Detroit powerplant opinion

Turbo engines have a lower compression ratio. The water pumps are higher capacity and the heat exchanger and oil cooler are designed to remove more heat than naturals produce. So the engine would run cold with the thermostats constantly opening and closing and the oil would be too cool for efficient operation.
If you buy a boat with turbo DDs, just take it easy and you'll get longer life, just not decades as with naturals. Running below 80% of hp and having clean oil makes a huge difference.

Regarding clean oil: how often should it be changed on a 12v71ti

"Regarding clean oil: how often should it be changed on a 12v71ti"

Depends how you chose to clean it.

Factory filters ? 100 hours or what Da Book sez.

Gulf Coast or other bypass filter perhaps double , but that's where oil sampling comes in.

Centrifuge ? the oil is clean "forever, but additives will only be added from oil consumption.


On our 6-71 the loop did great on oil ,factory bypass filter, one cold start per day 6-10 hours of running , no idle time , engine shut down in locks.
After a change the oil would be clear & golden (like a car) for 50-60 hours , never got that Black Black as the 8v-71 in bus did instantly..

Woodland Hills said:

Regarding clean oil: how often should it be changed on a 12v71ti

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How long the oil stays good depends highly on the engine duty cycle and sump capacity. Run hard or run too easy and oil can degrade quickly. Run at mid range and oil can be good for easily twice the book interval. I think the book interval is 250hrs or so, but often that spec does not account for the high hp versions, it covers everything that the engine might be installed in, from gennies to cranes, etc.

Oil sample is your friend here. Run it say 200hrs and pull a sample. Lab can tell you how much life is left. Make your decision from there.

Post the results and I can help interpret.

I centrifuge my oil. Before the centrifuge I used bypass oil filters in addition to stock filters. I test my oil and generally have particles below 5 micron. Stock filters only filter to 30 microns. And much bigger particles get thru with longer oil change intervals. Once a stock filter is plugged with debris, the filter has a pressure relief valve that opens and no oil passes thru the filter element.

The debris in the oil is what wears engine parts. Rings carry oil up the sleeve/cylinder to lube the top. The more crud in the oil, the faster the sleeve/rings wear. Sleeve and ring wear usually dictate when an engine gets rebuilt. I started using bypass filters in the 1960. I've run many engines thru several rebuild cycles. When I went to bypass filters, I doubled the interval between overhauls. Bearings would usually have less than .002" wear at the time the cylinder components were changed. With bypass filters I pull an oil test and often just changed the bypass filter because the test is good. The stock filters were still clean. So in the long run it was cheaper to have a bypass filter than pay for the additional oil changes and stock filters.

Now with the centrifuge I run the oil thru about every 50 hours and never actually change the oil unless I get a bad test. And I've never got a bad test. I run my Detroit naturals at max continuous rpm. Turbo engines I ran at 80% of hp and probably never at 100%.

671 dd

Check out bus diesel grease monkey.

Lepke said:

I run my Detroit naturals at max continuous rpm. Turbo engines I ran at 80% of hp and probably never at 100%.

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Isn’t max continuous for a 6-71N just about 80% also?

My experience with Detroit’s goes back a ways when Detroit’s were installed in many OTR trucks. They were solid engines but a bad driver could ruin them. What killed them was fuel economy. The 5.0 MPG was fine when diesel was $.29/gallon. The Arab oil embargo in the mid 70’s doomed the old Detroit engines. They also were not noted for being long lived. A Detroit at the time was good for maybe 400,000 before an in frame. The legendary Cat 3406B would go nearly twice as long. OTH, parts were much cheaper for a Detroit.

My old fishing buddy and I were talking about engines and I was ragging him about his 371. The engine is the main and drives his refrigeration also. He starts it at the start of the fishing season and only shuts it down for oil changes. He put me in my place by telling me has averaged 6000 hours a year for the past 15 years. The engine gets rebuilt every 5 years. Has right at 100,000 on it now but was told by his mechanic no more rebuilds for it. Guess what brand he will replace it with.

Downsides of these engines are they're loud, smokey, thirsty, produce soot, slobber blow-by, and can be nearly impossible to keep free of oil leaks.

Upsides are there's still lots of parts and engines around and they can be rebuilt over and over, they're simple and reliable and easy to keep running and plenty of people know how to work on them.

