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Old 03-05-2020, 02:17 PM   #1
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Detroit powerplant opinion

Looking at a boat with Detroit 6-71 - T1 engines:
If we proceed I will secure an expert to review the powerplant. In the mean time I would like to hear comments about these tough old powerplants.
Since opinions are like a________s, this should be fun. Thanks in advance!
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Old 03-05-2020, 02:38 PM   #2
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Yes, opinions are like a__h__'s, everybody's got one. Especially on 2-cycle Detroits. I had 8v71n's, the aspirated, low horsepower model. The are loud and leak oil, but last a long time. But that's not necessarily the case of the TI engines you're looking at, turbo'd with an intercooler which create high output and correspondingly less service life. If your engines have been run hard, probably shorter life. But if they've had reasonable use, they might just last you for a while. Your problem is you don't know their history, so it's something of a crapshoot.
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Old 03-05-2020, 02:54 PM   #3
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How much hp? And then how many gallons of fuel have run through them? Total hours may not be a good indicator of remaining life: lower hrs but run on the pins is not as good as higher hrs and run at hull speed.

Everyone says they have to leak, but if you take care to seal up the covers on the sides they can be pretty good. Get a survey and do a compression check if you really are concerned, but that will add to the expense of the survey.

Try this:

https://www.samsmarine.com/forums/sh...pression-check.


for a quick health check on Detroits.
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Old 03-05-2020, 03:23 PM   #4
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My opinion of Detroit marine diesels is formed by my experience running them. They are noisy, messy and less fuel-efficient than four-stroke engines, especially those with electronic fuel management. On the other hand, DDs are reliable to the point of being nearly bulletproof, easy to service, will burn nearly anything combustible, and once started, will keep on running for as long as you supply fuel, air and lubricant.

For some reason, I've always loved the distinctive high-tempo growl of a Detroit's exhaust note, and I don't even mind their persistent scent of incomplete combustion.

If they are in a boat that checks all the rest of your boxes, I'd say buy it and budget for eventual engine replacements, but in the meantime run 'em 'til they quit.

On the subject of running on anything, my former employer operated a fleet of party fishing boats that ran nothing but Detroits. On the way in from a trip, the mate reported that the fuel tanks were nearly dry - they might quit before the boat (loaded with passengers and their freshly-caught fish) reached the dock. Over the radio, the vessel's owner instructed the mate to empty the galley's deep-fat fryer into the fuel tank. No harm done, and trip successfully completed. Try that with your modern, fancy-schmancy, electronically controlled diesel.
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Old 03-05-2020, 03:38 PM   #5
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We can not form an opinion lacking one very important fact. What kind of boat and what kind of use have they had ?

A sport fisher with a charter name on the back will have lots of high RPM hours. A low speed trawler, not so much.

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Old 03-05-2020, 04:03 PM   #6
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Remember: you need a constant supply of electricity to keep a new electronic diesel running, but a Detroit only needs electricity to shut off......
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Old 03-05-2020, 04:33 PM   #7
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Not always. My Jimmies were shut off simply by pulling the throttle fully back (after lifting a small safety lever).

They needed electric to start of course, but a spring started could have solved that also.
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Old 03-05-2020, 07:32 PM   #8
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Parts, parts, parts!

Not to mention they can be easily overhauled in the frame.

I like my twin Lehman SP275s, but the DDs are beasts and ubiquitous!
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Old 03-05-2020, 08:15 PM   #9
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I saw a 54’ Boat with 400hp dd 671’s in it. I have nothing good to say about that package and yet, if operated correctly there shouldn’t be anything wrong. So like asked several times already, what is the boat, what is the HP and how was it operated.
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Old 03-05-2020, 08:30 PM   #10
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Turbo 71 series will go 3000 to 7000 hours on average between rebuild depending on how they were run and maintained. If you baby them like I do, clean oil and cruise at 80% of hp, then 10,000+ hours between overhauls. In a sport fisher, if you can run at 25 knots instead of 30, you should expect the higher number. If you run flat out for long hours then 1-3000 hours.

