Repowering 1977 42' Conv.

[traderron]

I am re-powering a 1977 42' Conv. with a used pair of 450HP 671-TI's. Will probably have to buy new trans. to complete installation as the 671-TI's presently have straight shaft without offset or angle. What model transmissions has anyone had experience with in a similar installation? I plan to use 1.5-1 ratio to avoid up-sizing my 1.75 " shafts. Also what prop size did you use? Appreciate all help.

[big1]

Why on earth would you consider putting an inefficient relic like a 6-71 of any horsepower configuration in your boat and consider it a positive move? They are a thing of the past,and at 450hp,a time bomb that isn't worth the effort.Big-1

[jkp1]

I would not be so quick to condemn the DD 6-71. While it is yesterdays 2 cycle technology, There are a lot of them out there and will be widely used for years to come. I do agree however that when cranked up to 450 hp, service life will be reduced. Most will need work by 1500 hours and 2200 is the longest lived 450 I've heard of.

Assuming you acquired a pair of good used engines at a good price, they should work well for you. You might want to consider having them de-tuned to 410 hp to increase service life. I'm told that de-tuning is just a matter of changing injectors. You probably will not even notice much drop in performance between 450-410 hp.

My 42' has the 410's with 1.75" shafts and 24" props.

[hATTISFACTION]

the comment on 671TI's being unreliable is simply not true.If run properly these engines will easily give 3,000-5,000 hours of service. There is a 42 Hatteras running now out of Gloucester ,Mass which has OVER 5,000 hours on the engines and the boat is used regularly to fish giant tuna. I spoke with an owner who has 671TIB's at 485 hp in his boat (not a Hatteras and not an engine I would choose) and they have over 3,000 on those engines. JUST DON'T RUN THEM WIDE OPEN.Parts for the 671TI's are readily available and reasonable inexpensive.
Check out Dale Pascoe's comments on his website on 671's.

[Genesis]

Not at 485 or 450 they won't make 3,000 hours. 1,000 is more realistic.

Detune them into the high 300s and they have reasonable service life. 320-350 is even better. Of course you won't like the boat's performance at those levels.

This is a 426cid motor folks. Pushing it beyond 0.9hp/cid is unwise. Its a great little engine.

[hattisfaction]

this is not a matter of whether they will or not,,as they already have exceeded 3,000 in many cases when run and maintained properly..THIS IS NOT SPECULATION IT IS FACT.

[jkp1]

I checked out Pascoe's website as suggested in a previous post, he seemed to be supporting the Genesis position. The following is a direct cut & paste from Pascoe:

Internal displacement is the best measure of an engine's ability to deliver power efficiently. And the ratio of CID to horse power (divide CID by engine HP is the simplest measure of how much service life can be expected. The inviolable rule for service life is that the more power is squeezed from an engine block, the shorter it's life span. A 350 CID block generating 260 HP is going to last a whole lot longer than the same block putting out 350 HP, whether its gas or diesel. That's why the old 6-71 Detroit Diesel will run darn near forever at 265 HP from 465 CID, but self-destruct in 6-800 hours at 450 HP. A ratio of 1:2 is about ideal for a marine engine, but at 80% to 90% at least yield reasonable service life. At 1:1 and above it should be considered a high performance engine with a very short service life indeed.

Full text can be found at:
www.yachtsurvey.com/GasDiesel.htm (http://www.yachtsurvey.com/GasDiesel.htm)

[Genesis]

Ok, this is probably more information than most people want, but what the heck :)

First, you can "detune" any diesel by simply underpropping it. Since diesels are governed, you cannot overspeed them (so long as the governor is working properly.)

Therefore, a perfectly acceptable way to "back off" from 450HP to 350HP, for example, is to simply prop for 350HP. The engines will spin easily to full WOT RPM, and you will like what that does for their service life.

You probably WON'T like what it does, however, to your boat's performance!

Now why is this ok? A big part of it is how diesels actually work. In an ideal world we would want the injection event (fuel going into the cylinder) to happen instantaneously. Of course we can't manage to do that, because you have a finite quantity of fuel to inject, and that takes time. However, the SHORTER the interval the better, all things being equal.

