6V92 engine questions

[garyd]

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

 

[Genesis]

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.

 

[DaveP]

DDEC's

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.

 

[RSYacht]

DDEC's

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.

 

[garyd]

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

 

[Traveler 45C]

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?
G

 

[captcoop]

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.

 

[Genesis]

Yes

.... 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.

 

[garyd]

6V92TA

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

 

[Genesis]

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.

 

[garyd]

6V92 DDEC 625HP

Excellent response, Having rebuilt many high powered gas engines and even a jet engine, I appreciated your disertation.

I have another simple question. When I repitch the props by 2" will the govenor keep the RPM from going tooooo high? I have never run the engines up to WOT not in gear.

And if I understand it, unlike gas and jet engines RPM in diesel is not really the bad guy its power/load. So I can increase RPM to a point and actually make the engine breath better and get complete combustion. Thus increasing mileage, reducing stress and carbon buildup. This not normal to either gas or turbine engines. This is an insight I was missing and thank you for a new mindset!

thnaks again garyd

 

[Walter Pereira]

6/92 engine life

After reading all the comments regarding 6/92's, I think I realize why when I tell people that I have 6/92's in my boat they sort of give me their condolances (sp). Truth be told, I am guilty of not running most of my boats hard enough. I like to cruise at displacement speeds unless we are on a timetable to be somewhere. We also don't want to be locked into trawler speeds. We like the option as the need presents itself.

My question is - how can I cruise most of the time at 9.2 K (1200 rpm) and still run up to 16-19 K when I need to, without hurting my engines.

I presently have 6/92TA's of unknown HP as they have been redone in the past. The history is a mystery, as I only recently aquired the boat. I don't know what sort of maintaince it had (if any). The oil resembled tar both in color and viscosity. One of the engines (port side) was actually replaced with a new long block a couple of years ago and I can't imagine why it is already worn. I need to get them majored or so I was told and I want to be sure of getting the right job for my needs. The boat is a 1982 48 MY with the usual "stuff" that is in the process of a complete rehab.

 

[Genesis]

Yes, and yes...

Gary, yes. The governor prevents overspeed.

Indeed, one of the tests you do on a diesel is to confirm the no-load speed. That is, you warm the engine up fully, disconnect the linkage, take it out of gear and advance the "throttle" to full, then measure the RPM. The no-load RPM on a 6V92 in a marine application is typically set to 2500.

Full-load RPM will be about 150-200 RPM off that, if the boat is propped for full power.

The DDECs are electronic rather than mechanical, but there is still some "slop" between no load and full-load RPMs - although not as much.

Typical "spec" from various marine engine people (e.g. Cummins, Cat, etc) is to run at no more than 200 RPM off the maximum for continuous operation. Due to prop demand curves this should result in an injected fuel amount of no more than 80% of maximum. This is for a relatively-conservative rating - for one that's very aggressive, as your boat is, I'd want to be in the 70-75% area at 2100.

In a marine application the usual arrangement for the governor corresponds the "throttle" (which is really not correctly named, as there is no "throttle" in a diesel!) directly to a RPM setting. That is, you select the RPM of the engine with the throttle lever. The engine varies the fuel injected to maintain that RPM across varying load (e.g. up a wave, down a wave, into the wind, with the wind, etc)

This is very different than in a "road" engine, where the throttle position roughly corresponds with the power output, with a hard RPM limit at the maximum specified.

Walter, the issue for these engines in terms of what you want to do is mostly a function of whether they stay fully warmed up. If they do, and you blow them out every few hours with a 10-20 minute run at cruise power (1900ish), they'll be fine. Trouble begins if the temperatures start to drop. Anything below 170ish is bad news; that's the opening temperature of the thermostat, more or less, and the engine has to produce enough heat to keep itself clean. If they run cooler than that its bad news, as you will get incomplete combustion and the carbon will build in the ring grooves. When a ring sticks it will either score the cylinder or break - either is very bad news as it will hose that cylinder immediately, and if you get unlucky with a broken ring the piece(s) can come back into the airbox port and then be trapped, doing catastrophic damage in that cylinder or even worse, wind up with pieces floating around in the airbox and going into OTHER cylinders.

In older motors one common reason for this to happen is the thermostat housing seals. The thermostats in these motors are kinda bizarre - they are a bypass design, and either run the coolant through the H/E or bypass around it. There are also two thermostats - one for each side of the motor! In the housing there is a seal in which the thermostat poppet rides - if this is worn and leaking the engines will run abnormally cool at part-power settings. If you've got this problem, you need to pull the housings and replace the seals, along with checking the thermostats to make sure they're fully closed at temperatures under 170F. Its not a difficult job but does require partially draining the cooling system.

