The other thing to proper air flow (intake and exhaust), the MAF will adjust the A/F ratio to match and may just adjust the HP back down to if you were running a standard intake. The only way to get around this is to add a tune, and for the 3.9L I imagine it would need to be custom.
The photo below shows the CAI (cold air intake) on my Durango. I moved the cruise control module to accommodate the filter and new intake tube. The rubber guard beside the radiator protect water from hitting the filter directly through the grille. I had a K&N installed originally but found the paper filter to be more effective and easier to maintain. I can clean this filter out with compressed air, where the K&N needed to be washed, dried, and re-oiled.
I have various modifications to my Durnago, including a custom SCT tune which matches the mods together to gain the best HP/TQ possible without worrying about engine knock while using a specified octane. Just FYI - The Dodge 5.9L (360) V8 does not have knock sensors.
A lot of folks think they can just 'throw on', or 'bolt on' mods to increase performance. After 50 miles or so the engine computer will relearn the change and you won't see any performance gains. If you're buying an air intake for mileage - don't. It'll take you 20+ years to see the potential fuel savings.
You can contact the guys here:
Tom Morana Racing Engines and see if there is anything for the 3.9L that can be done. If there is, a custom air intake, exhaust, and a tune will easily set you back $1200+. If the parts can be tuned properly you might get an increase of 18-20hp, at the HP peak of the engine (5000rpm or so). General driving might not see much performance benefit unless you're on the gas all the time.
If you're willing to get the intake, exhaust and a tune, your better off getting a freer flowing throttle body (can pull one off of a mustang or some other vehicle, knowingly) to aid in the low end and mid range where the power is mostly needed on daily driven vehicles.
As an example, I went with a 52mm Throttle Body on my Durango. Stock is 50mm, and it was one of the best bang for the buck modifications I've made. Depending on the tune, you can get 5-20+ useable hp from that alone.
Some folks have even made their own cold air intake take. You need to be good with a wrench and know your plastics. 4" poly pipe can be used within reason for an intake. Just don't cheapen out on parts as lesser plastics can melt with the heat the engine gives off. The other issue is the check engine might may stay on without a tune, because the engine may assume the intake is missing from getting too much air volume, or if you try to make it fit on a tube that is of different diameter, like the guy in the video below.
Stock throttle body on the right (50mm), 52mm on the left.
[ame="http://www.youtube.com/watch?v=aesMo705kGg"]
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