Maybe some of the factory ones, but I would imagine the aftermarket resonators would flow much better compared to a factory one, like comparing a factory muffler to an aftermarket muffler.

they all start out "free flowing", but due to their design, they don't stay that way. they're basically a bunch of interwoven screens designed to trap soot. when the engine is running, it produces soot. the more particulate it traps, the less room there is available to pass the exhaust. you don't need to dissassemble one to understand it's function-simply look through it. the inlet / exit is 4", but it only shows 1" of light. in short, if you want it to breathe, don't add restrictions. (kinda like the "cat-back" exhaust-it's effect on flow is negligible, but someone said it would blah blah blah) don't take my word for it though- do the math regarding flow characteristics, and decide from there. also- the location of the muffler has a lot to do with the resonance, as do the hanger brackets. the factory uses the most convenient locations for attachment, not the optimal ones. lose the mount at the bell housing, relocate it to the frame, and voila........you can turn the stereo down a bit. this info is the result of "trial and error". mine is loud as hell to those beside me, but the in-cab noise more than acceptable.

I went with the areo turbine 4040xl works great

seriously checkout someones take off out of one of the new 6.7s there are no screens through it, ou can hold it up and see right through it

I got a hushpower muffler and I really like it on my 06 short bed. Still have the good sound but sure helped on the drone. There is still a little there but not like it was

My '07 came without the cat and an aero 4040xl installed by the PO. It is deafeningly loud outside with the go pedal down much at all. Inside, dunno. Never rode/drove anything comparable make/model-wise. Does seem to drone under load, tho.

i have a 06 cummins with the cat and i put a glass pack on backwards sounds great and no drone and doesnt affect the air flow out the exhaust and glasspacks are cheap and you can get them at any muffler shop

The cat restricts airflow though :nope:

yes you are 100% right on that but in the part of texas i am registered in they do emissions testing and the cat has to be there but the glass pack is free flowing when put on backwards and its 4" in 4" out

Gut it? :humm:

Do they just look to see if its physically there or do they actually emissions test?