This is my current box plans for the 1 18"BTL and 1 Saz-3000d I have. I plan on putting all these into a 03 Tiburon(hatch) and competing in spl. Really my only question is would a bigger box better/ is 5.27 cubic feet enough. Thanks

box: (outer dimensions)

heighth = 16.5

width = 36

depth = 21.5

port: (inner dimensions)

heighth = 5

width = 7

length = 10

this port will tune the box to about 34.67 hz which is what i personally like to tune mine to.

after port displacement...this will give you about 5.27 cubes of air. so you have room to brace it and not go below the recommended volume. this is also assuming that you're using 3/4 MDF.

competition port: (inner dimensions)

heighth = 5

width = 9

length = 5

this port will tune the box to about 46.21 hz for comps. you can add to the width if you wanna tune higher, or take away to tune a little lower.

after port displacement...this will give you about 5.39 cubes of air

It should be fine. Mine have had between 8 and 5 ft, tuned between 32 and 36hz. I liked 36hz better, and they sound good in the current 5 ft.