E6s have a chamber designed by a crackhead. It is supposed to be "high swirl", but all it does is impede flow. E7s were a big improvement (part of a 25hp improvement which I believe included intake and TB as well)) done in '87.
Bottom line: E7s have potential to make decent power when ported. E6s have the potential to hold down a large stack of papers in a breeze.
I used to have a pic with diagram showing the differences. I remember that the main thing was that the valves are shrowded towards the center of the combustion chamber. There is a reason they were only made for a short period of time. E7's are pretty good heads except the small port size and the exaust port was designed by the same crackhead that designed the E6 head, I guess they just didn't have it to fire him. Some porting will take out those problems.