Voice coil choice

[Poeboy73]

I'm building a sub at the moment it's a 18" 640oz magnet  custom motor with a sundown tooled frame that has 10-11 & 12" spider landings ( I still need to get the bottom vent/spacer and there 6x5 bolt patern adapter), and I have a lord of bass mega roll surround. I'm trying to choose a coil, 3" 4 or 8 or 4"  4 or 8 layer, winding width size ect. I have a 5.25 gap depth so there's plenty of room for big x-mech I'm trying to build an over the top sub so decent power handling and 4" minimum peak to peak excursion is a focus. I've built subs but not one of this level. Lord of bass has a 3" 8 layer with a 3.6" we that is a tall we. I know jacob fuller likes 3" for some reason he uses 3" x3.12"ww(or something like that) on his big flagship subs even a 4 layer so there has to be a reason for 3" and x an Z's will take a lot of power is it just because of well designed cooling? I know 4" typically handles more power but there has to be more than that to it. I want dual 2 for 1 ohm final impedance.  Cool choice is sparse at the moment. I've thought about just calling apex and saying send me a zv5 18 coil n spiders but I would like to build something my own not just a sundown with a custom motor. My motors machining isn't done yet so I can go with 3" or 4". My ferrite has a i.d. of 4.27"  if I go 4"  is there an issue having the ferrite that close to the gap opening? I'm assuming no but not sure.