I think the main reason is air flow, these turbo diesels need plenty of air, and, in some cases, they tend to drag a lot of dirt with every gulp. I can Recommend Jim, we worked together at Cummins, and he does a lot of injector and pump work, and I've never heard a complaint about him from anyone. I have a 7.3 Ford, rattle/smoke as a friend calls them, and they don't like K&N filters either, most guys that are making them perform, use a big one, about like you see on OTR rigs, and just kinda lay it under the hood, nothing fancy, but lots of area to breath thru. I like my 7.3, and the Ford chassis, my 97 F350 has just short of 400k on it, and as far as I know, hasn't had any engine work, and the truck is as rust free as one half it's age, if I were in the market for another, and could find a Ford, with a Cummins, that's where I'd be. Bob