Airflow under the hood is managed on many modern cars. All of the air that comes through the radiator is subject to drag on its way over and around the engine. Air that passes through radiator baffle openings, such as headlamp bucket cutouts, A/C line openings, even the hood cable release hole...is all air that gets slowed down as it drags over underhood components. Simple engine covers are commonplace, these days, and have multiple benefits for MPG, component protection, and even (claimed) pedestrian safety.
Air pressure is higher at the base of the windshield, of course, but its also very high at those cutouts under the front bumper. That high pressure point is where the engineers have learned to manage the available pressure to aero advantage under the hood, as it lifts into lower pressure over the top of the engine.
Dont forget the openings into the front fender forward quarters. If those are open, inside the nose, more air volume is accepted through the grill opening (or under bumper inlet area). More air entering is more air slowed down by unstreamlined components.
Back in the mid-90s, I set up a demonstration for some visiting engineers, to help them understand why they were seeing so much damage to A/C condensors on a certain new luxury sedan. We had a line of near new vehicles, that were in company use on local freeways, for the demo.
We just took off the front wheels, pulled out the fender liners, and let all the collected stuff fall on the floor. The biggest "oohs and ahhs" came when 1/2 of a plastic Saturn hubcap fell out of the drivers side front fender of one car. That took some big airflow to bend and ram that piece of hubcap through the hole at the end of the radiator baffle.
Failure to manage that "after inlet" airflow was costing a lot of money and some efficiency. This is why motorcycles are such bad aero...too much lumpy stuff in too much airflow (and the older we get, the lumpier we get)!
JimL
PS: What I didnt say....venting the underhood will slow you down because it lets more air flow over more unstreamlined stuff. This is an issue that is carefully addressed on modern, high mpg cars. Go to a dealer, talk to the Service Manager, spend a little time looking at the bottom of the engine bays to see how much money the manufacturers are spending on this area of airflow control.
This is not a new concept...I remember seeing certain models, many years ago, with a large "heavy tar paper" type baffle just below the crank pulley area. I was told it was a lesson in how to keep the leaky front seals from blowing so much oil over the top of the engine and coating the bottom of the hood! Reduce the air blast getting in, and the oil just dripped off onto the ground where the customer didnt look (dont we always tell the wife, "that must have been somebody elses car that leaked where you were parked".)
