NACA vents are low drag intake vents, not meant for exhaust.
http://en.wikipedia.org/wiki/NACA_duct
I am considering streamlining the engine compartment, ducting the flow so it expands from the slit in the bumper to the radiator (diffuser) and then narrowing again to the exit and try to match the external flow velocity with minimum turbulence. I don't know if it can be done in practice, but I had something similar on an earlier saturn and that car would blow leaves out the back. I'll post progress on sc1 aeromods thread. I ordered more rivets, hopfully they will arrive today.