How does a port work?

Can you tune a ported box to the same frequency if you use one 4x14x12 rectangular port compared to two 4x14x6 inch ports?

I'm trying to build a box for a Kicker S12L5 and the manual sais that the port should be separated from the wall by at least 3 inches.  Can I build a port along a wall without affecting the way the port functions?

The manual for the kicker L5's in on this page:

http://www.kicker.com/06/tech-support/manuals/manuals/2006/2006%20L5%20Sub%20c01%20WEB.pdf

I think I can build the port along one side wall but the depht of the port will be hard to fit into the box design I wanted.  I wanted a box that was 60"x17"x~17".  The back seat of my car is slanted so the top depht is 14" and the bottom is 20" which equals ~17 which I used in WinISB.  Can I fit a port which has a slanted back edge so that my port wil not have to be so close to the back wall?

Tunning a port like this will require me to experiment with it's lenght right.  I was thinking of building a rail in which I could slide a piece of MDF in and out thus changing the port lenght.  I won't need to cut the piece of MDF to check different port lenghts because it will not be considered a port since it only has one side outside the enclosure.

Is it practical to have a rectangular port that you can alter, without dissasembling the box, to change the tunning frequency?

You can attach an external port to lower the tunning frequency of the vented box.  You can make the port area smaller by attaching more mdf along the lenght of the port thus giving you a lower tunning frequency.  How can you increase the tunning frequency after the box is built?

I've been using different programs to find the port lengt and they give me different values for the tunning frequency.  I've decided to just follow the manufactures directions for the vented enclosure but I was still wondering if anyone has a way to vary the tunning frequency easily. The values I inputed into WinISD for a kicker S15L5 in a 6cu.ft. enclosure gave me a tuning frequency of 26.9hz and the fs was 23.4hz.  Do they have to match?

Tuning frequency of the box and Fs of the driver have nothing to do with one another.  Don't worry that they don't match.

The ability to adjust tuning after the build is all in your creative mind.  You just have to figure out the details before you build the box, while double-checking every possible port change for problems.  For example, one way to change tuning is to insert a length of MDF into the port so that it adheres to one side.  This would make the port volume smaller, and also would make the opening smaller.  An extra piece or two of MDF would be cut exactly the same length as the piece you build into the box for the side or top wall;  then you would have them available if you want to try to use them.  Just slide them into place with some glue on the side.  You would have to first check for noise problems with this smaller opening size. If you build the opening size extra large to begin with it will allow for this type of experimentation.

Another way to change tuning is to add on an external port extension, since the port doesn't have to be built entirely within the enclosure.  This would lower the box tuning  and you could experiment with several sizes.  Here is a diagram I made awhile back that shows an external port extension with sizing options: