Formule Libre – or free formula racing.
Just before World War I, various A Grand Prix is a motorsport series sanctioned by the FIA and held in various countries for a world title.... There's more... started sharing some rules, mainly engine sizes and weight.
In 1924 the Association Internationale des Automobile Clubs Reconnus (AIACR) was formed to, unofficially, regulating Grand Prix racing. The regulations were abandoned in 1928, in an era known as ‘Formula Libre‘, when the organizers decided to run with virtually no limitations to allow teams to run with what they could get together.
Today clubs hold Formula Libre events allowing Formula cars of all ages, weights and engine sizes to take to the track together. For some of the unique cars, this is a rare opportunity for them to run.