9ff is a German car tuning company, based in Dortmund, founded by Jan Fatthauer in 2001, specializing in converting stock Porsches into street legal racing machines, either as fully tuned cars, or selling individual parts or kits. They are best known for building one of the first cars to exceed 400 km/h (249 mph) - the GT9, which is capable of speeds up to 256 mph (412 km/h).