The air up at cowl is denser than in the grill?

 

Sorry, got my threads mixed up and thought I was responding to something else. I have edited acccordingly.

The air from a cowl intake is coming from outside the engine bay, where it will be considerably cooler. Cooler air backs up against the base of the windshield and down into the cowl vents. It's designed that way to pump cool air into the cabin when you turn on the vents. The cowl intake takes advantage of that design to get cool, dense air to the engine. Cooler air is already denser than hot air, and theoretically you get an extra boost of pressure from the backpressure against the windshield.

In older XJs there was a rectangular hole in the forward panel on the driver's side of the radiator that allowed cold air to pass through directly to the airbox. Some of them even had a rectangular shroud that ran from that hole to the airbox opening to pump cold air to the engine and keep out the hot under-hood air. Chrysler removed all that at some point in the late 90s for emissions purposes. Now the stock airbox is just sucking hot air, and it's in an area where there's no ram air effect from air entering the grill.