[aerohead]

So far,in the literature,minimizing all 'internal ' flow is emphasized.
We want as much air as we can get on the outside body surface.
It minimizes wetted area and surface friction.
It minimizes super-velocities which increase surface friction and encourage vortex formation.
The only benefit a tunnel would provide is downforce,which in itself is induced drag which we'd like to eliminate altogether.
Since streamlined bodies don't produce lift there's no good reason to design in downforce unless you're going to race the vehicle.
If the body's right,the air will get around it without separation,delivering almost all of its kinetic energy to the wake area.
With all due respect to Bucky,the idea of lifting the rear off the ground could spell disaster in a split second.