depends on how far you want to go with the engine...
if you're simply looking for a lil extra pick me up, go with the K&N...
if you are doing other mods, are going to get custom tune done, or anything like that, go with the CAI...
I started with the K&N, mostly cuz it's super cheap compared to the CAI, and at the time I hadn't done anything else yet. Once I get a few more mods done, I plan on switching to a CAI and getting a custom tune done on a Dyno.
without a tune, the only thing you will see from the K&N (or CAI for that matter) is a bit quicker responce... I would highly suggest a tune...
K&N recently came out with a cold air intake for the GT. A version for the 4.0L shouldn't be too far down the road.
I currently have the K&N drop in filter and am very happy with it. Though, I'll be replacing it with a CAI once there are a few more offerings out there.
I am not sure I understand the air filter mod. I was looking at K&N's website and I see the Dyno on the Aircharger for the GT and how it gets 15+ HP. But that 15 extra HP is only at 6000 rpm. I assume the air filter for the V6 works in a similar way, gaining HPs (but not as many) at the higher RPMs.
I have a manual trans and never get to those higher RPMs on the gauge before shifting. I calculate about a 5 to 7% gain at 3K to 4K RMPs from the Dyno referrenced at K&N's website (http://www.kandn.com/dynocharts/63-2565.pdf)
My question is 5 to 7% gain that noticable or are there other benefits I am not understanding?
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K&N or CAI - 
