On my Jeep CRD, I removed the CAT and choke point in front of it and replaced both with a larger section of straight aluminized pipe and replaced the stock muffler with a Flowmaster. Jeep has a nice mellow tone and gets a little louder when you get on it hard. Noise is not bothersome at all in the vehicle and can hardly be heard with the windows rolled up and music playing!!!!
I am a firm believer that the open free flowing exhaust system helps in engine performance by lower EGT leaving the cylinders, faster turbo spool up time, and better fuel economy... I was able to confirm the EGT lowering due to adding a 5 inch turbo back exhaust system on my Dodge Cummins when I modified it several years ago since it has full gauges.... The quicker turbo spool up time and better fuel milage also improved...

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Supporting Vendor and Moderator of LOST05 Jeep Liberty CRD Limited 
Ironman Springs/Bilstein/Shocks
Yeti StgIV Hot Tune
Week's BatteryTray
No FCV/EGR
Samcos/ProVent
SunCoast/Transgo
Carter Intank-pmp
2mic.Sec.Fuel Filter
Flowmaster/NO CAT
V6Airbox/noVH
GM11 Bld.fan/HDClutch
IronrockArms/wwdieselMount98 Dodge Cummins 24 Valve