Something that occurred to me after thinking about this a bit. The Fiesta uses a front air dam that goes all the way across the front of the car. There is no smooth path for air flow to go under the car to the deflectors. Unlike a Boxster, 991, Vette, Mustang, etc, there is no way for cooling air to get to those deflectors. All of those cars have a smooth underbody setup that could use a deflector duct to catch some of that air. Or they use a designated duct from the front of the car through the fender liner that dumps air into the wheel wheel that could then be deflected into the center of the wheel, likle the GT350, E36, etc.
Air still flows through the wheel well, up from that front skirt/dam. It’s not a sealed piece to the leading edge of the engine compartment like an undertray so all it does is push the air downward before the airflow becomes unstable. The low pressure area of that skirt helps draw air down from the engine bay like wickers for hood vents it’s only to a certain point until they air dives back and up. A full under tray would need to have air flow paths added but since the fiesta has no undertray this isn’t as big of an issue.
It’s also not about ducted airflow to the deflectors. It’s about the available airflow already present and deflecting/re-directing that through the wheel well. You’re not taking anything from anywhere that already doesn’t have some sort of flow path. Wheel wells are known pressure spots and usually exhausting that through the top or back half of the fender can increase flow in that area. Just use what you already have within that area and direct it through the wheel instead of around the inner fender area.
Boomba has a great video showing the air flow path through the stock front w/ air dam and out through the wheel with a smoke test.
Also guessing from the amount of brake dust before and after on my car, it’s pulling as much air as possible. I have not tested with temp strips yet but can on my next track outing. It’s 1 bolt to remove the deflectors so it shouldn’t be too hard to show a before/after comparison. I do know that i havent boiled fluid or overheated a set of pads since installation.
Both the Focus RS and Porsche 993 use these same style air deflectors without dedicated ducting(they do have undertrays but they do not have dedicated path for flow to them) and they work perfectly for those applications.
Focus RS
The Performance Pack 1 Mustangs now have deflectors also with its air dam which has a dedicated flow path since the undertray seals from the front bumper PAST the area of the deflectors. Vorshlag has a set of plates which use the stock flow path to direct airflow. So the air flow stays the same they just direct more of the available air through the wheel.
The Corvette, M3 and certain others use dedicated ducting. That is understandable as a better solution but much more involved, more maintenance and more cost associated. For someone who isn’t tracking their car every single weekend or doesn’t feel like removing the dedicated ducting for daily/weekend use then the air deflectors is a great choice.
Here is the Vorshlag plates. The undertray on a Mustang makes it a requirement to add in those "tunnel" parts to make some path for airflow from the front of the car. I think with our car, airflow is still there just not as much as there could be with a full undertray and directed flow paths.
https://vorshlag-store.com/products/vorshlag-s550-brake-cooling-deflector-kit