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De-tuning a Detroit Diesel 6-71 TIB

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De-tuning a Detroit Diesel 6-71 TIB

Old 03-16-2008, 11:17 AM
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Default De-tuning a Detroit Diesel 6-71 TIB

I am interested in a few boats but all have 485HP 6-71 TIBs.Due to increased fuel costs and reliability issues I'd prefer a 350HP or so version.How hard will it be to do and what/where should I look to spend.
I worked on gas turbines in the USN,built racing motorcycles and was an electrical engineer til I became self-employed so I'm not afraid of parts scattered about and precise directions.TIA.
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Old 03-16-2008, 12:22 PM
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Default RE: De-tuning a Detroit Diesel 6-71 TIB

The quickest and easiest method is to not run full throttle. Remember than an engine only produces what you demand of it. You may need that extra HP at some point when heavily loaded, so just underprop it slightly and run it easier.
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Old 03-16-2008, 03:03 PM
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Default RE: De-tuning a Detroit Diesel 6-71 TIB

thefermanator,I had given that some thought but it really doesn't work well with marine diesels.They are designed to work best and deliver best GPH usage in a wide but defineable range.Planing hulls still require an engine to be in that range,in a displacement speed trawler it works to a degree.
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Old 03-16-2008, 03:13 PM
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Default RE: De-tuning a Detroit Diesel 6-71 TIB

Fastest way to detune it would be to let me work on it.



Seriously, you can compare the builds between the motors and generate a list of things to change
like injectors, maybe heads ect. I would think 350 hp from a 6-71 is still alot from these motors.
The ones that lasted a long time were about 165 hp.

If it were me I would run them easy as long as I could and leave them alone.




Steve
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Old 03-16-2008, 03:40 PM
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Default Re: De-tuning a Detroit Diesel 6-71 TIB

I had a pair of the 485 TIB's in a 41 Viking. I wouldn't mess with these engines if they are running fine when you get the boat.
They are very fuel efficient for there size and horse power. I could run 27 knots at 2200 rpm's all day. I was only burning about 28 gallons per hour combined. I would normally run them at 1900 and they where hardly working.
I would think if you had a REALLY good Detroit mechanic look at the engines and you do all of the required maintenance they would treat you very well. Mine always did..
Good luck with what ever you decide..



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Old 03-16-2008, 03:41 PM
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Default RE: De-tuning a Detroit Diesel 6-71 TIB

froggyfisherman - 3/16/2008 11:17 AM

I am interested ina few boats but all have 485HP 6-71 TIBs.Due to increased fuel costsand reliability issues I'd prefer a 350HP or so version.How hard willit be to do and what/where should I look to spend.
I worked on gasturbines in the USN,built racing motorcycles and was an electricalengineer til I became self-employed so I'm not afraid of partsscattered about and precise directions.TIA.
Disconnect the TIB (bypass apparatus) and just underprop the boat. Diesels are driven by load demand, so if the prop doesn't ask for the power, the engine won't give it.
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Old 03-16-2008, 03:48 PM
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Default Re: De-tuning a Detroit Diesel 6-71 TIB

Like said above, underproping the boat will basically de-rate the motors without having to do anything else to the motors.

They have mechanical govenors on them and when you hit your RPM's the motor will stop making power, easy, simple and cheap to do. You can always go back to 485 with a prop swap.

Just remember that one day you might sell the boat and the next guy might want 485's in his boat.

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Old 03-16-2008, 03:51 PM
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Default RE: De-tuning a Detroit Diesel 6-71 TIB

Thanks jenkinsph,
I need at least 250each to plane the boat I'd thing and probly 300 would be better.I work on a boat with a 400hp 12.5 litre John Deere that has almost 8000 hours.That's a 1/2 hp per cube and the 6-71 is 426cid so I will settle for few more HP if I can get a reliable 4000hrs out of a fresh rebuild that has been detuned.I get between 500 and 1000 hrs per year total running all the boats I'm on now.I won't consider anything other than the old DDs so I have to take what I can get and make the best of it.
A 485hp 6-71 just says handgrenade to me.
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Old 03-16-2008, 06:24 PM
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Default RE: De-tuning a Detroit Diesel 6-71 TIB

Froggyfisherman, Go to boatdiesel.com and post your question there. I believe its 25 dollars to join and it is well worth it. That questions been asked there before. Good luck with the boat.
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Old 03-16-2008, 06:37 PM
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Default RE: De-tuning a Detroit Diesel 6-71 TIB

froggyfisherman - 3/16/2008 4:51 PM

Thanks jenkinsph,
I need at least 250each to plane the boat I'd thing and probly 300 would be better.I work on a boat with a 400hp 12.5 litre John Deere that has almost 8000 hours.That's a 1/2 hp per cube and the 6-71 is 426cid so I will settle for few more HP if I can get a reliable 4000hrs out of a fresh rebuild that has been detuned.I get between 500 and 1000 hrs per year total running all the boats I'm on now.I won't consider anything other than the old DDs so I have to take what I can get and make the best of it.
A 485hp 6-71 just says handgrenade to me.
Don't fall for the 671 BS thats out there.
They have made those engines for a long time. They don't blow up like everyone wants you to believe..
I have fished 100 miles offshore for the the past 15 years and have been on plenty of boats with those engines and they have NEVER least us stranded......

