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OK yes, I remember that too, but I still think there's something to "venting" it with the spill hole. If the 'chute is solid (no spill hole), it will "fill with air", and the rest of the air will find its way around the 'chute, streamiling around the "bubble" that's created by the filled 'chute. It's my understanding that a pickup tailgate is much the same - - venting it will let air pass through it, creating more drag than if you allow the "bubble" to form, causing the rest of the air to "streamline" around the bubble.
Similar with the comments about the airplane speed brakes with holes, etc. I actually suspect the "parachute bumper" may be better than no bumper because of the air bubble it might create, causing the rest of the underbody air to "streamline" around it AND other dirty clutter under there, like the rear axle, etc.
But this is just out of my mind - no documentation or anything.
However I do remember reading in a car magazine when the new Camaros & Firebirds came out in the early '80s that the Camaro actually had lower drag, even with the open headlight buckets. And they concluded it was the same thing - an air "bubble" would form in those buckets that the rest of the air could streamline around easier than the cracks & seams in the closed headlight doors of the Firebird......
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