92 series Detroit Diesels

Figure 2.75 GPNM with 8v92's and 2.5 gpnm with 8v71's at $3 per gallon, divided by mileage , figure 900nm's just guessing on the mileage, including all of the in and outs of inlets. $7425/2475 gallons with 8v92's, and $6750/2250gallons. But this should be fairly close, if you're running the ocean and at cruise (except for ICW Norfolk to Beaufort,NC).

Like everyone else said, keeping RPM's at 1000 rpms (about 8gph for both 71's, and 10 gph for both 92's) or net you a lot better fuel burn per mile. BUT, this time of the year I'd get South as quick as possible. Then you have to figure all of the additional in and out mileage you're adding (going in and out of inlets, just to get to the sea buoy's to go South) if just doing 8-9 knots and days......additional marina fees, Captain (if you hire one), dinners out, additional days you're weathered in this time of year coming up, and on and on. On a boat this size, you really don't save a ton more on a long trip like this by running slow when you factor in your time and additional expenses.

Like it says in what you quoted. Cruise. He asked the most efficient speed after I typed that. The most efficient speed would be dead idle, in gear, but that wouldn't be logical, comfortable, or good for the motors. Read my later post regarding additional days, additional weather days, marina expenses..... on a 46' it's not logical to do a long trip like this at hull speed, unless you've got the crew and weather to do long pokes (overnight legs), which I just did earlier in the week by going from Fernandina Beach 350NM to Fort Lauderdale non stop on a 45' lobster boat. It only cruised at 10 knots. It took about 2 hours from the marina just to the sea buoy in Fernandina beach at 10 knots..... Same with at Hilton head....so you're wasting 2 hours out the inlet, 2 hours back into an inlet in a lot of places.....to only do a maximum of 7-8 hours South. When you add all of the extra mileage, days and hours, you're creating by going in and out over twice as many inlets because you're doing 9 knots everyday, dockage, etc. what are you really saving? That $3k in fuel gets eaten up really quick with an additional $1000 in dockage, food, and extra days. And, now you're getting into Fall where the chance of not having a weather window and getting stuck at a dock somewhere is even greater. AND, if he needs to ask this question, he needs to hire a Captain at $350-400 per day to help him get it to Myrtle Beach! AND, if you find cheaper fuel the savings are even less of a difference. I just paid $2.50 gallon in Hilton Head and $2.25 in Fernandina Beach at marina fuel docks with no discount (pump price).....What are the savings then? I just quoted $3 a gallon because in the NE fuel tends to be a little more.

I have a 1989 43 bertram with 6v92 great motors flash right up no smoke after 5 sec. 1285 hrs in fresh water always run 1800-1850 love them!!

You likely would get no smoke at all as long as you turn on your block heaters 24 hrs prior to starting. My 12v92's never smoked as long they were warm. If cold and cool ambient temp they smoked out the harbor.

Wow ! From start to finish, an extremely good read and education, with all the DD- YF heavy weights involved.

+1

All,

I have been reading so much about the famous/infamous Detroit diesels out there, I’m curious, if most of them issues on them (if not all) are related to the high horsepower, which in terms can cause excessive heat, thus cracking heads, braking rings, etc.

If no need for speed, an 8v92TA rated at 485hp can give you much longer life expectancy than the same engine at say 720hp, what’s to keep from detuning the 720hp to the original design at 485hp, it sounds to me that is a matter of injector and turbo sizing.

I’m curious as there’s lots of folks out there (like me) that are not in the need for speed as much as reliability or longevity of the engine.

The market is loaded with Detroit Diesels and lots of them are the high HP with lesser reliability, what’s to keep from bringing back to factory specs?

Good question. I'm sure someone here as a good answer.
Here is mine. Just go easy on the throttles. Never over 1500 rpms that will prolong the engine hours. Heck rig up some sort of stop inside your throttle controls or down at the governor. My 71's governor , new, had a stop ring in it to keep it from getting it's full RPM. It was set up to stop at 1950 RPM. for use on a 471 generator or what have you. I had to pop it out to get to 2500 rpm.
Perhaps your governor is similar and you can add a stop ring.

