At race speeds (say over ~150 mph), you don't even need a fan or any active mechanism. Just passive venturi channels on the underside can generate so much down force they are strictly limited in F1 because they can easily enable cars to sustain extremely unsafe speeds.

There are some McLaren street cars that use passive venturi channels which from their shape alone, generate more down force at high speeds than the car's weight. In theory, the car could drive upside down (if there was a Daytona-sized race track shaped like a ~30 ft diameter pipe).

Yeah, it turns out that in very specific and controlled situations, using a fan to make downforce is great. It's also a very obvious thing to do in those situations and doesn't do much for you otherwise.

Imagine a normal car having to carry around the added weight of fans, COPVs, and the plumbing to make it work. And only being able to use those only if the track is clean and there are no other cars nearby.

The Chaparral 2j is one of the most inventive and cool looking cars I've seen.

Not f1, Can-Am but yeah, I'm really surprised this isn't a more common approach to modern track cars. Obviously banned in most motorsport but I'm surprised with all of the one-offs these days that it's not more common.