Well, unfortunately, fuel is not compressable. Therefore, for a given size plunger in an injector (whether on the head or in an injector pump) and a given rate of motion of that plunger, a certain amount of fuel will be displaced per unit of time. Thus, the more fuel you want to inject, the more time you need to do so.

Fuel does not ignite instantly; there is a short delay. You want the peak combustion chamber pressure to occur just after the piston starts down on the power stroke. If it happens later than that, a LOT of power is wasted and goes out the exhaust; this also contributes to overheating problems and turbo meltdowns since the manifolds and cylinder heads are part of the cooling circuit, and still-burning fuel ends up in your turbo on the exhaust side. If the peak pressure happens too EARLY, then the maximum pressure in the combustion chamber can exceed design limits and you can blow head gaskets or, in the extreme, bend or even break rods or fracture pistons, as the burning of the fuel actually pushes "back" on the rising piston.

What this means is that for all power settings of significance you are wasting some fuel, as the injection takes longer than you'd like and so does the burning of the fuel. From about halfway down on the piston's stroke you are producing essentially no power at all, and ideally the fuel has finished burning by then.

Detroits of a given design (e.g. 6-71, 6v92, etc) all have the same SIZE plungers. They have to, since the injector tube size in the head is a fixed quantity. The amount of fuel they inject is determined by the helix that is attached to the rack on the side of the injector; as the plunger is driven down by the rocker arm a bypass port is covered; this causes the needle to unseat at the tip and fuel to flow, as the fuel has nowhere else to go. The end of injection occurs when a second bypass port is uncovered as the plunger travels downward. The location of and distance between those two points, and thus the exact timing and duration of the injection event, is determined by the rack position and the height of the injector (which is set during tune-up.)

What this means is that the LOWER power engines in a given series actually have MORE ideal injection timing, as the injection happens "faster" (less time elapses between the start and end of injection.) But you can "emulate" this by simply depropping the boat, so that the output demand is less - when you do this the rack will be pulled back out, since the engine will require less fuel to turn up the set RPM at the governor.

Do your engines a favor, and if they're set up for "high performance" levels prop to EXCEED rated RPM by 50 or so if you're aggressive, and if you want to be conservative and derate the motors, prop for that 50 over and then take out ANOTHER inch or so of pitch.

That will nicely de-power the engines and greatly extend their life, if you can live with the performance that results.

[tony 275]

This seems to be a lively subject.I know that there are tons of Detroit fans out there that romanticize about the sound of a Detroit Diesel,and their legendary durability.They were durable in their day,but in their greed to make as much money with an existing product line as they could,Detroit,and their associated marinizers shot themselves in the foot.As a guy that has repaired these and every other popular diesel engine made,and been to most of their schools,I will tell you what I have learned.Detroit 6-71's V or inline @ 450 to 485 horsepower do achieve 3000hrs,at modest use in their origonal,"factory" condition.I have never seen one go more than around 1500hrs, of normal use,"not trolling for giants,or idling" after an in the boat major.Why?As you dis-assemble one of these high horsepower engines you will understand.As the liners are pulled you will notice areas of blue on the outer circumferance of the liner.These areas indicate poor heat transfer with the engine block,which has deformed and the counterbores,that the liner fits into are no longer round,the compression rings lose their temper in that area,due to overheating, and the engine loses compression and power.The engine might not use a lot of motor oil,because the oil control rings in a Detroit are much lower,and not exposed to this heat.Now someone comes in and rebuilds the engines in the boat,sounds reasonable,but in 1000hrs'they start to smoke,you play the old injector replacement game,use block heaters to reduce the smoke,but the problem doesn't go away.The engine is on the way out,if the blocks had been removed,and the counterbores bored out and fitted with oversized liners,the engines stand a much better chance of reaching the origonal hours,on the overhausls.Detroit has always had this problem with their engine blocks exposed to high horsepower,and now I am seeing it in the 60,series as well.The duty cycle of the high horsepower,2-cycle Detroits is very low when compared to more modern engines.Run'em easy and take care of them maybe,"if they were properly overhauled" they will take care of you.Hope this is helpful. Tony

[jkp1]

For what it's worth, I have some benchmark data of my inline 6-71TI. (410 hp) I have 1400 original hours on it and compression on all cylinders is 465 ( /- 5). New rebuilds come in around 500 psi and I am told a rebuild is due when compression drops to 430. Assuming compression loss occurs at a linear rate (?) AND I remain diligent on oil changes & cooling system maintenance, these engines should be good for 3500 hours or so.

