This seems to be a topic of great mystery. Well after reading many posts about the subject of wether or not to remove the cats from the newer LLY Dmax when installing a high flow muffler or straight pipe I have come to these conclusions:


1) Removal of the cat will in most cases cause the engine the set fault codes p0401 & p0404, most likely emmisions related.


2) There is no noticable difference in performance with or without the cat in place while a high flow is in stalled.


3) There is a noticable gain in performance by removing the stock muffler and installing a 4" high flow muffler or staight pipe. (this is 4" piping only behind the cat flange and back)


All are welcome to post observations and experiences so that others may gain knowledge to make these bad trucks even badder!

In my opinion I agree with number 2. with that being said I would say that about a truck with no performance upgrades! But I also believe that once you start adding some programming into the equation The cat will hold you back somewhat in the power department(due to flow restriction), or at least will run you at a higher egt level than a truck with no cat! Personally I have not seen any codes set on the dmax due to removal of the cat!


Anyways these are my thoughts and experiences!


Thanks DEREK

I have removed my muffer and replaced with a straight pipe and gutted my cats. I have about 5k on the truck since the work and have had no error codes, truck quite, and runs great

The only LLY exhaust mod I have done is the Dead Kitty mod. No dyno report, but it did drop 0.08 seconds off my ET at the same track, same weather.


I did it to reduce EGT's, because I tow with a "tune" in the truck.


No codes, but I run the stock muffler.