The rules varied from country to country and race to race, and typically centered on maximum (not minimum) weights in an effort to limit power by limiting engine size indirectly (10–15 L engines were quite common, usually with no more than four cylinders, and producing less than 50 hp).
As regras variaban dun país a outro e dunha carreira a outra, e polo xeral centráronse nos pesos máximos (non mínimos) nun esforzo por limitar a potencia limitando o tamaño do motor indirectamente (os motores de 10-15 L eran bastante comúns, polo xeral con non máis de catro cilindros, e producían menos de 50 CV).