A good bypass filter in addition to the stock oil filters will ad many hours.
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Old 03-05-2020, 08:36 PM   #11
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We has DDs in a previous boat. Other than the fact that they leak oil they are a good solid engine. Parts are available and are relatively cheap compared to current engines. Also any diesel mechanic can work on them. Just buy a lot of diapers...
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Old 03-05-2020, 08:57 PM   #12
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DDs are reliable to the point of being nearly bulletproof, easy to service, will burn nearly anything combustible, and once started, will keep on running for as long as you supply fuel, air and lubricant.
I think pretty much all the mainstream diesels can be extremely reliable if given clean fuel and proper maintenance. However I've often thought that if you were powering or building a boat for bluewater cruising to remote locations, and didn't have backup propulsion, then a Jimmy natural would be the most reliable and trustworthy engine that you could literally bet your life on. Of course if you were building new, would you really want to install a rebuilt engine that was a minimum 40 years old?
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Old 03-05-2020, 10:22 PM   #13
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Old 03-05-2020, 10:53 PM   #14
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Originally Posted by Lepke View Post
Turbo 71 series will go 3000 to 7000 hours on average between rebuild depending on how they were run and maintained. If you baby them like I do, clean oil and cruise at 80% of hp, then 10,000+ hours between overhauls. In a sport fisher, if you can run at 25 knots instead of 30, you should expect the higher number. If you run flat out for long hours then 1-3000 hours.

A good bypass filter in addition to the stock oil filters will ad many hours.
I somewhat disagree here. A DD run at 80% is a 3000 hour MAX engine....potentially less. A turbo DD is a 2000 hour engine and anything after that is bonus. BUT!!!!!......any 671 run easily could be an engine that will outlast all of us.
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Old 03-06-2020, 12:15 AM   #15
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I had a yard that took care of a number of party boats with Detroits. From naturals to twin turbo 12v71s. The operators that took care of their turbo engines were routinely rebuilt somewhere after 5000 hours. Myself, I've got 11,000. The hot dog operators got as little as 1000 hours.

I've used bypass filters and now a centrifuge for about 60 years covering many engines. Keeping the oil clean adds thousands of hours.
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Old 03-06-2020, 07:52 AM   #16
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One of the delights of DD is by selecting the Engine cylinder count 2-71 on up the engine power and therefore service life can be selected.

20HP per cylinder is a forever engine 30 per cylinder is close..

IF the boat was built as a displacement cruiser the engine heavier weight is a non issue.

A short service life comes from a turbo engine working 100% most of its life, as on some J&T hot rod sport fish.

Most oil leaks come from indifferent service , cheap unskilled labor & poor sealants.
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Old 03-06-2020, 08:08 AM   #17
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I somewhat disagree here. A DD run at 80% is a 3000 hour MAX engine....potentially less. A turbo DD is a 2000 hour engine and anything after that is bonus. BUT!!!!!......any 671 run easily could be an engine that will outlast all of us.
This is purely and simply not true. And I speak as an owner of a pair of turbo 8v92's and someone who knows a lot old DD owners (being an old Hatteras owner and aficionado) along with three top grade DD mechanics.

There is more dockside legend about these engines than just about anything, save anchors, maybe.

I should also add that they only leak if you let them.
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Old 03-06-2020, 09:34 AM   #18
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This is purely and simply not true. And I speak as an owner of a pair of turbo 8v92's and someone who knows a lot old DD owners (being an old Hatteras owner and aficionado) along with three top grade DD mechanics.

There is more dockside legend about these engines than just about anything, save anchors, maybe.

I should also add that they only leak if you let them.
Do you run yours at 80%? No need to answer because I know you don't. I know DD people and mechanics as well. I'll be the first to say a DD will last forever in an application like your boat and the way you use it. But if you ran that thing up on plane in the upper teens all the time, then I would say it is a 2-3000 hour engine. I am making assumptions of load and the weight of your boat...but I am probably close. Run it at 9-10 knots and rust will probably kill it faster than internal combustion. A buddy of mine just rebuilt his 8v92s....Hatteras 47 SF....2200 hours. He bought the boat with "high" hours and budgeted rebuilts into the price. Very nice boat....but "high" time engines. He got it cheap.
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Old 03-06-2020, 10:52 AM   #19
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I just turned 5300hrs on my 12v71’s with 650 hp. Last compression test at 4700 hrs had all 24 cyls above factory minimum. That’s 50+ hp per cylinder at WOT, but I rarely run them long above 1250rpm.
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Old 03-06-2020, 10:58 AM   #20
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OP has not come back with details, so not much to add here.

Basically on a TI, TIB, TA or TAB detroit with high rating, if you run it at high power setting (like planing speed on heavy boat), they don't last that long. 1500-3000hrs is typical.

But it is not the whole engine that needs to be rebuilt, the weak spot is the piston rings above the intake ports. Just hard to get good lube in there without excessive oil consumption. And the oil that does get there does not like the heat.

Tear one down and usually everything is in excellent condition except the top rings and liner bore.

Run any detroit (NA, T, TA, TAB, TI, TIB, does not matter) at modest power setting and they will run dang near forever.

I'd just throw out an opinion: Cruise any 671 at 150hp or below and you are in the "forever" category. Run any at 350hp up and you are not. Grey area in between.

Other DD's similar if you apply a proportion to hp and displacement.
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