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Recently, I read someone's advice about avoiding a boat with maxed out Detroits (485 hp) 6-71tib. The boat I,m looking at & considering has these very engines. What is considered maxed out? What financial impact could I be facing? I like the looks of this boat and would seriously consider it, but that comment makes me a little nervous.
I guess it all depends on many hrs the engines have on them since the last rebuild. 485hp is a lot to pull out of a 465ci engine. If they've got over 1500 hrs you could be looking at rebuilding these with in a couple of seasons. Try using it as a bargaining chip.
A 6-71 is 426 cubic inches. A 6-92, displaces 552. The "71", "92", "53", etc. designation in a two cycle Detroit Diesel is the cubic inch displacement per cylinder.
Multiply that by the number of cylinders (3,4,6,8, 12, etc.) to get the mill's total displacement.
More then likely, from Pascoe's site. I noticed that mistake in one of his diesel articles.
I've spent 11 years working on DD-powered motor coaches (buses). I wrenched on inline 6-71s (older GMC coaches) 6V-71s (GMC "city" coaches),
8V71s (GMC transit coaches, Flxible city and transit coaches), and 6V-92Ts (Grumman-Flxible and MCI coaches).
So, it's safe to say that I've earned my "Time in Hell" when it comes to mechanically injected (non-BDEC and DDEC) Detroit Diesels.
BTW, another useless bit o' trivia: 53 and 71 series Detroits have "dry" cylinder liners, 92 series have "wet" liners. Basically, a 92 series mill has it's cylinder liners in direct contact with the engine's coolant, whereas 53's and 71's have their liners housed in a bore, with coolant circulating around the bore's water jacket.
[This message was edited by Fubar512 on 10-01-03 at 06:08 PM.]
FUBAR- do you know anything about the Detroit series 60s. I have a 1998 MCI Motorcoach with a DDEC series 60.I sent it over to Featherlite because it was smoking a bit, their diagnosis was One cylinder has bad blow bye caused by a faulty ring.The recommendation was a new detriot Reliabilt Plus long block for $15k. Isn't it possible to go in thru the oil pan, drop the crankshaft and just do the one kit?
Guys I have a set of those 625hp 6v92tabs in my Tiara. I have heard the stories about the high performance Detroits, but I must admit these things have proven to be strong and reliable. Mine went 1500 hours before the first rebuild (about $40k)
Tiara, I don't have any experience with DDEC DDs, but I still have connections with my former employer. I'll run your question by him.
One thing though, AFAIK, you'll have to pull the head on that side as the liners have a lip on top, so they go down through the fire deck of the block.
Check out Boatdiesel.com. If you don't have much experience with diesels, join it. I tis money well spent. Someone mentioned getting 1500 hrs before a rebuild on 6-71's 485 hp. 1500 hrs. is what you should expect from a gas engine which you can likely replace with new for less than $10000 and rebuild for much less. Diesels should and do run much longer with proper maintenance. Most of the 6-71's at 485 hp get to 2000 before needing rebuilds. To get more you'll need to be very meticulous with maintenance according to what I've read about them. That ain't much time before you have to spend 40-50K to rebuild engines IMHO. Check out the boat diesel site..you'll be glad you did.
Maximum rated HP does not mean shorter engine life unless you use the high HP. If the engine is normally run at 2/3 or 3/4 rpm it should last a long time. Most owners don't cruise at anything near WOT. The fuel consumption is so high at max speed few people can afford to run at full throttle. Try to find out how the boat was run.
I think 1500 hrs out of a high hp diesel is probably very close to the truth. Look at Tiara. He only got 1500 hrs out of his 625hp 6-92s', a high hp diesel. My neighbor has a pair of the 485hp 6-71's in his 36 Blackfin. They've already been rebuilt twice and I'll bet there's no more than 3000 hrs on the boat. Both of these engines have hp/cu ratios of 1.13:1. Thats a lot.
It is true that most diesels should get 2000 hrs or more between rebuilds, but there are some that never will. I think the 485hp 6-71's and any 6-92 with more than 550hp are a couple of them.
