I had a friend that raced a NHRA stock class 69 Camaro that said he taped off the air opening for fresh air to the interior of the car, then replaced the metal cowl grille over the area. He claimed it was worth a tenth and a mile and a half by doing that.
We tried the same thing on another 69 and guess what? One tenth and a mile and a half improvement
. Never did try just a plain old non cowl filter setup in comparison though.