So I decided to actually measure my cargo area in my Trail Blazer last night. Guess I'd just been lazy and not got around to doing it previously. I was making my enclosure and driver decisions based off what I had seen in other Blazers and similar sized vehicles. I planned on doing two 18s and now I'm not sure I can. I want to keep the box at a reasonable height so I can still see out my rear windshield. My max outer dimensions are 44.5" wide, 27" deep, and 23" tall. I could go a little taller but I'm already sitting higher that the rear seat with 23". I have a notch measured for the bottom front corner edge of the box for the batteries. It looks a lot like Meade's box but his batts are to the rear and mine are forward of the box. Subs would be up and port back. With a double thick baffle and considering the loss of air space from the notch which is 7.5" tall (batt height) and 3" wide (distance from edge of batt to terminal post) that leaves me with 11.83 cubes before sub and port displacement. That is 11.25 after sub displacement.

Just spent the past 10 min doing some math. Looks like after port displacement (140 sq in x 11 in long for 35 Hz) it comes to 10.19 cubes. After a couple 2x4s for bracing it is 10.07. So what now? Will it work? Would I be better off with 2 15s in a reasonable 8 cubes or will the 18s wang in 10 cubes with about 5 inches clearance from the port to the hatch? I could possible add another inch to the top height and gain another .5 to 1 cube.

BTW, see my sig for the setup I'm working on. It says 18s but who knows now. May end up being 15s.

If i go two inches taller I'm at 11.6 net

154 sq in port ( 7 x 22 and 8.25 long ) for 35Hz

With 4,000rms a sub, definately go with a smaller box and less port. Look for something around a 10 cubic foot common chamber plus about 120in^ of port area tuned to whatever you want. Id suggest anywhere between 30-40hz, depending on where your car peaks.