1 - Don't be "that guy" Loud obnoxious exhaust coming from a 6 cylinder isnt cool, you are not fooling anyone into thinking you have a massive V8, it causes noise complaints which can lead to public areas being shut down or you being asked to leave most private wheeling spots.
2 - You could actually be hurting your self in a number of ways. I am not sure about Jeep years, but if that is a F.I. engine and has O2 sensors you need the exhaust system to ensure proper fuel delivery. Next you do not want exhaust fumes sitting under the vehicle, they will creep in and you will get sick as hell, there is a reason all vehicle manufacturers and every sanctioning body known to man kind for any auto competition requires exhaust pipe to extend outside the vehicle body. Finally, When you go from near WOT to no throttle the quick change in exhaust pressure creates a temporary vacuum that will actually suck air back in the exhaust system. If the pipe is too short (I'll let you determine how short too short is) the air drawn in will be much cooler than the temperature of the exhaust gases which will warp your valves and damage your engine.
Look we all like a little rumble, but making you XJ sound like a fiull out race vehicle is irresponsible, unecessary and certainly not cool, dont do it.
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