[Vman455]

Originally Posted by Ecky

Right, I imagine this helps fill the low pressure zone in the wake, but it probably also does it in a very turbulent manner. A kamm or boat tail + belly pan would be ideal, but there are plenty of situationally effective less than ideal solutions.

Originally Posted by aerohead

I found what I was thinking of,from a 1986 SAE book on recent progress in fuel economy.
The paper is from Subaru's R&D of the Cd 0.29 Subaru XT.The particular citation is on page 20 of the book,with Fig. 19- Influence of flat-bottom.
The image shows three different under body configurations.
*The lowest drag is for that of a sub-trunk which fills the void between the gas tank and the rear bumper,with the bottom of the bumper even with floor of the trunk.
*The second lowest drag is for that of the same configuration,excepting that the bottom of the bumper has been raised above the elevation of the sub-trunk floor.This additional void is all turbulence,and adds 2% to the drag.
*The worst-case scenario is with an even higher bumper bottom,and removal of the sub-trunk,which adds 12% to the drag.
-------------------------------------------------------------------------
When air loses a surface to flow over,it immediately turns into the void,triggering turbulence.The sub-trunk prevents the deflection and helps shape the flow smoother.
With the XT,the concept is further promoted by including the bottom of the bumper as a means to shape the flow.
--------------------------------------------------------------------------
Dead air pools inside the perimeter of the bumper,with the active flow passing underneath,just as air flowing over a pool of water.
--------------------------------------------------------------------------
By cutting holes in the 'dam' your inviting flow separation,turbulence,and higher drag.
------------------------------------------------------------------------
It looks like you can't change the drag more than 2%,so the most your fuel economy could change is around 1% which I submit to you,cannot be measured outside of a wind tunnel.Your car,with zero modifications will have a seasonal mpg variability of 12% anyway.Any minute change,as with the holes,will be lost in the signal-to-noise ratio.
I have the graphic with me and hope to get it scanned before the store closes.
Here we go

I found myself thinking about this the other day, and I'm not sure that the conclusion that airflow through the holes would help fill the wake is correct. The Subaru figures indicate a low-pressure area behind the spare tire well which causes turbulence. If holes are added in the bumper, for the air to begin flowing it would need to be higher pressure than the low-pressure wake behind the bumper. Do we know this is the case? Seems to me the first thing to do here would be run a manometer to in front of a stock bumper cover and behind it and see what the pressure differential is, if there is one, and thus if air would actually flow into the car's wake or not.