Genesis or Tony, are my assumptions reasonable or am I off base?

[Genesis]

I and I bet you run a "real" power output around 200HP.

The problem comes when you set up for 450 or 485 and run 200 RPM off the top (or at 2300), or worse, when you can't reach 2500 and run at 2300 anyway.

[jim rosenthal]

Having been through one repower, and not wanting to do it again, I think to go through all that trouble to put in an obsolete overstressed engine is a mistake. You would be far better off with something like a 3176 CAT, which IS available on the rebuild market at 600 hp. Since you have to make up new engine bearers etc better to do it for an engine worth putting in. Propped liberally the 3176 will run for a long time, it weighs less than the 6-71, it will use less fuel, etc etc etc. Or some comparable engine, but 6-71s are not by any means your best choice. They may be the worst.

[captddis]

I guess I will weigh in with my thoughts. I agree there is nothing like an out of boat rebuild, However there is no reason you can't get a quality in frame overhaul. There are two critical things to pay attention to first on 53 and 71 series it is very important to get proper liner fit . this involves hours of trying liners and honing and fitting again until a proper liner to block fit is achieved. 92s have wet liners and fitting is usually not a problem. The next is to properly clean the air boxes, which involves removing the blower and airbox covers and clean until the inside is clean. This step is often left out and is one of the reasons for premature ring wear. This is why a lot of overhauls only go 1000 hrs.
The 671s are good for a budget repower and give reliable performance with reasonable use and care. They also do not have any of the issues the 6 cyl. 3126, 3176, 3196 timebombs have. They are also easily maintained by a do it yourself owner. The computer controlled engines are not.

[DLCameron2]

Check out the January issue of Motorboating, page 94. This is the story of repowering a 1977 42C which had Cummins 903's. Cost was $100K for Yanmar 440's and many additional upgrades such as shaft logs, freshwater pump, etc. Draw your own conclusions....looks like a nice effort to me.

[jim rosenthal]

The 3126, 3176 and 3196 are not, as far as I know, computer controlled engines. They are also not available any more, I think; they have been replaced by computer controlled engines and I would agree with you that the more complex an engine is, the more potential to go wrong. I have had good service from my 3116 CATs; not perfect, but perfectly acceptable. The gentleman above is looking for a budget repower of a 42C, which is why I suggested the 3176 or 3196- they have been around a while, they are mechanical engines, and they are available as guaranteed factory rebuilds from Alban Engine here in MD. At a substantially lower price. If propped prudently and maintained well, they should perform well in that boat and last a long time.
I feel that the best predictor of engine life is how much hp you are trying to wring out of its' cubic inches and how well it is maintained. Put another way, if the FIRST Detroit engines on the market had been turbocharged intercooled motors with 140 series injectors, they would not have the reputation for reliability which was established by unblown low-output versions of the same motor. It is somewhat unfair to blame modern engines for their lesser reliability which is in turn due to their higher specific output, which is in turn due to the expectations their builders have to meet to sell engines. We as boaters are the holders of those expectations.
It is absolutely a fact, as detailed above, that Detroit was unwilling to retool for a 4cycle engine until they lost all their market share and had to. I agree with the person above who suggested running an engine deliberately underpropped and taking it easy- it will last ages longer, and when you look at what these toys cost, that is important. I spent more to repower my Hatteras than I did to buy the boat. (leave out for a moment the cosmetics and the new InterProtect bottom...)
It is interesting to me that the muscle boat crowd- Cigarettes, Donzis, etc, pretty much accept that the cost of going very fast is huge fuel consumption- and frequent engine consumption as well. Maybe they don't mind because it is so much easier to remove and replace those motors in those boats. Even their diesels are very high output- and break a lot. I guess they feel it is normal. Me, I am willing to go slower and hope that my engines last a lot longer.

