[Fr3AkAzOiD]

I have a behind the grill plexiglas lower grill block.

What I have noticed is that at even with a piece of metal bar behind it securing it to the grill it will flex at highway speeds letting air around edges.

I used my air compressor at a low psi to confirm this.

So not only do you have smooth surface vs "toothed" surface, when a block is on the outside and gets higher air pressure it just pushes the block even further into place where as if it is behind the grill it may flex and allow air into the engine bay.

I too am thinking of setting up a system to direct the air into the radiator but that is a little farther down the road.