Originally Posted by NorthStyle

The RS is a street legal track car. It was purpose built to go fast around a track...

So yes, while the rear bumper vents/ducts do aid in engine cooling, they do aid in down force by cutting down on the "parachute" effect (I don't know the proper term) of the rear bunper. However, you also need to realize the undercarriage, wing, front fender vents,etc. all play a role. Doing only one will upset the characteristics of how the vehicle handles at speed.

I had contacted Verus (formerly Velox) about producing undercarriage panels for the Z similar to the ones the make for the 86s, WRX/STI and Miatas. But due to the piss poor reception of the diffuser they produced for us, they won't be making anymore parts for the Z34.