Torque converters have 2 stages, a fluid coupling that occurs first when you are starting, accelerating or pulling up a big hill. When you are cruising at a constant speed the lock up portion of the converter is engaged and 100% of each rpm is transferred through the converter, just like through a standard clutch. BD engineers have redesigned the fluid coupling by reshaping the stator fins to achieve a higher ratio of rpm transfer through the converter when in a fluid coupling. The lock up clutch material and surface area is enhanced to hold the high engine torque on performance Diesel engines. Many of the models contain a new billet front cover that eliminates clutch surface warping and mounting pad weakness.