[Christ]

This post:

Originally Posted by aerohead

Ollie,imagine that the air captured within the boundary between both bumpers and the rocker panels just travels with the car.Like a pool of water upside down,with no influence from gravity.
If air WERE moving there,it would be like a literal torture-chamber for the air,with nothing but separated flow and mass quantities of turbulence.
By "capturing" air underneath there,all the air flowing below,will pass by undisturbed,with an un-obstructed path to the rear where it can inject it's kinetic energy into the wake to dilute it's drogue effect,That's where the drag is,behind the car.
If you cut a hole,now you've let the air out of the dam and re-opened the torture-chamber.And then there is the chance that ALL the air under the car will be turbulence,adding to the wake resulting in higher drag.