The 6V92 is not a bad motor, but you have to be aware of its heritage and what a REASONABLE output level for it is on a continuous basis. Anywhere beyond 300 HP on a continuous output basis is trading power for time in terms of durability. You have a 552cid engine - the general "rule of thumb" is that for really good service life you want the maximum output at WOT to be somewhere around 0.8hp/cid, which would be right around 440HP. This would make the output at 200 RPM off the top in the neighborhood of 300-350 HP.

0.9hp/cid is the "wall" is IMHO "the line" for good service life in "heavy recreational" (over 100 hours/year) service, which is 500HP.

Beyond those ratings you're playing with hand grenades with loose pins.

You can loosely determine output from fuel burn - diesels develop roughly 16hp for every gph consumed, give or take a bit (the newer full-authority electronic motors - DDECs are NOT in this category - do a bit better due to varying injection timing, but for Detroits these numbers are reasonable solid.) So if you know what you're consuming from the tank, you can roughly determine how many mustangs are being requested.

The final thing to be aware of is that these are "wet liner" engines. The cylinder liners are held in place in the block with O-ring seals. If you EVER overheat these engines, you are almost certain to compromise the seals, which will result in cross-contamination of the cooling and oil systems. As such the cooling system condition is critical, and it is very important to not allow these engines to ever exceed 185F. A wise man installs alarms that sound at 195F, and never hears them ring.....

 

[DaveP]

Governor?

Gary,

The DDEC's don't have a mechanical governor but a microprocessor controlled fuel system. I believe that whatever parameters the tech sets into the computer regarding idle, no load speed, and WOT will not change if you change wheels. The only thing you will do is reduce the amount of fuel you need to run the same RPM's. As Genesis said anything you can do to help your engines live and breathe you should do.

We have 550 hp 8V92TI's in our 45C and we propped her to run 1900 at cruise because our mechanic told us that was the low end of the best sustained operating speed for these engines. He mentioned EGT's, and Load, and breathing, and increased lifespan if the engine is getting the right amount of fuel and air for clean total combustion. J&T likes you to run 1900-2000 continous cruise with 92 Series engines.

As for low speed cruising with 625hp 6V92's I am not sure that you are helping your engines out. As long as your water temp is around 180 you should be OK, but I know even with my detuned engines and small injectors I load up after trolling at 900-1100 RPM's for a few hours. My engine temps drop down to 165-170 and I smoke when I accelerate. I changed my thermostats to 180's and that helped but J&T said to put back in 165's because the 180's will be constantly opening and closing and will mess up the cooling if I tried to run with the thermostats in the closed position. If you look at your DDEC readout for EGT, just make sure they don't rise above normal temps. I will say this...at those slow speeds you aren't letting your engines breathe, in fact your turbo boost is probably in the neighborhood of 5-8% if that.

 

[garyd]

6V92

Ran the boat last night. at 2000 rpm white/clear mist exhaust. At 1900 a very slight smoke at 1800 a little more, nothing major. It looks like I can run about 1900 at 26.5 kts and accomplish what everyone is saying until I get a chance to reduce the pitch. Turbo boost is: 1800 13 p 16 stb. 1900 17 p 19 stb. 2000 20 p 22 stb.

Trolling doesn't really load them up I guess the ddec system takes care of that for the most part.

Temps stay at 170 at just about all RPM's.

Someone told me to let them cool down about 15 minutes before shutdown due to fuel cooling the injectors. I figured he really meant to cool them down so I let them drop until they stabilize around 145 then I shut them down. See any problem with that?

thanks gary d.

 

[garyd]

6V92 DDEC cool down procedure

When shutting down I let the engines cool down to around 145 degrees it takes about 15 minutes. Is it necessary or harmful to do this? thanks for your responses, learned abunch so far.

garyd

 

[Genesis]

Good idea....

.... to cool them down before shutdown.

15 minutes is more than you probably need, but won't hurt anything.

The big deal is don't shut down hot.

 

[divernc]

Engine temps

First year of Hatteras ownership. I have 450hp 6V92TAs in a 1979 43C. Engines have 2050 origianl hours. As winter water temps have risen from low 50's to low 80's, both engines' temps have risen from 175 to 181-182. These are from bridge gauges and mechanical gauges in engine room. Is this normal?

