Its technical design was an evolution of the successful Fiat 600. The internal name for the Fiat 600 development project was “Project 100” and consequently, the internal Fiat codename for the 850 project was 100G (G was a follow on of model designations for the 600 which ran from A to F). The engine of the 850 was based on that of the Fiat 600, but had its capacity increased to 843 cc.