06 C13 Pete straight pipes?

They use a catalytic converter for a muffler, I've read from people that changed to a high flow muffler or straights getting better MPG. I have not heard anything about CEL or other issues, it's no different than any other pre-DPF engine. I'd consider a high flow muffler, you can get them loud or quiet. Pittsburgh power sells them, so does Summit racing.

 

For some reason, on those ACERT CATS they do. I'm not knowledgeable on everything, but I'm pretty sure everything EPA 07 and new rated has them too. DOC aka "Diesel oxidation catalyst." I'd get the high flow muffler because you wont get a ticket for not having a muffler, it's loud but not as obnoxious as the straight pipe.

 

You can go to Pete and get the same muffler, for an earlier version without the catalytic. They will flow a whole lot better than what you got now, plus you can reuse all your existing main parts(elbows, shields, outlet stacks, grab handles etc.) if they're in re-usable shape. Just done a 06 379 C 13 a couple of weeks back. Customer supplied mufflers and I think they paid around $140 for the pair through Pete. If you go that route, there is a rivet at base of your existing set up that you'll have to drill out or grind off to separate muffler from elbow.