[captddis]

My point was that for a reasonable investment a person on a budget can get a good pair of GM engines that will give reliable service. The 3116, 3126, 3176and 3196 all have a bad reputation. That is why Cat dropped the 3116. And renamed the others. The new models are supposed to have the problems corrected. Some of the 3126 engines were electronic and all of the 3176 and 3196 that I have worked on were electronic. I know of people who are on their THIRD set of 3126s. I also have a customer who after the second set gave the boat back and ordered a new boat with 3208s. The 3176 renamed the 3196 were not any better . I have people who sold their boats to get rid of them!
Sea Ray had so much trouble with these engines they switched to Cummins. As I previously posted I think if someone has to scrounge for the money to repower, a tried and proven engine is the safest way to go. If money is not an issue than there is nothing like new. But carefully research what engine you are going to use. I have always believed a bigger engine run conservativly will give the best service.

[jim rosenthal]

I don't disagree with you about propping an engine conservatively and letting it lead an easier life. I think that's smart and I do exactly that...I do, however, know numerous folks who have CAT diesels of the models you are speaking about who have had good service. I would say they have had somewhat more than their share of problems- turbos, heads, soft French blocks- lots of items. Ironically, in workboat service they seem to do better, at least locally in the Chesapeake. Mine have behaved themselves, but I don't do as much boating as I would like to...
Would I buy them again? It depends on the application, but I very well might. There are a lot of 3176 and 3196s running around in workboats around this area that seem to be doing fine. I still feel it would be a good budget repower for a 42C.

[traderron]

Thanks for your info regarding service life for the 6-71's.
Could you give me the specs on your gears; maker,ratios etc.
Also some performance data would be helpful; rpm vs. speed and fuel economy.

[mike]

go to www.goldcoastpower.com they have a lot of data, including DD dimensions for the motors and trans, plus fuel consumption curves.

[BADBOYBILZER]

Been a mechanic 25 years
Detroit's and cats and Cummings
Gasturbines, Millwright,Welder,large megawatt powerplants,Oilfield,Dozers,Scrapers,Boats,Ships

Each type of engine has its strengths and weakness

a Detroit's are a leaky and temperamental beast
a little tlc and you can double and treble the discussed
life span it will guzzle fuel and smoke until the bitter end.
a 71 will last twice as long as a 92, we put the first
92 pistons in aluminum Detroit blocks for the Captians Gigs in the Navy, you do pay for performance.
a 71 with a turbo is a must the blowers lose effectiveness after a while and the turbo is the longeivity diffrence.
a Detroit that has more powerfull injectors and has the maximum RPM limited is the ideal in a marine application. i have often seen detroits that spin 1000 RPM greater on water than land, a detroit has a narrow operating band that will provide longevity.3600 and 3800 are too fast , in over the road trucks we see from 2600 to 3200 RPM and that is a major difference
for a Detroit i would not decrease a pitch unless there is a massive ocean going or upstream load thats my preference.
being that im a mechanic i would still have my governor set so that i could make 3400 to 3600 but i would use my head in operations.

that said the cathodic protection and the extra Jacob's impellers for the water to water coolers , water to water coolers are a must , never run river water or lake water to cool a Detroit, to many places for electrical potential difference .

I like boat anchor Cummings.the most durable and
reasonable to repair, few specialized tools.broad power-band. needs, cathodic protection.

new smaller motors are options like 7.3 (electrical nightmare in salt water) or even a rare bird non electrical 6.5 . both turbo charged and in the economy range.

cats are for the rich and foolish, if i had money to spare
id have a cat, barr none the best money can buy, they
last a long time without repairs,but when they need repair you are going to pay,even if you buy the specialized tools to be a do it yourself er, you have a large complex and mostly unsatisfying experience.

i finalize my statement about cats saying this,i have spent 3 days and many dollars on specialized tools to
just change injectors on a customers propane truck
in the end i had to put the pretty tools down and remember how i was taught to fix Detroits.

