. Detroits use blowers.

    If WOT in neutral is ok then it would not be a bad idea to set the injector timing and valve clearances. You will need the correct timing tool. If it is not ok I might suggest you have a well experienced DD mechanic set the governor, rack, injectors, etc. I do mean WELL experienced. The procedure is fairly straight forward but it needs to be done properly. Under normal circumstances its only a few hours labor and sometimes a mechanic that is familiar with your engine may spot a potential problem.
  14. Capt J, Sep 1, 2010
    #14
    Capt J

    Capt J Senior Member

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    According to the Detroit Diesel website and service manuals the "N" stands for natural not normal.
  15. ArielM, Sep 1, 2010
    #15
    ArielM

    ArielM Senior Member

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    If it was reaching full rpm under no load then I thought everything is adjusted no? If it reached full no load rpm then it should be another problem like the props. Maybe I'm wrong about that. Anyway I will do the no load test tomorrow and let u know what happens.
  16. jhall767, Sep 2, 2010
    #16
    jhall767

    jhall767 Senior Member

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    DOH - my bad. I'm so used to hearing it incorrectly that I guess I double corrected.
  17. jhall767, Sep 2, 2010
    #17
    jhall767

    jhall767 Senior Member

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    It takes relatively little fuel (power) to reach 2650 rpm under no load. The only thing the no load test really "tests" is the governor setting. It's just the first step. Since the boat is new to you some of the next steps would be.

    Bottom is already verified as clean - correct?
    1) Clean Fuel filters
    2) Make sure the engine is getting enough air (run with engine hatch open)
    3) Test (short-out) injectors and set injector timing, Check rack for full travel
    4) Check fuel rail pressure.

    In short make sure the engines are performing properly before changing anything else.

    Run a full speed trial before removing the props. Two directions. I suggest you find a prop shop you are comfortable with before running the speed trial and pulling the props.

    In the meantime assume these engines are overloaded. They are pretty stout at their rated hp but if the load is the reason they can't turn up rated rpm then they may be overloaded at lower rpm's also and have insufficient heat transfer. This can be happening in local areas even if the average coolant temp is not showing an overheat.
  18. ArielM, Sep 2, 2010
    #18
    ArielM

    ArielM Senior Member

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    Alright guys i did the no load test today. Max NO LOAD RPM was 2550. SO i am 100 rpm under the 2650 max no load RPM. So i dont think i have a prop problem. I guess it's time to do a tune up and see if that helps. thanks to everyone for the help.
  19. K1W1, Sep 2, 2010
    #19
    K1W1

    K1W1 Senior Member

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    Hi,

    JHALL767 - How would you "short out" a mechanical injector?

    I would also check the valve clearance
  20. jhall767, Sep 2, 2010
    #20
    jhall767

    jhall767 Senior Member

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    It's a standard test any DD mechanic can do. While the engine is at idle you put the injector to full fuel briefly. A sharp sound will be heard. If the injector is bad you may hear no change or a more muffled sound. Generally working through a bank of injectors you may hear one that is not as good as the rest. This is where an experienced mechanic knows what to do and what to listen for. Other than taking the valve cover off the whole test only takes a minute or two.
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