Opinions on Negative Pressure vs. Better Flow
 

I have been reading and learning more and more about aerodynamics lately, and one thing that I haven't given much thought before is Negative Pressure in the wheel wells.

My 2004 Taurus has ducts in the front bumper that basically blast the front wheel wells with air. This seems like a terrible place to direct air flow. I imagine the wheel wells to be very turbulent and also act like a parachute as air enters into them.

However I also imagine that if I blocked these vents off, the would be an increase in negative pressure in the wheel wells. But smoother air flow.

Which would be the better option, to block or not to block?

Here is a picture for reference.

Yeah, totally, block off the vents that aren't for cooling... Also, block off(partially) the ones that are meant for cooling... They're mostly oversized anyway...

I think those are for brake cooling. But if you aren't racing you don't need them.

Purely aesthetic, and certainly no benefit to aerodynamics. The smoother you make things, the better.

vents
 

Ford may have measured a constant Delta-P between the engine bay and wheelhouse at this location at all driving speeds,and chose it specifically do to this pressure relationship.
Cabin ventilation outlets are designed this way to save pumping energy of an air mover.
Without a complete pressure survey of your Taurus,you're shooting blind.

Try blocking them off from the inside.

If there are any adverse affects you can always reverse it.

I'm trying to form a structure for under my front bumper that is very similar to the part in the lower half of your attachment.

My thinking is to block the outer opening with a smooth plate on the outside to create a bluff body front end. The center section will have a diverter floor and spats across the front of the tires. The spat will form a converging duct that terminates in a slot at the front of the wheelwell opening.

What this does is form a curtain of air across the wheelwell that acts like a virtual fender skirt. The technique is used on new BMWs and Fords.

The front wheelwells are more problematic than the rear due to the extra volume to allow for steering. To understand what's going on with front wheelwells, observe cars and trucks in pouring rain (like so it's bouncing off the pavement) on the freeway. You'll see a lot of variation, but generally your 'negative pressure' is boiling out of the opening. Sometimes like straight out, especially commercial trucks.

Manufacturers have to design their cars for any extreme conditions. That is why the grille openings are so large. Imagine low speed operation in Las Vegas middle of summer a/c on high. You need as much airflow as you can get. If you don't do that you can block a lot of grill opening. Or driving over rural mountain roads and no one ever taught you to shift to a lower gear coming off the mountain. They try to build enough brake ducting into the car to make it survive. We don't drive this way because we understand this wastes fuel, so get rid of the open nose and see how it does.