If you're not a commercial operator the downsides can be pretty weighty. Sportfishers and 'power' boat operators tend to like the Detroit's authoritative rumble, and small Jimmys in a large boat can be fairly isolated from the main cabin spaces, but in a lot of boats with them you can't have a conversation in the salon while cruising. Relatively few yacht owners see one round of engine rebuild, much less more than one.

So consider the comfort factor at cruise and whether it is important, and things like diesel smell & smoke on deck, soot around exhaust, these are more prevalent with these engines even when they are running well.

makobuilders said:

Isn’t max continuous for a 6-71N just about 80% also?

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About 85% of the pleasure rating. 1800 or 2100. Detroit's ratings have a higher rpm/hp for pleasure use limited to 500 hours/year. Naturals don't create the high exhaust gas temps so you can run them at max forever. My current engines were run by previous owners at max and they went 6 decades before overhaul. Much of that time was as a commercial tour boat. I run them at continuous because 10 knots is fast enough for me.

Cat 3406: I know a log truck driver with a 3406 that gets a million miles between rebuilds. And that's probably tougher service than the average over the road truck.
I like Detroits for the reliability because I use them on the ocean. An injector pump and tubes are more things to go wrong. Price an injector pump for a 3406, even having one rebuilt, that makes up for a lot of mileage.

DD of a number of sizes had aluminum blocks (mine sweepers) so weight can be kept down.

A modern material like Soundown does a great job of quieting.

If dry stack, a "hospital critical" rated muffler keeps it almost as quiet as a modern truck.

A 6-71 is so smooth hard engine mounts work just fine.

If I wanted a propulsion engine taking all the reciprocating parts to a machine shop that does racing , and balancing everything for the cruise RPM, it would be hard to notice the engine on.

I think you got this all correct!

FF said:

DD of a number of sizes had aluminum blocks (mine sweepers) so weight can be kept down. I HAD TWIN ALUMINUMS, THEY WERE EXCESS GENSET ENGINES.

A modern material like Soundown does a great job of quieting. THAT IS EXACTLY THE MATERIAL I DIDN’T HAVE, AND SUFFERED NOISE FATIGUE BECAUSE OF IT.

If dry stack, a "hospital critical" rated muffler keeps it almost as quiet as a modern truck. DEFINITELY FOR ANYONE CONTEMPLATING A NEW INSTALL - SPEND THE MONEY FOR SURE.

A 6-71 is so smooth hard engine mounts work just fine. 100%

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Yep, you can hard mount a 671 and it is still social.

No point in doing the race-car guys balancing thing, though. The two stroke six is not inherently balanced like the four stroke six. All six crank throws are 60deg apart and don't exactly cancel out. That's why the cam and balance shaft have counterweights on both the gear and free end. But even with those, there is a little side-to-side "waddle", a first order vibe. Not obnoxious but you can feel it when engine is at say 1200rpm.

Want to feel smooth? 12V-71!!! Twelve hits per rev! A bit off as the 12 hits are not evenly spaced, but with that many, who cares? A similar sized Cat C18 makes more power and has 3 hits per rev!

I think the 8v71 and 12v71 are in complete balance, read that somewhere, but not 100% on that. The in line 671 is not in complete balance. Pretty dang good, better than a four stroke, but still has a first order wiggle.

Given the hours people are quoting for Detroits in various forms of service and loads, the longevity doesn't seem legendary to me. It seems normal, compared to most other engines run in similar service. So I wouldn't buy based on that. Cheap parts - yes. Easy to work on - yes.

Long before the environmental issues sunk the 2 stroke DDs, they were being replaced in the high hour industrial markets due to excessive fuel burn and weight. Doesn't mean you shouldn't have one in your low use boat where replacement is generally not justifiable.

Two years ago I was privy to the professional analysis of replacing a pair of 16V 92s with a modern four strokes. The fuel savings combined with the great new engine warranty needed about 2,000 years of use to come close to making sense. The DDs were totally rebuilt instead and work great.

But, buying a boat with DDs requires some careful thought IMHO. Especially if there are some decent 4 stroke alternatives.

"Cheap parts - yes. Easy to work on - yes."


For many folks that suffer with engines with unobtanium or insanely expensive parts the fact that DD parts should be available ,cheaply, worldwide, for another 50 years is a draw.