Ported is for people who like to bump in the 30-50 Hz range and have nothing else. At least in cars :P
As for the box calculator... i think you should upgrade to a REAL one:
LinearTeam
UniBox - Unified Box Model for Loudspeaker Design
I use the latter as I find it has more features out in the open which you can quickly tweek unlike WinISD.
Heres my view on Xmax vs. design's Xmax:
Xmax is the when the Bl of the motor is at 70% of its rest value, that means that this is a good idea of the limit for distortion granted the spider and surround are up to that level of excursion. A good design will have used most of the Xmax of the driver while still having a small safety net.
The safety net in car audio is CRUCIAL because people like to turn things up past where they should be and have no idea if their amps are clipping or not. For my home audio rig I have Vu meters on all channels and I know at what level which amps will clip. The speaker sounds like **** before it clips so thats safe but the sub has an Xmax of 30 mm and the one that was designed for the box is capable of 22 mm. The 22 sounded like **** when being pushed (the box was designed to push that woofer to 22 mm of excursion with the full 1200 watts). So clearly the speaker wasn't enough in my design so I finally reconed the other 30 mm Xmax sub and threw that back in and voila, no longer have headroom issues. Downside? That sub was designed for sealed use in a car so the response curve is more than messed up (drops off after 30 Hz instead of being close to flat to 20 Hz)
Another thing with headroom and all that jazz... My car rig is fused through a 60 amp fuse and this under max load will start to drag the system voltage down by 0.5 or a full volt for a fraction of a second. With that knowledge, clearly the stock system can't handle enough power to throw an 18" sub in there. Or even a decent sub of any size other than 8 or 10. By decent i mean like this:
[ SoundSplinter Subwoofers - Ignition Series 8" and 10" Drivers ]
Sure they're kinda bling with the aluminum spun cones and rubber surrounds but the one I had was the cleanest sub I've ever heard.
If you want good subs for decent prices, I've got links, but keep in mind, you won't be able to say "I've got kickers!" "I've got JBL!" but you'll be able to show 'em up none-the-less.
Subs really are the easy part, I mean you walk into a Subway or Mr Sub...
But seriously, they are. The tricky part is finding a solution to midbass, midrange and high end.
