the size of the abc box depends on the thiel small parmeters of the speaker. The way I model them is i run the specs and set the speaker for a maximum flat curve to the fs of the speaker. Then i split the total in two parts 66.666 percent and 33.333 percent and port the largest chamber to the fs of the speaker and make the second chambers port the same length, Now because each chamber has a different volume they have different port tunes and what seems to happen is that the second port tune peaks an octave apart form the first port tune. So if your first port tune was 35 hz your second chamber would peak at 70 hz. The other intresting thing is the effect it has on box rise. Box rise is the measure of resistance a given frequency has over the actual bridged output of the amplifier. (bridged in this case) As you can see the lower this is the better cutting it in half doubles your delivered power to the speaker. Couple this with your cars cabin boost and resonant frequency and you got a sick amount of spl goin on. The box design isnt a perfect one but has alot of potential.