Originally Posted by rslifkin
To clarify on the power requirements you mentioned in the first post, I'm gather than the 220hp at 20 kts was 220hp total, not per engine, correct? If so, running the engines at 1/2 power shouldn't stress them much at all. Even a typical gasser would live a long life at that relative power output.
Originally Posted by rslifkin
If the transmissions are an integrated trans / v-drive there might be fitment issues. But I don't think it would be anything un-solvable.
It looks like the 8.2s turn anywhere from 2800 to 3200 RPM depending on rating, so plenty of other engines would be in the same ballpark. And there are enough marinized diesels out there that at least one of them of sufficient power will fit into the holes the 8.2s are in now. Looks like the 8.2s are 31.4 inches tall, so depending on how much space you've got above them, you likely have choices. A QSB 5.9 is 33.8 inches tall, for example, so only an extra 2.5 inches. The newer Yanmar V8 diesels are just over 30 inches tall, so they'd definitely fit.
Looks like the Detroits are just over 1500 lbs, so the QSB or Yanmars would be lighter.
From what I can find, the 8.2s in marine form were rated anywhere from 130 to 300hp, depending on who marinized them and turbo or not. I'm seeing mentions of Detroit calling the turbo versions continuous rated at 160 or 200hp at 2800 RPM. NA continuous ratings were 130, 145, or 160hp at 2800. So I wouldn't worry about cruising at 20 kts with the 8.2s if the total power demand is only in the 110hp per engine ballpark at that speed. Doing that for days on end wouldn't be pushing them too hard, IMO. I'd personally consider that to be standard fast cruise with the ability to sprint a bit faster.
Now you have me stumped . The V drives I have seen were mounted to hull stringers to take the thrust and the engine was a distance away with a short shaft to the V drive.
All the more modern engines spin at a higher RPM , which might demand a different tranny , or a different set of props.
A marine exhaust manifold for a converted engine could be avoided with a dry stack exhaust.
Yes , unquestionably keeping the stock DD 's would be best , and a 10-12K cruise would be just fine with us .
I'm trying to figure worst case ,just in case, as I don't want to end up too broke.[/QUOTE]
Don't all of us with old boats do a little dreaming and planning for "what if" scenarios, specially with engines that are possibly past their best. I just happen to have some measurements handy for a Cummins 6BT 5.9 that would fit in my boat
As rslifkin says the QSB is 33.8" high. Some other 6BT engines are slightly less than that but they show a turbo sticking up above the valve covers 3-4". If you used a water cooled marine exhaust manifold and turbo, the turbo seems to be at or below the top of the valve covers. In my boat that would make the difference to fitting in the space. Another important measurement is height above crank center line, which is about 21-22" on the 33-34" tall engine. That leaves about 12" below CL.
The 8.2s did run faster than a lot of diesels but they are running the Dodge truck engines over 3,000 rpm now, although I don't like to. So a 6BT could be run with the same ratio and props as you have said you want to.
27kts-2800 rpm, 20kts-2100, 12 kts-1250. All good engine rpm range for what you want to do with the boat.
As far as V drive transmissions, there is a variety of Twin Disc 506 gears in different configurations of V drive, rear drive with 5 1/2" down offset, and 8-10 degree down angle output shafts.
It would be a big job to repower, coming from one who has done a few. If your 8.2s are running good for your low cruise speed requirement, I might just enjoy it the way it is. Go through everything and do some good preventative maintenance. I think my GM 6.5 turbo in a 1993 2500 truck is in the same engine family. I know of lots of them that melted down in the area of the back two cylinders on the right side where the crossover pipe and turbo meet because of a lack of cooling capacity. There were aftermarket kits that had high volume water pumps (83 gpm to 130ish gpm), high volume thermostats, Duramax 9 blade fan and viscous drive that engages at a much lower temperature than the stock hub, and on the pickup we put in a thicker rad core. At the same time we added a 4" exhaust system and you would think it must be a totally bigger engine with heating only when pulling on the hottest days now. 250 k miles and still running strong! Gets great mileage. It was mentioned to add a vent line in the cooling system so thought cooling might be concern on these engines too.
Good luck and keep us in the loop!