[lunarhighway]

I wouldn't worry to much about the scoop effect, if the air cant get out at the back it'll just be filled with static air travling along with teh car, also it's open face is shielded by the tire so it's not directly exposed to the airflow, and this is improtant because any difference in drag between an open and closed face largely come from the fact that with a closed face there's a stagnation bubble that acts as a crude nose cone, making the air pass cleaner over the edges... as the tire has already pushed the air asside, an open or closed boattail won't matter much... however there's not benefit either to having them open so, if you can close them go ahead