HEWLAND MK 8/9, LD 200 AND WEBSTER GEARBOXES. EARLY. It has been proven that an Open Differential is faster on oval tracks than a spool, cam and pawl, or salisbury differential, as the open diff frees up the car to run fast on the straight, without binding in the corners. An aluminum Differential Housing can save as much as 7.5#. For classes requiring an open differential, the lighter weight insures faster acceleration and better braking.