I believe the block, heads, head gaskets, and reciprocating components are the same for all of the turbocharged models. Sixteen years ago in preparation for making the decision to purchased our boat, I called Johnson and Towers to see if any of their original 8.2 certification engineers were still around. Lo and behold they connected me with an "old" gent who was part of the original JT certification team. If I remember his comments correctly, the original turbo truck engine was at 180 HP. He explained that the subsequent horse power increases to 220 and the 250 were achieved by simply increasing injector size. For the 300 engine they upped injector size once again and added an intercooler. But nothing changed in the core engine....even the same cam profile for the injectors. So percentage of rated power is somewhat misleading. They ultimately loaded that head gasket to the failure point. And from my discussions with two rebuilders through the years, the preponderance of head gasket failures were in smaller sport fish boats with the 300 HP engines....running near max power for extended periods. Lose a raw water pump at very high power and the gasket would likely pop pretty quickly...
Regarding carbon buildup from running at slow speeds...yes it does concern me. So I do the standard run up to higher power once every couple of hours. And that does visibly clean up the exhaust. The other thing I did a couple of years ago is switch to Rotella synthetic T6. That seems to have helped. Something else that I've wondered about is downsizing to the 220 HP injector spec. I don't need the high end power anyway. Think I'll call J&T again....
PS Detroit Diesel, J&T and Stewart and Stevenson each used a different exhaust manifold casting. I suspect that this has been a source of parts availability issues over the years. In any case, Mesa Marine Exhaust can make any of them.
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