sealed or ported?

There are some gross generalizations and inaccuracies posted in reply to your question above. 

Here's how I would look at a woofer's enclosure type:  find the EPB, which is a simple calculation of Fs / Qes.  The parameters are listed in the manual.  You'll find that with this sub, when you divide the Fs of 26.1 by the Qes of 0.556 you will get the result of 47.  Commonly, anything below an EBP of 50 is best suited for a sealed enclosure...anything really high, like above 70 should be ported...in between is a gray area where you should look at both ways and build according to your tastes.  This sub should go sealed.

In the manual you will see a recommendation of sealed box size as small as 1 cu. ft and as large as 4.6 cu ft.  Big difference.  This gives you an indication that this sub will perform in the space available in most car applications.  You'll also note that the larger 4.6 cu ft net airspace size is labeled as "SQ".  You will achieve best low end rolloff response and minimal peaking at mid-sub frequencies with this larger enclosure.  But the tradeoff for this better sound is that the sub will take only 300 watts input instead of 400.  You can prorate that power handling, such as assuming that with about 2.5 cu ft the sub will handle about 350 watts.

Use sealed, for one.  Then, use as large a space as you can but not exceeding 4.6 cu ft internal.  But if it were me, as I would never want to give up nearly half my trunk space for a sub and haul the weight of such a big box, I would find a suitable middle ground like 2.5 cu ft.

There's a lot of information to be found in Kicker's sub manuals.

Not all subs can be downfired... How can you tell? It's always been better for me -especially with the back seat down. I figure its the same principle as facing the subs towards the back, about 4 inches from back