Hi, trying to decide if a 2004.5 and newer will offer the same MPG as 2003 - 2004.5 and how bad the catalytic converter is for economy and longterm maintenance.
I wonder if the new features described below offset having a catalytic converter. I may be mislead by thinking cats are horrible but it just seems like one more thing to replace and service, etc. Also, I heard that in theory any additional HP equates to less MPG.
Thanks for the help. Chuck

quoted from dieselpowermag

That all changed on January 1, 2004, with the mid-'04 model "Cummins 600." Inconel valves and high-cobalt content Stellite exhaust valve seats complemented revised ports with less swirl, a bigger turbo compressor wheel, and electronic wastegate control for the HY35 turbo. To feed all that power, a new fuel lift pump and intake air heater that needed no ground strap found their way into the engine. This brought the top engine rating to 325 hp at 2,900 rpm with 600 lb-ft at 1,600 rpm, thanks to 30 pounds of boost-regardless of transmission or state the truck was sold in.  

The Cummins "600" is a good engine. I'm not sure when cats started showing up on diesels, but I want to say around '01. To me, the bigger issue in terms of maintenance headaches and mileage loss is the DPF. Those didn't really start becoming common until the '08 models.

Generally, the emissions crap on the newer diesels lowers economy, ironically enough, in the name of cleaner emissions.

Probably the worst thing for long-term durability of diesels is EGR. The newer trucks tend to flip out when you block it off or disable it, but you can re-flash the computer or run an inline box that trick the ECU. The EGR makes your oil dirty a lot quicker. Hope this helps.