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).