Irvin-Generally speaking(and some may disagree) the WIDTH of the coil is about what you can expect for depth. Meaning,your football shaped 8.5" WIDE coil will most likely NOT get the depth of your stock 10.5" coil. However,it will get 2" more COVERAGE. These observations are mine and other peoples,but nothing is exact. For example,how deep a coil will go is NOT proportional to its overall size. I have a 5" Sunray. By someone's calculations someone may think,it being 5" round,that it should achieve 5" in depth. It can achieve 7-8" in depth pretty easily. Why? The TX power(transmit signal) on most machines,and certainly the Explorers,is fixed. So if you have a large coil with a lot of wire in the winding the signal gets "diluted" in a way. A small coil with not as much wire in it is more "energized" by the same available transmit power,thus making it proportionally deeper than its huge counterparts.
Slightly off track....in MY experience,for MY Explorer2,the stock coil is very smooth,reaches deep and just WORKS. I have also had ALOT of good finds with the Sunray X-12,which I also have,along with the X-8 and X-5. I'm sure there are other coils that will do a good job,that's just what I usually use for an arsenal on mine. Let us know how yours works out....