I'm still missing the $20K+ that says you are talking from principle, not practice.
Just the "simple" process of re-fitting of drive plates will inform you that there is a bit more involved than horsepower calculations on a 4 cylinder machine as compared to the 6. Those two extra cylinders can really matter in spreading the load in some situations as a sort of buffer. Not all, but some. While some might get lucky, some (me) will get unlucky. Well, actually luck had nothing to do with it. Mine were defective the day they were built, so were a number of 6 cylinders but their casualty rate was demonstrably lower in the same design than the 4.
I can tell you my engines were never overloaded and had the Pyro/boost data to prove it long before I paid a metallurgist to cut a piston in half and make some other conclusions before we found the data Volvo themselves published that gave away the secret to what was going on after 3 rebuilds by the same dealer.
So, in short NO, a 4 cylinder diesel is not always as reliable as a 6 if taken care of and not over propped in real life. That's all interesting from a mechanical perspective, but the real conclusion is to never trust Volvo with your hard earned cash.
Everybody who makes engines has issues from time to time, just a part of complex machinery. What really matters is how they step up when they do have issues. The support network and how it is managed is far and away more important than the engineering itself. THAT is the real problem.
