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  1. #1
    garyd Guest

    6V92 engine questions

    I have a pair of 6V92 625HP in my 1986 41C. It cruises at 24.5 Kts at 1,800 rpm, tops out at 33kts at 2350 rpm. Almost toooo fast. I would like to know if anyone has experience with these engines and if running them at 1800 rpm at 170 degrees is ok, they do load up a little and I like to run them up to 2200 rpm to blow em out. Any input would be appreciated. I have about 100 hours on them so far without any trouble.

    thanks garyd

  2. #2
    Genesis Guest

    I would....

    .. take several (as in 2 or more) inches out of your props.

    This will effectively unload the motors.

    I do NOT like the 6v92s at that rating. At all. They fall into the "loose pin grenade" category, and I know of way too many people who have had them blow up long before they should have - just because they're being pushed so hard.

  3. #3
    DaveP Guest


    Your DDEC's are good motors but as Genesis said they are operating at the very upper end of their performance spectrum. I am assuming you have DDEC motors as I believe those are the only ones that came rated at 625hp. Just remember these little motors are very good when not pushed, but when you are getting 625hp from only 552ci you are definately running a HighPerformance motor. Most of the current 600+ HP engines are 12 liter or bigger. Yours are more like 9 liter engines. The old 8V71TI's were 556 ci but rated at 435hp and had a very good lifespan, but when tweaked to 500+ hp they usually only lasted just over 1500hours. I would not only do as Genesis suggests, but also make sure that ALL service requirements are met, and that you REALLY watch your temps, nothing too low or nothing too high. Either way you will shorten an already short lifespan. Baby those puppies or you'll be doing a rebuild before you can ask what's wrong, and unloading the motors is one of the easiest ways to de-tune them.

  4. #4
    RSYacht Guest


    The above responses are right on. Just another reference point - I'm running 8V92TI's at 550 hp each in a 58MY. Running the 92 series above their "base" hp rating significantly shortens their life span. Unfortunately, this is (was) a common practice and ended up nearly ruining Detroit Diesel - even the legendary 71 series.

  5. #5
    garyd Guest

    6V92 625 hp

    The DDEC engine load parameter shows about 70% on port engine and 75% on starboard at 1800 RPM. I felt this was running them well below what most captains push there diesels. The engines seem to be happy with this RPM (170 degrees) but do load up a little. Therefore I feel I need to blow them out at 2200 rpm. I do not want to take them up to 2350. My question for anyone is this OK over the lifespan or will I be creating another problem down the road.

    The boat comes up on plane easily. The engines are so strong that it doesn't really matter weather it's fully loaded or dead empty. Though I can only get .57nmpg.

    This boat originally had wedges to get the bow down, those were removed with excellent results. I would suggest if you are repowering to check for wedges and remove them if you are bumping the power significantly.

    I read Genesis thoughts on a crap can and wonder if this has to be vented or does it add diesel smell into the engine room etc.

    thanks garyd

  6. #6
    Traveler 45C Guest

    Re: 6V92 625 hp

    Taking 2" out of the props will increase the engine's life span. I understand that, but does this also effectively lower the HP? This will also lower the cruising speed, correct?

  7. #7
    captcoop Guest

    6V92 rpm

    I have 6V92 Detroits and the recommended continuous cruising rpm is 1900. I would run at 1900 as is, rather than re-prop to slow down to speed the engine up. WE all would like to have your problem of too much speed.

  8. #8
    Genesis Guest


    .... it does effectively lower the HP you can demand from the engines.

    Unloading the engines is a good thing. Running at anywhere from 1900 to 2000 rpm is not a problem provided that you are not loading the engines excessively.

    RPMs by themselves are not bad. All things considered I'd rather have 400HP out of the engines at 2000 RPM than at 1500 RPM, because to get the HP out at the lower RPMs requires more torque to be developed, which is harder on the engines.

    The paradox with diesels is that at lower RPMs you get little airflow and boost, and the consequence of this is that they run hotter combustion chamber temperatures.

    That spells trouble.

    By running at a higher RPM you increase the AIRFLOW through the engine. Remember folks, the air pumped is more-or-less controlled by the RPM - the more times the piston comes down in a unit of time, the more air is blown through the cylinder. Airflow is GOOD, as it controls EGTs, valve and seat temperatures, and piston crown temperatures. The controlling factor of course is that as RPMs increase reciprocating loads increase, but so long as you remain within design limits on RPM you're ok there.

    In 625HP DDEC trim, the 6V92 is WAY beyond its output power design limits. I would not own those engines at that level of tune, because I'd be scared to death of the overhaul monster showing up every 500-1000 hours, with possible catastrophic failures in the middle, and that's if I treat them nicely!

    If you HAVE those engines, the way to get out of that box is to depitch the props by 2" or more. This will drop the output demand significantly and allow the engines to breathe. You will run a higher RPM for a given cruise, but if you're ok with the speed you develop at that RPM, and keep cruise RPMs to no more than 1950 or so or even 2000 I'd be happy with this.

    I would consider pyrometers MANDATORY on those engines in order to detect incipient problems before they burn huge holes in your pocketbook. If you have 'em, you'll notice that the EGTs go down significantly for the same speed if you depitch the props and run a higher RPM. The reason for this is the increased airflow through the motor.

    The 6V92 industrial engine's maximum CONTINUOUS RPM level is specified at 2100. Now granted, this is at far lower power outputs, but the point holds - there's nothing wrong with the RPMs provided you're not loading the engines too heavily.

