We’ve always had great luck with Detroit Truetrac units, so we saw no reason to try something else. Its planetary helical side gears and pinions make this the perfect differential for a street car that likes curvy highways and the occasional road course. Under normal conditions the Detroit Truetrac performs like a regular open differential and, when required, automatically transfers torque to the wheel with the most traction. The Truetrac is capable of transferring up to 3.5 times more torque to the high traction wheel. This torque transfer ratio, called the bias ratio, is accomplished by using parallel-axis, planetary helical side-gears and pinions that mesh much like worm gears. Since the Truetrac uses hardened gears against hardened gears and not clutches or cones, there is nothing to wear out, so it won’t need maintenance. Eaton offers a preloaded and non-preloaded version for the 9-inch Ford. Because the preload helps the differential react faster, Currie offers it exclusively.