port design

Of your two examples, the 13 * 2 * 13.5 is the only one that is acceptable.  Use the ratio 8:1 to consider the limit of port length to width.  A ratio of 1:1 (square) is better while an extremely narrow port is worse.  This consideration takes tuning accuracy into account.  The 13 * 1 opening is too narrow at a 13:1 ratio.

A second consideration regarding accuracy has to do with the port length.  You want the port to be long enough to allow wiggle room for both ease of build and for inherent design inaccuracies.  While it is sometimes possible to design a port at the desired tuning freq that is merely a hole cut into the MDF wall, it would be far too short for tuning accuracy.  The 13.5" length allows for more of this wiggle room than does a 4.5" length and so is the better length to use.  There is no ratio guide of box volume to port volume to follow;  the design has to be a balance of tradeoffs of overall box size and reasonable port length.

The consideration for port noise has to do with the opening area of the port.  Use the guide in the stickied threads for my comments on that.

I didn't consider that as I'm not accustomed to seeing changes in group delay (with varying port lengths) using the two programs WinISD Pro and Unibox.  A port tube can certainly turn into a "pitchfork", though, if it is extraordinarily long and especially if it's not stabilized along its length.  If you are lucky enough to be able to get the port diameters "just right", there will be no bends and the port will fit easily one way through the box.  Your main consideration, however, is port noise.  The length will necessarily be what it has to be when you get the port diameter large enough to overcome noise.

I can't think of any acoustic problems that would be caused by allowing the port to be closer to the side of the box than its diameter.  The fact is, most slot ports are built so that the port wall actually IS the sidewall of the box.  But the space at the free end of the port is important!  It is there where you want to make sure you leave that extra space.  There is a small amount of space at both ends of a port that combine to function with the air in the port.  This entire length of airspace is known as the "acoustical port" and is necessary for proper function and tuning.  That means to leave plenty of space (the diameter of the port is the basic guideline, if the port is round) at the inside port opening, as well as to leave clearance at the port opening on the outside of the box. 

Then what if the port is 12" X 1.5" slotted?  Use the equivalent diameter of that opening if it were round, which would be about equal to a 5" round port...so you would want to leave 5" of free airspace at the open end of the port.

I built my main home speakers with the bass woofer ported to the rear.  I think they sound just great, but then again I didn't design and test them any other way to do a comparison.  It's a case of design tradeoffs:  I didn't want the port on the front because I didn't want to see it.  I think it would have messed up the whole Feng Shui of the design ;).  But its a case there where the tuning is ultra low, into the sub range, and isn't critical to the sound of the woofer.  It's mainly there because the parameters called for it.  In the case of a car sub, however, the tuning is often an important part of the sub's impact and so it's more important to have the port firing alongside the driver, on the same baffle and in the same direction.

Know this, while you are designing the box:  the driver and port act in harmony with one another in producing the sound.  It makes the most sense that they should have the opportunity to send their soundwaves along basically the same path to your ears.  Design as such if your box will permit it.  But porting directly into the cabin while the driver is turned toward the trunk?  No.  At least, if the port has to be on a different baffle, face both the driver and port into the same airspace;  if it's a trunk, they should both fire into the trunk.