3116 are a cat with Detroit style helical adjusted injector height timed . all the rest of these cats are temperamental but a sharp guy can repair them, a stupid mechanic will cost you money by not listening and understanding the beast in front of him. you wont know you had a bad repair until the engine burns a piston or grenades 3 to 6 months later.

lastly these maxims
detroit, always need repair
cummins need good maintenance
cat never touch a cat, if it needs fixing, overhaul it

kinda sounds lame but i know many shops and mechanics that preach this, i dont have that luxury

[Genesis]

I have 6V92s. They've got about 750 hours on them since they were overhauled (to an unknown level of competence) before I bought the boat.

I got them with essentially zero time on the overhauls.

Have I had issues? Sure. There were things not done. All them had to be done. Most of it I knew about before I wrote the check, but not all.

Will I curse at 1000 hours? Maybe. If I do, they'll probably come out and get replaced with Cummins. Maybe. Or maybe I'll pull 'em, rebuild them in my GARAGE, send 'em to a dyno shop, and then reinstall once I KNOW they're run-in and ready to rock and roll.

I like the 92s because I can work on them easily and in my experience when they're right, they tend to stay that way. Getting them right requires competence at the outset, but its not impossible. They are known as being cantankerous engines however....

71 rebuilds fail because, as was noted, people don't check the liner/bore fit. It takes a LOT OF TIME to get it right, and if you get it wrong, the liner overheats and there goes your compression. 71s at 450 or 485HP are just too far blown up. At 380HP or less I like 'em a lot; indeed, at that power rating or below they're one of my favorite motors.

92s fail due to overheating most of the time. The liners are wet and sealed with O-rings top and bottom. Overheat the engine, even a bit, and the O-rings are compromised, resulting in oil/coolant contamination. Anything over 195F on the coolant is bad, and anything over 200F is almost guaranteed to cook a seal. Don't catch that IMMEDIATELY, you buy an engine, because usually the mains are damaged, and sometimes the crank goes with it. But they can be rebuilt easily, because there is no liner fit issue to be picky about - the only real problem there is making sure the counterbore is correct for the top of the liner's fit in the block, and that's a simple measurement to check. 500HP is the top rating I'd own these in - the 550s are ticking bombs, the 650 DDEC IIs blow up if you look at them wrong. 475HP or 450HP is even better for these.

92s with aftercoolers (rather than intercoolers) also have their aftercooler in the block, under the blower. A real ball-buster to get to for cleaning, as you have to pull the blower. Not hard, but a lot of time and work, and a mess besides, as the blower leaks oil all over the place when you disconnect and remove it, and of course you have to drain the cooling system to get to it all.

ALL common Detroit configs in boats have the airbox drains plumbed back to the crankcase. The first thing a smart owner does is remove this, and remote the check valves so he can get to them, running the output to a crap can instead of back to the pan. A stuck open check valve will trash your oil quality, a stuck closed one will trash your cylinders due to the crap in the airbox being inhaled. Those valves are critical and need checked when you do oil changes - no exceptions.

The next big problem is the Walker Airseps. They have vacuum limiters on the breather connections. Those like to stick, and if they do they will pull over gobs of oil which will foul your turbos and aftercooler, and oh, by the way, they return to the crankcase too (another bad decision.) Those nice limiters are non-servicable too - they're riveted together. Thanks Walker - the fix for this is to pull them and put in RACOR CCVs instead, or if you keep them, pay close attention to those breathers and get rid of the crankcase return (again).

Electronic engines? They're great when they run.

Frankly, Cat has lost my trust. I know too many people who had trouble with the 31xx series engines, some of whom ate a LOT of money. Cat took care of some of them, but not all, and unfortunately what they never take care of is the downtime of having engines pulled and replaced - that's time you pay for in moorage and loss-of-use that is never compensated for. The 3208s are great motors provided you don't crank 'em up too far - yeah, they're parent-bore, and get smoky as they age, but they'll run forever if you keep the output reasonable.

Cummins? Depends. I know people with trouble with them too, especially the smaller ones (4bt series.) I haven't heard too much cursing over the 6bts, and none over the QSMs - at least not yet, but the QSMs are pretty new in the marine market. Time will tell. Those QSMs are damn pricey though.

