Hi everyone, im back and about to leave again..
(I'll just be honest, i like it on the countryside with a new engine in my bug...)
Did a tiny bit of research thought...
most importantly
the air cooling the brakes is the air flowing under the car.
This is true for every vehicle that has enough ground clearance ( us for example). the opposite is true for purposely built track cars (i.e lmp class, gt2/3, and road going versions such as the radical sr3 etc)
Imagine top view of the car and the front at the top. the air goes under the front valance/splitter, under the front section (along the front wheel wells) and then it gets sucked outwards each side, due to the vortices created by the rotating wheel. Going super low requires another means of providing the brake disks of cooling air ( naca ducts on the bottom of the front splitter/ air intakes on the front surface of the car). In plain words the wheel works as a "fan" and sucks air out through its self (weird wheel covers on 934/5 P cars etc)
Jadewombat: if I understand your testing correctly there is a possibility that the positive value that you get is due to the flow reduction caused by the louvres on the front valance. Sounds weird doesn't it?? The louvres actually by letting air flow through them act like a "see through" fabric...its there supposedly covering up something but actually you can see through...
but what you see is a differentiated image of the original. If you had the normal front valance maybe you could still gain negative values (due to high turbulence) in the same area...
Now as for the rear diffuser discussion, aartjan thank you very much for the pics, on the new cup car we get a very nice idea for a diffuser.
IMO that would be, fabricate a symmetrical 4-1 exhaust system that fits and has EQUAL primary tube lengths.
Chop the rear part of the rear fenders and enclose the exhaust manifold within the diffuser.ie place two vertical pieces (one each side) somewhere between the rear upper shock mount and the inside fender wheel well (where the Z bar attaches on swing axle beetles).
I strongly believe that much is to be achieved by splitting most of the air around the engine (still providing airflow to the sump) and then expanding the air flow in a sideway's manner since the engine sits in the middle
Sooooooooooorrrryyyyyy for the long post.
I hope it makes sense
Chris