Originally Posted by DAlgozine
The older 6.5 GM diesels are not very powerful. Do a quick web search and you will find out the details. They can be worked over to perform and be reliable, but it will take some skills and knowledge.
The 6.6 Liter Duramax was a huge improvement both in performance and durability .
6.5s are not really reliable, they get hot quick and are quicker than the 6.0 powerstroke at blowing head gaskets and cracking heads that was something you could rely on. The PMD in them were fickle, blame things would get to where they wouldn't restart once hot, mine left me stranded a few times that way. Only diesel I ever dealt with that didn't want to start once hot.
I wouldn't push it to run any harder than it's already set up for. I ruined one once that was in a prison van, made a good tool truck but when trying to use it to tow it was gutless pulling a 5500lb rover.
The 6.6 isn't an improvement, it's the difference between night and day comparing it to the 6.2/6.5s Chev had to do something drastic because the 6.5 wasn't even up for comparing to what dodge and ferd had in their platforms.