The Mk.XII was developed into the Mk.XIII for Formula racing with the addition of a Cosworth 12-bolt forged steel crankshaft, Cosworth forged conrods, wilder camshafts and 45DCOE Weber carburetors. The Mk.XIII became a big seller in 1965 when the SCCA created the Formula B category in America. The dominance of the Mk.XIII in Formula B was nearly absolute against its main rivals the Satta/Hruska-designed Alfa Romeo 105/115 DOHC unit, and the Alex von Falkenhausen-designed SOHC BMW M116 engine. This in turn allowed the now-viable engineering firm to reduce its previous near-total dependence on Lotus. The proliferation of the Mk.XIII triggered the establishment of new European style racing-engine builders in the US performing rebuilding and maintenance work needed on the Mk.XIII while also contributing to Cosworth's revenue.