8v92 Johnson & Towers 735 HP Good or No?

Space.

Weight and size to hp.

Well my various Detroits that I have experience with are 8v71s, 6v53s and now 471s. The 8v71s would smoke till warm until the were rebuilt. Then no smoke , even when it was a cold start .
My 6v53s , one would smoke like hell when cold. The other motor would not which was overhauled by previous owner. I had the other motor done, and no more smoke cold start. Both motors just crank a few seconds and the fired right up no smoke.

My current J & T 471s, port can be a hard start when it drops below say 60 degrees out. It will smoke for 30/40 seconds and clear out.
Starboard fires right up with no smoke.
I'm living with the port for now . Oil pressure is good etc. Zero smoke when under way full load at any rpm.

That's what I have to go by. 92s series is probably different?

It gets below 50 out then it will take a longer cranking period to fire up. But darn those 6v53s would almost fire up like a fuel injected gas motor. They were great.

The 8v92 at 735Hp is a 2000 hour motor if treated right, less if pushed hard imo. I had two and rebuilt both. never had any issues with them till about 1800 hours. I rebuilt both around 2000 hours. A little smoke on start up is ok, but blue smoke is what you want to look for to indicate wear issues.

Parts were all available at the time of rebuild. Johnson and towers are of little help though. They told me in the 90s they had a data base failure and any DD before a certain date they have zero information on.

Here is my experience based on having bought a boat with twin JT892 TA's at 790 hp. When I bought it, the engines had a little over 4,000 hours since MOH. They ran fine, but created quite a bit of smoke on cold start. I rebuilt the engines and the smoke went away. At the time of the rebuild, the cylinder wear was visually evident (inside cylinder walls compared to inside top of chamber, above ring level, where there is no wear). Once warmed up, the engines ran fine. And upon tear down, there were no problems with any of the bearings, etc. The only problem was cylinder wall and rings. But everything was replaced.
I overheated both engines, warping the aluminum heads, at about 1700 hours. Until then, they ran fine. I had both rebuilt a second time and put another 1500 on them before selling the boat. Last I heard, my buyer was over 3,000 hours, running fine, no smoke.

So my conclusions are 1) smoke at cold start is a good indicator of wear and remaining life; 2) the bearings and other internal parts are not particularly failure prone, compared with other mfg engines; 3) high horsepower diesel engines produce a lot of heat when running hard, and the heat transfer capacity of the J&T cooling system is marginal at best. I suspect that the most common reason for needing to rebuild J&T DDs is overheating, and but for overheating they can be good for, perhaps 5,000 hours (when the smoke upon cold start becomes intolerable); and 4) "Pyros", aka EGTs (exhaust gas temperature gauges) are essential to not overheating these engines; and 5) I don't think overheating is a function of engine hours, though it helps to clean the cooling system every few years. In my case overheating was from fishing close to shore, and unwittingly clogging my cooling systems with kelp, then running home at my typical 2100 rpm.

Personally, I think DDs, in general, have a worse reputation than they deserve (oil leaks are common, they are old technology, etc.) which diminishes market value more than it should. So, if you want the best boat for the best price, DDs may be something to look for.

Have you done compression tests on them at 4,000 hours to see if they were even close to allowable spec? The JT 892ta maxed at 735hp not 790 but in any case, 4000 is far beyond the typical life expectancy of these motors at the higher power band. Maybe previous owner was at hull speed a significant amount of time? 2,000-2500 is probably the general consensus and what I would expect if I bought a boat with them again.

I've been running Detroits for 60 years and rebuilding them for about 53 years. I used the engines both commercially and in yachts I like them because they are old technology. I found them to be the most reliable engine for the ocean. Never had DDECs. Overhaul parts are readily available. I recently completely rebuilt 2 - 1947, 671 naturals and had no issues finding parts. It was their first rebuild and they had run decades and 10s of thousands of hours.
When you wring out all the HP, turbo DDs do about 3000 hours or less. If you run at 80% or less hp, you can probably double the time between overhauls (and keep the oil clean) . I once made 11,000 hours on a 480 hp 671. Usual was about 7-9000. It's all in how they're run and maintained.
Parts are still available all over the world.
Compared to "modern" engines with all the electronics, my Ford diesel PU has been towed 4 times because of failed sensors.

If the engines you are looking at are 8V92s and 480 hp, that sounds like a low HP 8V92. I would expect a longer life because the HP suggests Naturals.

I've got 892s at 730+ hp that Covington did their thing with new. I've got about 4000 hrs on them with no major issues. I just now have a turbo beginning to leak a drip of oil, but thats about it. We push our 55' MY to about 15 kts occasionally but for the most part run about 1250-1300 rpm for long 10-12 hr days. They use about a quart of oil every 30 hrs or so.
cold start produces a fair amount of blue smoke but nothing like a barn afire. Doesn't matter if she sits for a week or a month. No black smoke, even at 1800 (I'm chicken to run higher)

Domie
1988 Viking 55' MY

BTW... anybody know exatly what Garrett turbo this takes?