Klignz, true, most people probably will never see wide open throttle (WOT).
However, to achieve higher output, diesel marinizers resort to proven hp producing techniques: more air and fuel in = more hp. They use larger turbos that increase cylinder pressure and temp (even with after & intercoolers), and a richer fuel curve. That often means larger injector cups (potentially, more unburned fuel washing down the cylinder walls at idle and cruising speeds). Now, electronic controls keep a better handle on fuel delivery, but you can only lean out the fuel delivery curve so much...
Also, boat manufacturers try to run higher output/smaller displacement diesels to achieve better performance and milage.
Years ago, if you wanted 500 hp, you got it by going to something like an 800 cubic inch mill...lots of weight, and a tight fit in even a 45 foot hull. So now, they achieve that same hp level with an engine that's 50% lighter and 50% smaller in physical size and displacement.
So, if operated at the same load that the larger displacement mill handled for 10k hours, the new "light weight" mills will wear out faster.
Why did boat manufacturers go this route? Because they realize that most boat owners don't operate their vessels more then 100-200 hours per year. At that rate, you're talking 7-10 years between rebuilds.
Besides, their reasoning is that if you have the scratch to buy a half million dollar yacht in the first place, you can the afford $40-50K to rebuild two engines 7 to 10 years down the road!
Doug, That was my point. Diesels should get far more than 1500-2000 hrs. before needing to be rebuilt. The high hp 6-71's just won't get that. Their life expectancy is no higher than a gasoline engine. While it is true you can get more life from them if you run them easy, why have high hp if you're not going to be able to use it? I wouldn't by a boat with high hp 6-71's approaching 1500 hrs or more unless the price were adjusted to reflect a repower or a rebuild. Even then i don't think I would rebuild them, rather I'd think about repowering with something witha proven track record of performance and longevity. The high hp 6-71's just don't have that, except a some eceptional cases.
Dan
On your Detroit dilemna, I'd have an oil analysis and survey the motors. Bargain as if you were going to rebuild them and allow 15 or 20 K in the price.
If you get the boat, run 'em till they need a rebuild, they're not likely to blow up, only smoke, blow more oil (if that's possible with a Detroit) and lose power. Find a reputable independant and use DD parts. Rebuild can be done in a few days. Buy Airseps.
Its easy to knock the old 2 cycle Detroit 6v71s but how can you question their durability. The
US Navy has been using them since the 40s. We still have some boats that were used in World War 2, now running harbor patrols on the original 6v71s. Granted they aren't high horsepower, but 60 years of service is hard to question.
I think the one post hit it on the head, if you can afford 500-800k for the boat you are not worrying about a 1500-2000 hour rebuilt, hell the guy that comes every other week to service them will take care of it and all you see is an end of the year invoice written off as a business expense.
You have gas engine pocket books analyzing big dollar toys....
Nothing beats the sound of a good old American Detroit firing up!
Tiara, It is true the 6-71 has a history of being a great engine in terms of legevity and rekiabilty, just not at 485 hp. The 6-71's the Navy used were likley the 6-71 Naturals at 210-250 hp. The Navy didn't use the new 6-71's at 485 hp to my knowledge. There's a big difference in getting 210 hp from a 426 ci block and squeezing 485 hp from the same block...cololing system for 485 is marginal and withoput scrupulous maintenance and/or propping light or use at lower rpm's so you effectively derate the 485 hp version to a lower hp thier life expectancy just won't compare to the lower hp versions of the 6-71.
I had 671 tib's j+T 485 hp in my 43 egg..Great motors, i really had no trouble with them...They did smoke a bit.....When i sold the boat, it prob had around 1000 hrs..the mechanic checking them out had said they had some wear on them....But in all honesty, i loved them....never gave me trouble..But i do agree, they stretched the hp enough......I personally like big block diesels at lower rpm's these days....Im not a big fan of these jap diesels putting out high rpm's....
Detroit 6-71TIB 
