I recently had to fix a water leak, and decided to pull apart the factory intake to see if there were any weak points. I was pretty shocked at what I found. The whole thing is looks good with the exception of one area - the 2 small inlets to the airbox. First of all, the size of the 2 holes is questionable. And the bigger one up top is partially blocked by the side of the filter. But the worse thing was that there are 2 corresponding holes in the fenderwell, and they were not lined up with the airbox well at all!
I removed and reinstalled the air cleaner base, but the positioning was the same, the holes in the base of the airbox did not line up with the holes in the fenderwell, and the inlets were partially blocked. So with the base installed, I put a Sharpie through the airbox holes to mark on the fenderwell where I wanted to cut. Then I opened up the holes in the fenderwell a much as I could while still being able to keep them sealed to the inlets on the box. I'd guess I increased the area of those holes 50% or better. Then I went through the plastic airbox. I opened up the inlets on the lower half of the box with a die grinder as much as I could without the thing falling apart. On the upper, I noticed a lot of plastic flashing right by the MAF inlet, so I smoothed that out. I also ground on a few ribs on the upper near the MAF that I thought might be blocking flow.
The seal on one of the inlets was partially gone, so I used some A/C insulation to make a new outer seal.
Looking at the factory airbox, it is easy to see that if you really want lots of cold air, you can just cut out the bottom and optionally install a tube, which is what some of the aftermarket kits do. Forget the "Swiss Cheese" mod, that is a disaster for airflow. Yes more air will get in, but coming through several small drilled holes it will be "dirty air" (sailors and pilots will know what I am talking about).
I decided against opening up the bottom. It would probably be fine, but I recently got caught in the floods in Baton Rouge. And I decided that if I ever need to go through high water again, I want that extra margin of safety. It is hard to determine where the air comes in to the holes in the fenderwell. Shining a light into the holes was of little help. It is definitely a convoluted path, but in the interest of keeping water out, that is probably a good thing.
Without a doubt, the truck has a little more intake growl to it. It feels faster up top of course, but that is always the case when you are expecting it.
Also, I picked up 1-2 mpg. Ha ha, just kidding! I always laugh when I hear that with an EFI car, because at part throttle, the throttle is holding airflow back and the airbox is not even a factor.
If anyone is curious about this, just remove the top of the airbox and then remove the air filter. Then slide your hand through the inlets in the lower 1/2 of the airbox. If you hit metal, there's your answer.