Facts Only wrote:What negative effect (if any) will an un-tooled surface have on the airflow at the rear of the car. It appears that the Force has some sort of tooling or cleaning on the upper surface.
Not much to be honest... Comparably, the decrease in dynamic pressure from the acceleration of the air below the car (and inside the venturi tunnel) will cause a much larger pressure differential than any sort of "top surface" panel design would - so you can get away with not having a properly tooled or formed outer surface. Obviously there will be some amount of turbulence or dirty air generated simply because it isn't a nice flush surface, but it will be minor compared to the wheels around it.
Facts Only wrote:Also for tooling and support many diffusers like this have a large return lip pointing upwards, I assume this will act as a gurney flap but would using a larger lip like a blocker have positive or negative effects?
Gurney flaps only work up to a certain height which is related to the thickness of the boundary layer which has formed on that surface to that point. Beyond that, any extra height just begins to become more draggy. You want a gurney flap to induce a vortex that is rolling in the 90° angle of it followed by two counter rotating vortices on the other side of the flap (off the vehicle) which then serves to virtually increase the effective angle of attack of the surface. But again, most of the performance gain that would come from those tunnels would be due to the internal and underneath geometry, rather than any sort of top surface finish or design.