You don't want to run 92's at 1500 rpms or less, you will surely wash out the cylinder liners with the big injectors. Key to keeping 92's live a long life a few things are critical. Under prop them a hair so they make 50 rpms over rated WOT, run them hard 1850 rpms or more at cruise, and change the coolant every 2 years. I managed a 63' Ocean SF with 12v92's that went almost 6000 hours pulling a 35' Marlago around at cruise the majority of the time. A lot of the other issues with 92's not lasting were a lot of out of spec blocks. Rebuild the motors but never fix the out of spec issues, Either the dowel pins making the blocks not properly aligned, out of spec cylinders and/or loose fitting liners, as well as other stuff.

A bit on the fence on that, there are many 8v92’s that have lived 10k plus hours at a very low rating of 300-400hp, these are continuous duty 1600-1800rpm, I’m pretty sure is about the loading or how much fuel the injectors put out.

What’s to say you drop from 720hp to 485hp, prop correctly to say continuous duty of 8-10kn at 1600-1800 rpm, you would no longer run low rpm which could washout the cylinders, not an expert here but it seems to me like there’s so many boats out there that have those engines but most are afraid of them due to their reliability.

Im looking at a 52Hat C with 100 hours on the 8v92’s at 720 by Covington, I’m a trawler fan myself so no need for speed or extra ponies, I really don’t care about fuel consumption, but I do care about getting to the destination lol!

I come from a 42’ trawler with twin Lehrman 120’s so 8kn is blistering fast for me.

You could derate the engines and do that, but the cost of new props, injectors, etc. etc. would be cost prohibitive. You could run 92's for 4-6 hours at 800-1000 rpms (closer to 1000 rpms would be best) and then run them at 1900 rpms for 30 minutes to clean everything up, and this would be your most economical situation and not effect resale etc.

Thanks Capt 3

so it could be done (money aside), pretending rebuilt was done properly you could possibly run say 6000 or so hours instead of 2000 or so hours. Unless you are commercial it would probably take me 15 years to rack up 3000 hours. At this point it’s a matter of who goes first, me or the boat.

However, my understanding is the 8v92 is a reliable engine at spec configuration, less prone to overheating, washing cylinders, etc. no need to clear up every 6-8 hours.

The question is, if you were doing a 700+ mile journey, would you be more comfortable with a 8v92 at 720hp rating or a 485hp factory rating?

For a trip that's all of 72 hours, I wouldn't much worry about it either way. But,It depends how you look at it. If you're doing slow speed with the 720 hp engine and then running it every 4-6 hours at 1900 rpms, then the boat is on top on plane. If you're running the 485 hp version you might be turning the engine 1400 rpms......but then at 1900 rpms, you'd simply be at the absolute possibly worst running angle and strain on the engine. I ran a set of 12v92 TA's just that, 700 miles and ran them 1000 rpms for 90% of it and ran them up every 6 hours.....then 12 hours at 1000 rpms overnight..........as well. Boat was a total pig, did well at hull speed- 10.5 knots at 1000 rpms, did 13.5 knots at 1900 rpms......they were actually pretty clean when bringing them up after a long slow speed and didn't use a drop of oil the entire 700+ mile trip. The newer 6 port injectors not only add HP, but helps them a lot in regards to running cleaner on the bottom and not washing cylinder liners.

I also did the entire great loop with a set of 12v71 TI's and 90% of it at 1000 rpms.......

The biggest thing I've seen with 92's is they last a lot longer in a 25 knot+ boat.....like the Ocean SF many were in, .the ones that seem to last the shortest are heavy boats that are barely on plane and have constantly changing loads/speeds at cruise and one minute are doing 22 knots and next minute 17-18 knots and trying to climb back up the hill.

I would just run what's in there, provided they're in proper tune and then worry about it when the time comes to overhaul.