In addition to the restrictions on the engine, it is required that an engine operating on gasoline as a fuel type, must have an air intake restrictor through which all of the air entering the engine must pass. On the intake manifold system, the restrictor has to be placed between the throttle mechanism and the engine itself. It must also not be movable or flexible in any way. This restrictor has to be circular in shape, and limited to 20mm in diameter.