Diesel Pro Stock Tractors are limited to one turbocharger and diesel fuel is the only allowable source for power, in keeping with the 'spirit' of the original tractors. The maximum engine displacement is . These engines can achieve around and of tourqe In recent years, new classes have been created to appeal to different groups of pullers. There is now a class called Limited Pro Stock that is limited to engine and turbocharger. This class typically pulls at and is slightly restricted, as opposed to the Pro Stock class, which can run up to engine and an unrestricted size turbocharger, along with intercoolers.