I'm planning a build of 1 Fi BTL N2 18s off of two Crescendo BC3500s walled off in my truck, and I'm using the Torres Speaker Box Tuning Calculator. I'm wondering exactly what the "# of Common Port Walls" option means. I understand that it refers to how many sides of the box are going to act as sides of the port, but I don't know why the back of the box isn't considered a side of the box. I have about 52" of width to work with, so I'm looking for the port to be 50.5 inches wide, so it would span across the entire top of the box. I've been messing around a little bit with some of my options and I'm not quite sure what the option I mentioned is referring to. If someone could clear this up, I'd appreciate it.

Is it simply because the "4th" wall is just an extension of the third?

My guess would be yes, the back is not considered a separate "wall" because it's simply an extension of the 3rd wall. For the purposes of that question in the programming it's probably indifferent whether or not the port bends and follows the rear wall since at any given point the port is only touching 3 walls of the enclosure.