Always remember the 4.0 needs a certain amount of backpressure to help build torque (its not a torque monster anyways). If your looking for more performance run a high flow cat into a good quality catback exhaust system.
No engine in the world needs backpressure for anything. That is an old myth that comes from hotrodders who couldn't understand why they would lose power when they put on hugely oversized exhaust.
The truth is, you require exhaust of a specific diameter and near a specific length for optimum scavenging. Too large, and you get turbulence because the gas expands instead of moving through the pipe. When it moves through the pipe, it sucks the pulse behind it along, if it is expanding instead of pushing down the pipe, the pulses mix and move more slowly causing.... wait for it.... backpressure.....
When you have exhaust that is real short, say open headers, you have nowhere for the pulses to meet, so you have nowhere in the pipe for each pulse to produce a suction on pulses from other cylinders. Therefore, you lose power this way on a normally tuned engine until you reach high RPMs.
Anyways, point is, all backpressure does is keep exhaust gasses in the cylinder where they don't belong. This means some of the energy of the piston moving is used to push this gas out instead of being converted into rotational energy for the crankshaft.
By the way... at 225 ft lbs of torque.... I would definitely call this engine a torque monster for its size, because that is quite a lot from a v6, especially when you compare it to its measly 155 hp.
back on subject:
Open headers are friggen LOUD. They will make your ears bleed, your neighbors hate you and will probably make you sterile. I would do it in a heartbeat
kidding, but yeah.... for the noise and how little gain there is, you are better off just having an exhaust shop bend you up some pipe with a couple glass-packs on it. That will mellow the noise a WHOLE LOT, and will be nice, cheap and free-flowing if this is a trail rig. Anything you want to put on the road, put real mufflers on it, the noise will drive you insane.