Ischotti, MAN, etc? Got a lot of money? A guy at my dock spent 25 big ones on "routine" 1000 hour maintenance on a pair of those MANs. How's that sound for a bill? Eeeeeekkkkk... Yeah, they run good - but that per-hour "sunk cost" is way up there.....

Yanmar? I dunno. The one in my genset sounds like a sewing machine. How do you argue with that? 700 hours and all I've done is change the oil and filters. Of course its a little 3-cylinder - does this extrapolate to their main propulsion? Hard to say.... they get some impressive ponies out of some small packages - will they stay together for 2000, 3000, 5000 hours? And what about parts costs?

There 'ya go, in a nutshell as far as my views on the various engines....

[DaveP3]

This thread is very very interesting. To get this much imput from so many knowledgable people is GREAT! I personally, not having nearly the experience or knowledge of some of you still tend to agree with most of the comments. Personally I would rather have a mechanical engine (any mechanicla engine) over an electronic one anytime. Electronics and seawater just don't mix well and if your electronics take a hike so do your engines.

Personally I am a fan of the old two-stroke DDC's, as long as they are mechanical and not stressed out (Ie: the 625 6V92's, 735 8V92's and 900 12V71's). I know a lot of you out there are going to say the DDEC upgrades are great for maintenance issues, fuel economy, and less smoke, but the only engines DDC saw fit to electronically control are the ones that tend to hand grenade.

When we repowered our old 45C (1970) we looked at Cummins (QSM's), Cat (3176 and 3196), MAN (680 V8) , and Detroit (Series 60), and when I started getting estimates starting at $100K and heading to ($150 for the MAN's) I turned my attention to finding a set of rebuildable two-stroke DDC's. I looked at 8V71's (what was in the boat) at 500hp, 6V92's (in the newer 45C's) at 550hp, and 8V92TI (older 1st generation intercooled motors) at 550-575HP. I finally found and bought a set of really old (out of a crew boat) 8V92TI's at 550hp. We had them purchased, factory rebuild including dyno'd, and installed for less than $65K. I now have new motors, more speed, good dependability (700 hours without anything more than oil changes, routine winter cleaning (of the coolers, cooling system, and airboxes and filters). My son (3 1/2yrs old) just last week took my port engine out of gear while running at 2000 and we sucked some seals out of our port supercharger, but that is my fault and not Detroit's. Above all I am really pleased with the performance, dependability, and cost of operation for my engines. I know of some people with electronic engines that cannot do any maintenance at all, they have to call factory service at $100.00 an hour. Me, a friend who services trucks does most of my engine work (with me helping) for $20.00 hour and fishing trips. Good trade off.

I don't bad mouth those people with other motors. Heck, usually they are faster than I am, quieter, and cleaner. But I look at my checkbook and just smile as the big 61 Vike runs by me at 30kts while I'm putting around at 23-24 kts. So my advice....if you have the bucks, go for the high performance diesels and enjoy the ride. But be prepared to pay and also have down time when you break down.

[pascal bertram]

hello

my name is pascal i am french a mleave in thailand and i need advice with similar story
i got an old bertram 42 with detroit diesel 671 but 325 hp my gear box ratio is 2-1 and i got by now ghorrible 22 inches thai propellers and go 14 knots i really need to go about 18 knots if i want to work with this boat it mean i have to if i want to keep it even a 1984 boat is expensive for me
i was thinking change for 29 propellers for exemple and youpi but my shafts are 1 3/4 like yours and it looks like even 325 hp with 2-1 gear box and 29 prop are too much for my shafts
in thailand i dont found until now professional for give me advice or even for make the work it looks like i have to make it by myself and i know very little about those allison transmission i even didnot know if it is possible to put bigger shafts without big fiber work

may be a polivalent idea with only prop change or a very detailed explainartion (i am french and my technic vocabulary is very poor) draw are much better and help the writing
i mean i dont know how to do and what to do

a little help from some friends will be welcome

thanks and kin d regards

pascal

[BUSTER]

Hi Pascal
I believe that particular boat with 2:1 to 1 gearing and 325hp each engine should be turning 24 inch diameter propellers with pitch from 23 inches to 26 inches depending on how the boat is loaded.1-3/4 inch aquamet stainless steel shafts are plenty for that boat up to at least 480-500 horsepower. Good Luck.

