[aerohead]

Originally Posted by wdb

The reason leaving the tailgate up on a pickup is better than leaving it down is that the swirl created aft of the cab improves aero, and that effect is lost when the gate is down because the air does swirl. I don't see how that translates cleanly to holes in rear bumpers.

I see the modern rear bumper -- which is effectively a big, empty, sealed plastic shell --having much more of a parachute effect than a swirl effect. I'm sure some air gets dumped out of it and into the low pressure area behind the car, so there may be some swirl taking place, but I would think that the drag generated by it trapping the air in the first place would overcome any such advantage. Besides which, cutting holes in the bumper will also dump air into the low pressure area behind the car.

In short I'm still looking for something substantive.

There is a lot of 'swirl' in the drag racing / time attack ranks around the topic, in parallel with the 'parachute effect' of driving a closed cockpit vehicle with the window open; is it better to let it fill with air and remain a high pressure area, or to vent it and have a continuous flow through the cockpit/cabin of the vehicle? There does seem to be some consensus in those communities that if you're going to vent the air, you should do it in particular places, and not just drill holes willy-nilly.

I recommend you get a copy of Subaru's SAE Paper from 1986 on the development of their Cd 0.29 XT.They explain fully,the peculiar airflow with respect to rear bumpers and the importance of depth of the bumper to achieve lower drag.