Actually that's not true. Ideally they don't want to be spun more than about 2800 RPM's. They have a governed limit at 3600 but you will find very few that survive repeated hits anywhere near that mark. Big problem is cracking main webs and cranks. If you can gear things up to keep them in the sweet spot of 2200-2400, they will run just about forever but they don't like high RPM.
Out of the box, Cummins appears to be a lower RPM limit but that limit was more place for warranty purposes. These can run up to 4100 if the limiter is removed but after that they can become ticking time bombs. They are way stronger blocks that will run all day long at 3500 RPM, if you have things setup to allow it.
This is based on the research I have done, with currently owning both.
. I wasn't talking about ultimate RPM capability, I was referring to the "sweet spot" where the motors are happiest/perform the best when driving and/or towing. A Cummins motor is happiest cruising at around 1800 to 2000 rpm motor. As you stated, the GM diesel is happiest in the 2200 to 2400 rpm range. Towing, the motors like to be around the upper end of the respective rpm ranges (based on my experience with a 90, 92, and 99 Cummins, and a 93 Chevy 6.5). Yes, the 24v Cummins motor can go to 4100 with different electronics, but more than 3000 rpm on a consistent basis is hard on the factory valve springs (at least the 12v motors, which had lower pressure valve springs). My comments on the relative preference of the GM motor (and Ford diesels, too) is based on the sweet spot for cruising and towing with these motors. The V-8 diesels "like" about 400 or so more rpm than the Cummins does (for a given road speed).