[REBrueckner]

Pascal: the 1970's era 41 Berts are just plain underpowered with 8V71 naturals,,,they needed TI's to get out of the water....a drawback of the deep vee design.....size your props for 2300 RPM WOT.

I think Genesis correctly noted about propping a bit small then running reasonable RPM....and someone else noted that running at 2300 RPM, for example, 200 off top RPM will shorten a 450 or 485 HP 61TI and standard propping is a life shortener...

These tie in with David Pascoe's correct idea that the rate of fuel consumption (power developed) primarily determines engine longevity....run even a 485 HP 671TI at say 1800 or 1900 RPM instead of 2300 RPM and see how much less power (and speed and fuel consumption) results....run a 485 HP engine at say 300 or 350 HP and it will easily give, say, 2500 or 3,000 hours....run it near WOT (say 425 or 450 HP) and you'll be lucky to get 1,000 hours before smoke and reduced compression is apparent.

As for repowering with 671TI's, sure they are older and less efficient technology, but figure the cost with fuel over, say, 10 years and likely you'll have spent half the money you would for brand new "fancy" electronic control engines in typical recreational application.

But what has not been mentioned, as usual, is annual hours of use: at 100 hours annually a bit less efficient fuel consumption and a bit shorter engine life, if incurred, still means many, many years of reasonable cost. Most recreational diesels lose half their service life sitting idle in the corrosive marine environment.
For a commercial application, say offshore fishing, and racking up a thousand or two thousand hours annually with ten to 20 times the fuel consumption of a 100 hour boat, and the annual cost picture changes dramatically...there a brand new high cost and high efficiency engine makes a lot more economic sense.

And who is the guy who posted about being a mechanic for 25 years and claims Detroits are "tempremental"....that's so far off base as to be absurd...

Anyone who has owned older detroits knows once they are set up properly and reasonably maintained they'll be incredibly reliable and trouble free. Thats a major reason they have been so popular commercially in such a wide range of applications on road, off road, and marine.

[TripleThreat]

[...... When we repowered our old 45C (1970) ....... We had them purchased, factory rebuild including dyno'd, and installed for less than $65K. I now have new motors, more speed, good dependability (700 hours without anything more than oil changes, routine winter cleaning (of the coolers, cooling system, and airboxes and filters)....[/QUOTE]

DaveP3,

How has your experience been in the 6+ years since your overhaul?

I am looking at the same boat and re-powering the same way and am trying to get a feel on the cost (above $65k) that you have spent on other non-routine maintenance issues since this was originally posted.

Thanks,

Gary

[Will]

I noticed in my Detroit manual that 6-71Ns are rated at 265 HP @ 2000 RPM (not sure on that one), 6-71 TIs @410 HP at 2300 RPM and 6-71 TIBs@485 @ 2500 RPM. If you compare the curves, it looks like all three are about 185 HP @1800 RPM. So, why wouldn't you have about the same service life on all three if you ran them at 1800 RPM?
Will

[daniel r morrison]

Foolish move even if the engines are free. Fuel consumption poor and life short unless detuned. Look at cummins c series. I have a 1977 42c and re-powered with cummins 480ce with 1.75 zf gear, 2'' shafts and 24x28 four blade props. Slow cruise at 18k with fuel consumption at 1.33 miles per gallon.This is at 1800 rpm. fast cruise at 2400 rpm is 23k-24k. full speed at 2670rpm is 28k. At trtolling speed i get 3 miles per gallon. I under-propped on purpose as rated rpm is 2600. The savings is the difference in weight, and efficiency of a newer 4 stroke. With my old 671's my exhausts were 1/2 under water now they are 3" above the water line. cummins 450 re-mans would be a good choice with similar results

[jim rosenthal]

I agree completely. YGWYPF Sometimes you don't even get that. I took out my Caterpillars, by the way, and put in Cummins B series 370s. Much better choice. The boat is lighter, faster, quieter, and uses less fuel. And the transom stays clean. It didn't before.