You can figure out the tuning frequency using a program like WinISD
This would be the size port necessary to give that box a reasonable tuning frequency:
http://edesignaudio.com/Products/13Ov2Subwoofer/BoxDesigns/twoidealvented.gif
See how long it is? If you make the port too short it will be tuned way too high. Say it was tuned to 50hz, that means that at 50hz the subs would be moving the least and you would have the most output (the port is doing all the work, the subs' cones and the air mass in the port are in resonance). Above this the output would slowly decrease until you got to about 100hz where the output would be similar to those subs in a sealed box.
As you passed below 50hz the output would slowly drop off until you reached about 35hz and then the output would drop like a rock and the subs' cones would go psychotic. This is called unloading, the subwoofer cones are moving so slow in comparison to the port's resistance that the port, instead of helping to tame the subs' excursion and increase output, it actually functions as a huge hole in the side of the box and there's practically no pressure inside the box keeping the subs excursion in check. As a result they start moving a LOT farther on a given amount of power.
Mix this response with the transfer function of the car, and what you get is an ungodly huge output spike at 50-55hz, not much output above that, and virtually no output below that. On top of that, if you feed your subs frequencies around 35hz or lower (which many songs have) at anywhere close to med-high output, they'll bottom out and possibly damage themselves.