Power-glide is a two speed automatic that uses a torque converter as the hydraulic coupling. Hydra-Matic (as original in our old trucks) is a four speed automatic that uses a torus. The arrangment of the torus is such that there is no slippage in either 3rd or 4th gear. The complete Hydra-Matic probably weighs 200 punds more than the Power-Glyde.