    A common fallacy is that you can compensate for overpropping or too-high output by just throttling back. This is only true up to a point. The problem with throttling back is that you're demanding the same output but doing so at a lower RPM, which means you're demanding more torque from the motor (since HP is torque over distance per unit of time, and distance covered by a unit of time is controlled by RPM.) High torque output from low displacement engines at low RPMs can only be achieved with the byproduct of creating a LOT of heat due to fewer "breathing" cycles (which cool things.)

    Drop the pitch and run the RPMs up a bit. Your engines will be happier and live longer.

  9. #9
    garyd Guest


    As you are aware on the ddec's you get a few extra parameter readouts. I will list each at 1900 rpm.

    I certainly can run at 1900 rpm all day long, that means, if the load % is linear, that the engines are running at 80% or 500hp. I thought 75% would be better which equates to 1800 rpm and 468hp. But it starts to smoke a lille over time about three hours unless you run them at 2200 for a short blast then the smoke clears. the Temperature doesn't change from 1800 - 2200 rpm always stays around 170 degress.

    If I have to rebuild them sooner than later as long as I know when the gig is up I don't necessarily have a problem with doing the work. It seems everyone is overhauling at 1,000-1500 hours anyway. Unless theye put 3,000 hours on every year.

    They do not seem to be under any real strain on this boat. Weight of the boat with full fuel, full water, and some equipment is only 35,500 lbs. Props are 26 x 30 x 2"

    Thanks for your input really appreciate it. Came out of Gasoline ally diesels are relatively new.

    Though the DDEC computer "fly by wire" system saved us the other day when one of the passangers kicked the engine out of gear at cruise rpm then put it back. The DDEC system slowly dropped the engine RPM to idle and would not let it back into gear, GOOD STUFF. don't try that with mechanical linkage, BOOM!

    thanks again

  10. #10
    Genesis Guest

    80% is NOT good!

    Gary, let me put this in a bit more concise terms for 'ya...

    First, load percentage is NOT what you think it is. Its not "percent of 625HP"; you can only develop rated horsepower at rated RPM. Its percentage of possible fueling on the injectors - which correlates to fuel burned, but not necessarily power developed. Only at rated RPM is there a direct correlation.

    For a given amount of fuel injected out of the total possible, you only turn all this into power if the engine is turning up rated RPM. If not then you get something less in terms of actual horsepower, and if you go too far down the curve you get heat and smoke instead of power, because the airflow is insufficient to cleanly burn what is injected.

    The way this works on a diesel is that the governor is asked for a given number of RPMs. It controls the fueling of the cylinders to achieve that RPM level - no more or less. If the RPMs begin to drop due to additional load, the rack is advanced so that more fuel flows. If the RPM rises above the desired level, the rack is retarded.

    On mechanical engines this is all done with a set of weights and springs, and pushrods to the control tubes. On electronic engines you have an encoder (usually optical) that reads the RPM and controls the injection either via the same kind of mechanical connection or by direct stepper motors in the injector(s) or injection pump themselves. The mechanical engine could bring out a pointer that represented "load percent" by simply showing you how "open" the fuel rack is, but they typically don't. The electronic engine of course makes this easy. All DDEC has done is replace the governor with a microprocessor and the control racks with motors, and they stuck a bigger turbo on there to increase the boost. More air = more power, if you can inject the fuel to burn in the air avoid melting things, which is where the extra monitoring comes in (to try to avoid melting things.) It is still a pushrod engine in terms of actually pressurizing the fuel in the injectors, and it is still a 2-stroke with all of its good and bad points.

    The 625DDEC 6V92s were and are beyond where Detroit should have pushed things. They're just too much from two few cubic inches. They exist because boat buyers want a small engine so they have a large saloon, and they also want their boats to go fast. WE, as boat buyers, are responsible for the mess that gets created by applying pressure to manufacturers to build products that simply can't be expected to live long.

    Detroit knew that the average "boat guy" runs his boat 100 hours a year or less. They figured that if these engines blew up at 500 hours they'd probably get out of warranty, and if they blew up at 1000 hours most people would get 10 years out of them, and be reasonably happy. The unfortunate few that run 300 hours a year, however, got (and still get!) really pizzed off by this kind of "service life."

    Don't ever think that if your load meter reads "80%" on a 500HP engine this means you're pulling 400 ponies. It only means that if you're turning up at full WOT RPM. What that "load percentage" means is that you're at 80% of "full fuel" for the RPM you are currently operating at.

    Second, 500HP is WAY TOO MUCH to ask of those engines continually. Continuous industrial ratings for the 6V92s are in the 300HP range! I have an ORIGINAL (late 70s) Detroit service book right here that shows the continual rating for a 6V92TTA (twin turbo aftercooled) motor of 275HP @ 2100 RPM! You have substantially the same motor - there is precious little difference in terms of the reciprocating parts between your motor and that motor. Don't think for a second that you can pull 500HP continuous out of those engines and have them last. They won't. That's like me (who has the 6V92TTAs @ 500HP) running with the throttles firewalled all the time - I'd be lucky to get 500 hours if I did that before something blew up.

    You'll have those motors for a lot longer if you pitch the props to show 80% load at 2300 RPM. Your boat won't go as fast, but it will burn less fuel and the engines won't fly apart. If you want to go for 85 or even 90%, go ahead, but realize what you're trading off. Beyond 90% on these engines at WOT you are playing with fire.

    If you're loading up at 1800, its because your boost (airflow) is low and you're not getting clean combustion. This is how you stick rings and ultimately break them or score cylinder walls. The solution to this, assuming you're ok with the speed at 1800, is to take 2" of pitch out of the props. You will then turn up about 1950-2000 RPM at the same speed, will not load up, and your load percentage will actually DROP. More importantly, your EGTs will drop significantly, which is a good thing in terms of service life. Finally, with the improved airflow you'll get better combustion and the engines won't load up on you.

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