Install a cat back system

Hi guys, I had a 96 ZJ 4.0L, tranny wasn't shifting right, running kinda rough. I had a Dynomax catback system on hand, so I decided to install it before I got to the other problems I was having. The other problems disappeared after the Dynomax was on.
The ZJ is history thanks to rust.
Now my ride is a 99 XJ 4.0L. Mileage started going down to 14 - 15 mpg, with sluggish performance. Went and installed another Dynomax catback on it and everythings back to normal again.
Don't know why this is, or if it's just my rides, but thought I'd share this with other members here. 4.0L Jeeps really need to breath I guess. And no this ain't a dynomax commercial, it was the cheapest system available

I had a blown out hole in my muffler on the OEM system and some leaks around by the cat connection. Once i put on the dynomax catback, aside from a much better sounding exhaust, i did notice a marked increase in torque at idle to about 1500rpm. aside from that nothing else, but i thought it was interested the definite increase of very low end torque.

As a note i'm sure if i put on a dynomax or not, just fixed the exhaust the effect would be similar, but still.

The D-max super turbo is a cheap/ kinda decent upgrade, but not my first choice for a "great" exhaust system. Can't beat the 75$ rebate check