i'm kind of curious too...but most of you have not posted what size motors you have...i have a 95 2.3l 4x4 5 speed and the first thing i did was to go to my muffler guy and had him check out my system, california being a big smog state....i did find their is a reducer between my catylitic converter and the muffler...he said by changing that it would definately increase horse power...he also informed me that to upsize from my standard piping would not do much at all on my 4 cylinder motor...my truck has no passing gear in the throttle and i would like more power on the lower end...it takes about 1900 rpm before the power picks up...if anyone knows anybody who has mod their 4 cylinder exhaust and improved performance and fuel, please get ahold of me....and i like to keep it quiet to...i know, i'm asking alot, lmao