Facing the sub up isn't nearly as effective as boundary loading the woofer. This is why you see alots of rear-firing subs in sedans. In the case of your 'stang, this isn't possible with a 15 inch woofer. However, you can get even better results by facing both the sub and the port at the floor. Boundary loading the sub and port to the floor of the vehicle will result in a significant gain in output. In your case, this could be pretty simple if there is any clearance between your existing box and deck now as you could simply turn the box over and wallah. You'll need a minimum of an inch and a half between the woofer and the floor. This could be done easily enough by sliding some 2x4 spacers under each end of the enclosure. If you like the sound, then permanantly attach them to the box.
Try it and post your results.
later tom
