For all of you who are confused about the resonator box removal on the S13, I removed my Y pipe, resonator box, and bottom half of the airbox. If you do all of that and you have a SOHC like me, it should look a little like this:
I had to remove part of my inner wheel well to acces the bolts to the resonator box and remove it, but overall it was fairly easy, the engine definately revs more freely. And the "growl" sounds quite nice.
I did this free mod in stages, first I just had the Y pipe off, the growl was good, then I finally found the resonator box and took it off, then I removed the bottom of the airbox, bent up a metal frame to put pressure equally around the filter, and zip tied it to the top of the box in little grooves I cut into it. This made the gave me better throttle response and even better sound. All this for $0
Next i plan on finding 2.5" dryer hose or shop vac hose to duct some cold air into there. making this whole set up about $5
I would imagine doing this with a high flow filter like K&N would be comparable to an intake for a fraction of the cost.
-Matt