The torque converter has a third item called a stator that redirects the fluid as it leaves the driven torus and re-enters the drive side. Before the driven vanes start to move, the fluid velocity actually helps the engine turn and increases torque. Fluid couplings only have two parts, a drive torus attached to the crankshaft, and a driven torus splined to the transmission input shaft. The vanes of the torus are straight on a fluid coupling, curved on a torque converter. The stator has a one-way clutch built in so it cannot turn as it's redirecting fluid. Once the driven side of the converter starts to turn, the stator rotates with the assembly and the torque multiplication goes away. Brcause of the convoluted fluid flow, the torque converter cannot achieve a good lockup. There's always about a 10% slip between input and output speed.
Jerry
