The lineage of KZ traces back to 1974, when Formula C (FC) was introduced as the first gearbox class at the Karting European Championship. FC used a 125 cc engine configuration and reached the Karting World Championship in 1983, remaining part of it until 2000. After 28 seasons of competition under FC regulations, the CIK-FIA replaced the class with Super-ICC (S-ICC) in 2002, though that transition came with a demotion to World Cup status rather than World Championship status.
In 2007, Super-ICC was itself succeeded by KZ1, which restored the gearbox class to a higher profile in international karting. In 2013, KZ1 was rebranded simply as KZ and returned to the Karting World Championship, the position it continues to hold.
KZ karts are built around a 125 cc direct-coupled, water-cooled two-stroke engine with mixed lubrication. The carburettor has a maximum diameter of 30 mm, and the engine uses a valve inlet in the piston skirt. A six-speed sequential gearbox is operated via a manual clutch. Braking is handled by both front and rear hydraulic foot-controlled systems. The minimum combined weight of kart and driver is 170 kg, and competition tyres are premium medium-compound rubber.
All major components — engines, chassis, bodywork, brakes, and tyres — are subject to the CIK-FIA homologation system. Each season, tyres, fuel, and the KZ carburettor are selected through a formal call for tenders. In 2024, the CIK-FIA mandated the use of Dunlop tyres in international KZ competition, a move that drew criticism from several drivers and teams ahead of the Karting European Championship.
KZ has produced some of the most accomplished names in motorsport. Gianluca Beggio holds the record for the most KZ World Championship titles, with five. Max Verstappen, the four-time Formula One World Drivers' Champion, is a KZ World Champion and remains the only driver to win both a CIK-FIA gearbox karting title and the Formula One championship. Two-time World Touring Car Champion Gabriele Tarquini also features among the class's World Champions.
At the European Championship level, Gianfranco Baroni and Francesco Laudato share the record for the most KZ European titles, each having won four.
KZ occupies the top of the international karting pyramid on the gearbox side and serves as a credentialing ground for drivers aspiring to professional single-seater careers. Its technical demands — sequential shifting, clutch management, combined with the physical loads of a 125 cc gearbox kart — distinguish it from direct-drive classes and make it a benchmark for raw driving talent and mechanical aptitude. The class's longevity across five decades and multiple regulatory eras reflects its central role in defining elite international karting competition.