My '02 dually is pretty stock. Can't enlarge exhaust system without redoing the PacBrake. I'm told it still would be good to improve my air intake. But what would that do for the engine or turbo? Anything?

 

The problem with the stock airbox is that it is not unusual for it to warp slightly along the sealing areas of the filter box allowing unfiltered air to enter. Check your intake and turbo fins for any 'dusting' or 'sandblasting'. Feel the passage just before the turbo for any fine abrasives. If you have any of this, you have problems with the stock box and you're slowly ruining your motor. So the greatest advantage to a good aftermarket set up is clean air. Some claim they see slight MPG increases and/or lower EGT's, but that also depends on what other mods have been done. I used to have the 6637 and it filtered very well, (as verified by Blackstone OA), but just recently replaced it with the S&B set up. For me it was simply wanting a more enclosed style filter set up.

 

the 6637 is louder and filters better.
i did not see much gain in it but the better filtering is what i was looking for. the exhaust helped the most.

 

I have a 6637, but want an AIS. I have a source on them for about $225. I'll get one eventually.

 

I'm happy with my AIS it keeps my 38R nice and quiet.