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Old 03-15-2020, 01:28 AM   #41
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Isn’t max continuous for a 6-71N just about 80% also?
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.
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Old 03-15-2020, 02:48 PM   #42
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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.
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Old 03-15-2020, 04:07 PM   #43
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I think you got this all correct!

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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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Old 03-15-2020, 08:34 PM   #44
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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.
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Old 03-16-2020, 06:47 AM   #45
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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.
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Old 03-16-2020, 10:39 AM   #46
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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.
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Old 03-17-2020, 07:07 AM   #47
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"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.
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