In a video about the Chaparral 2J, they point out that it would do 40mph with the jets from the two powered fans alone.
Donut Media: How Vacuums Made This Car Fast
What made it competitive was a complicated linkage system that decoupled the skirt from the body roll.
I was involved in the 1990s with an RC race car team. We had a car that would passively suck dirt 3" (the height of the wake) out of the pavement and leave a trail hanging in the air just like the 2J.