Imagine if you will that your car has an engine behind the driver and a radiator up front in the nose.
In a perfect world of choices, do you vent the rad into the airflow above the hood, or under the car?
Below is a video where they chose to dump the air under the car. Just watch it for a few seconds, sorry about the ad.
https://youtu.be/63-oQq6XLhI?t=164
And below is venting for a front mounted oil cooler, a situation seen on many exotic racing cars (
radiator typically) going back to the 1960's.
My version of the thru the hood venting... - Pelican Parts Forums
I imagine that dumping air below into an already compressed and often disrupted airflow and under low flow high pressure as shown in the first example with video is better than introducing more flow over the hood - up to a point. That point comes at high speed...........guessing normal highway speeds and beyond where uplift can be a problem.
At that point, safely and handling trump smooth aero over the top of the car's hood.
Down-force, it's a good thing in moderation.
Above or below, that is the question.