So I got my flowmaster 40 deltaflow muffler installed this weekend. PN: 942442. $67 online, and this is my experience so far.
Installation: The muffler is shorter than the oem muffler so the shop installed about a 5" extension on to the tail pipe. The cost for the installation was $40 from Mieneke.
Sound: When I first got in it and drove away my stomach dropped and the first thing that came to my head was, "Man I hope this doesn't sound like a honda civic". So anyway I drove it home and as soon as I got outside of my Jeep the my thoughts changed completely. From the outside it sounds good. Deep, not 8 cyl deep but far, far from sounding like a civic. I had my friend drive it around the block and definitely sounds good.
Interior noise is louder, but only between 2,225 to 2,750 range. At idle its not loud and on the highway you can't even hear it.
Driving: Obviously I wasn't expecting huge gains from installing this, and if you do....
, so with that clear, I must say on the highway I did notice more umph going up hills. It seemed like I had a little more power in 5th. I can honestly say this because I drove home 2 hrs on Thursday before the install and did the same trip back after the install.
Verdict: I would definitely recommend putting a flowmaster on a 4 cylinder. At a cost of $107 parts and labor it is definitely worth it.
Hope this helps anybody thinking about installing one.