Clean fuel, clean oil and they will run along time....
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Old 03-16-2008, 06:55 PM
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Default RE: De-tuning a Detroit Diesel 6-71 TIB

Moby1 - 3/16/2008 6:24 PM

Froggyfisherman, Go to boatdiesel.com and post your question there. I believe its 25 dollars to join and it is well worth it. That questions been asked there before. Good luck with the boat.
This is good advice.

Also, may I suggest again, that you take the time and thoroughly understand the effect of underpropping a high HP engine for increased reliability, BEFORE you start switching parts out.

It will save you a lot of money and aggravation.

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Old 03-17-2008, 06:20 AM
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Default Re: De-tuning a Detroit Diesel 6-71 TIB

In the old days with 15 cent fuel we used the 71 series for everything from lightplants to main engines for 23 boats. This was
a mixture of small motorized barges to tugs, the more power you try to get out of them the shorter the life between overhauls.
Today I would be more concerned about getting the most efficient setup you can, the fuel will cost you much more than the
overhauls over time. I would spend the money for flowscan equipment to determine the best speed to run to save fuel.



Steve
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Old 03-17-2008, 08:40 PM
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Default Re: De-tuning a Detroit Diesel 6-71 TIB

The engine that won ww2. My brother runs one in a 42ft. lobster boat. Slow as can be he only runs about 8nmph but he put the engine in used and ran it for 12 yrs. before overhaul and he plans on running it till retirement now. His is an old 165hp.
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Old 03-20-2008, 06:53 PM
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Default RE: De-tuning a Detroit Diesel 6-71 TIB

read this
http://www.yachtsurvey.com/comparing_diesel_types.htm

It won't tell you how to do it, but will give you lots of info about the old Jimmy now DD.

The engine was develpoed around 1937 to go in landing craft LSTs etc.
the war dept wanted a diesel engine that not explode if it took a round like a bosch injection pump and hi pressure lines.
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Old 09-16-2014, 06:31 PM
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Yes....I replaced an old GM Diesel (Two valve head, rounded valve cover) with a four valve 671N.

Running at 1800 rpm a 71 series will outlive you and me. It will leak oil. But that 2 stroke sound is sweet and unmistakable. The blocks are all the same. At least they are the same enough that you can make a 671N, a 671TI, 671TIB or a TAB - all you are changing is injectors, and the coolers.

As said before, longevity is directly proportional to the RPM. 1271's in the oil patch used to regularly spool up to 2400 and run that way for long runs.... Then you hear of the shrimper that ran his 671N for weeks on end and lasted 20 years.

I would guestimate that an 1800RPM 71 series will hit between 12K & 15K hours before major. I would cut the TAB in half. Just guessing though. I know that my 671N used 6GPH at 1800. Almost like clockwork, 1 gallon per jug per hour.
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Old 09-16-2014, 06:57 PM
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Like many others said , under prop them and easy on the throttle . They will produce only as much HP as you will let them .


AND it works real well on marine diesels .
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Old 09-17-2014, 05:01 AM
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A 485hp 6-71 just says handgrenade to me.
And how old are the actual engines in question? 30 years? 40?

Yes that is too much horsepower for that block. As others have said, so don't run them that hard. The whole business about loading them at a certain level is dockside urban legend. You can run them slow for thousands of hours, every now and then, say at the end of the day, run them up to say 80% for 15 or 20 minutes. It is not a simple or cheap proposition to de-rate them much beyond changing injectors and underpropping a little; find a really good Detroit 2 stroke guy (they are still out there) and have him go through and survey the engines thoroughly and give you some advice . I too recommend BoatDiesel for your internet questions.
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Old 09-17-2014, 05:12 AM
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Guys, cap'n dean through you a curve ball by bumping up a 6 year old thread...
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Old 09-17-2014, 07:03 AM
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Detune at the throttle hand interface. Any fuel savings you MAY get by changing parts will take loooots of hours to pay for the changes. Lower flow injectors COULD help but only a very qualified tech or DD engineer can answer all the questions that arise.
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