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

[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?