Last Friday, the port engine overheated, but resumed normal temp when I pulled it back to 1000 RPM. Hours later in a heated engine room, I found the pencil zincs in the Jabsco raw water pump broken off and laying on the intake strainer fingers of the pump. This seems to resolve the issue.

I have cleaned and replaced all parts of raw water cooling system except exhaust showers. The exhaust shower can is welded to the exhaust piping coming off both turbos(about 5' long with a Y for both sides of engine). There is not enough room to slide the rubber coupler downstream to get a look at the shower head while in place. I tried to remove by taking off the clamps holding the female to female mating of the turbo and exhaust tubing. I could not ge them loose with a rubber mallet or steel hammer on a 2X4. There is a set screw or gauge sensor port on top of the exhaust tubing immediately downstream of the turbo, which I could not free either. Any hints on how to get access to the shower head for cleaning? Is there a way to tell if these are obstructed without disassembly?

 

[Genesis]

Exhaust issues....

OK, you have the old style (original) single-piece exhaust collectors.

You have to dismount them to get them out.

To do so:

1. Remove the blankets over the turbos and the collector. Close the seacock and remove the raw water hose going to the shower head. This will make a saltwater mess - have a 5gal bucket handy.

2. Remove the Monmon (V-Band) clamps on the turbine exhaust outlet. DISCARD THESE. It is very important that you NOT reuse these clamps - they heat-harden and if one breaks underway you are almost certain to have an engine-room fire. New clamps are roughly $10 (each) These clamps also use castellated nuts which self-lock but are not nylon-based (too hot for nylocks)

3. You should now have the exhaust collector attached to a brace that goes to the transmission rear housing and bracketing system roughly in the center, and at the shower head in the hose that goes to the elbow leading out of the boat. If you have Pyrometers (wires coming out of the collectors just behind each turbo), remove them. PB Blaster is likely going to be required on these connections - be careful, however, not to get too much of this on the pipes in general, as it contains an oil and is flammable! Take your time on those lest you break the probes and get to buy new ones or worse, shear them off leaving you with no pyrometers.

4. Loosen the clamps on the hose on the shower-head end. Using 409 (yes, the cleaner!) and a straight-blade screwdriver, work the screwdriver between the hose and shower head, spraying 409 in the gap. Work around the entire shower head to free it up as much as is reasonably possible. This will take some time, be careful not to damage the hose.

5. Place your FEET between the turbos on either side of the flex section in the collector pipe while sitting on the airhorn. Try to avoid pressing on the flex section - you may not be able to, but do your best. Kick STRAIGHT BACK with moderate force - not too hard! The collector should separate from the turbos. The idea is to just break the joint loose - don't go nuts with the force here or you'll bend the support bracket at best and could rip things up in the exhaust beyond the shower-head at worst.

6. Now get some help to support the pipe. Its kinda heavy and awkward (perhaps 50-60lbs, long, etc) With the turbo end supported by a helper, unbolt the support and remove it.

7. You now have the pipe held on one end by the hose, and the other by your helper. The helper now must push his end aft while you use your legs to lever the pipe forward and up slightly to clear the hose. Beware that if you have blower-mounted alternators that there is a hot 12V lead back there that is on EVEN WITH THE MASTER DISCONNECTS TURNED OFF AT THE PANEL! It should have a boot on it, but..... Don't short it! Once the pipe is free you can maneuver it out of the engine room.

Installation is more-or-less the reverse of removal. Use Silkolene on the turbo-to-collector joints, and use NEW band-clamps. The first time you put the engine under load the Silkolene will smoke like hell - do your sea trial after repairs with the floor up and all windows open! Silkolene seals during that heat-curing process and will prevent exhaust leaks, which is very important to avoid serious engine damage due to ingestion of "recycled" exhaust.

 

[Genesis]

Temps...

181-182 is normal, provided those are real temps (get an IR thermometer or mechanical gauges if you don't have them - electrical gauges are notoriously unreliable)

Do not allow these engines to run over 185F. That is "thermostat fully open" temperature, and you will quickly get overheats beyond 185F, as you have effectively reached the end of the cooling system's reserve capacity.

Thermostats start to open at 170ish; so normal range is between 170-185. These engines have marginal cooling systems for hot raw water (over 75-80F); be wary and attentive, as regular maintenance is required to avoid disasters.

Overheats are DEATH to wetliner motors, and the 92s are. Check those zincs regularly, at least every three months, and replace long before they get worn and can break off.