The vent on the cowl

Noticing a lot of the guys with the early Mustangs block that off. The vent on the cowl. Most guys install a thin sheet of aluminum over top. is that something that would improve ET or mph on a 68 Dart. If not what is the purpose

cow???

cowl??

cowling?

I did it on my Valiant to keep some air out of the car with the heater box and everything else removed.

Common Practice in SS cars back in 70's to block off the cowl vents.
IF you can cause a vacuum inside the car, it will go faster.
Proven by guys with Gliders. When they installed an outlet at the tail of the plane to draw air out of the fuselage the plane was faster. Also, they could fly farther and higher than the gliders without this mod.
Try it and see if you like the results. Easy to remove if you don't like it.

I covered mine up on my 76 dart and saw no difference, but then again my car is kinda' slo.

I got a piece of the magnetic signs from a print shop and put it over mine.

good idea

I always wanted to do that even just to keep the maple helicopters out when parked.

I did it to fit my air cleaner

I covered mine with duct tape and saw no difference

Blocked mine off years ago when I had a cowl scoop so all of the air would go in the scoop instead of the cab and just left it after I changed scoops........