I will have ~5.4 cubes and 87" of port. What will get louder/perform better, 12s or 15s? BTW they will be BTLs with ~3kw each. I know it would be starving for the 15s, but not sure if the cone area would conteract that.

BTL 12

Well the box size is a lil over the reccomended for the 12, and right under the 15... so not exactly sure what route I would like to go.

I'd go with the 12s.

Did you calculate in driver displacement cause the 2 12's should take that .4cuft up.

Yea that is with all displacement.

I guess I'm a little confused. You already built a box but haven't decided on woofers yet. How bout a little more information on the box, dimensions, port length, tunning frequency?

No I don't have the box, it is just designed, and that is pretty much the biggest box I can build for my car. It is tuned to 32hz, port length of 34.7" 39w x 16h x 24.5 d

With the numbers you gave me and using 2 12 inch BTLs I got 5 ft^3 @ 33 Hz with 17.4 in^2 per cubic foot of port volume. Id definitely go with the 12s.