Just How Does This Thing Work?
Torque converters are something of a mystery for most of us. What's basically happening inside is fluid (and fluid alone) is converting that 500 horses and applying it to the back tires; there is no direct mechanical connection. The converter assembly is inside a circular housing that can be bolted or welded to the starter ring, which is, in turn, bolted to the flexplate and the crankshaft. Meanwhile, the transmission's input shaft is spline-fit to a floating turbine inside the housing that won't turn if there is no fluid. This turbine moves only when transmission fluid is thrown off by centrifugal force from a similar-appearing piece called the impeller assembly. As the impeller spins with the engine rpm, steel vanes, or blades, are directing and angling fluid outward in a clockwise direction. This fluid will be pushed toward the outside edge of the impeller assembly and forced into the turbine.

The vanes in the turbine are similar to the impeller, but are reversed to direct the fluid inward counterclockwise and, in turn, cause the turbine to spin; it's this force that will begin to turn the trans input shaft. The fluid is then directed to the center again, where it's forced back into the impeller to begin its travel to the outside edge again. A half-round trough called the split ring is mounted to the center of both the impeller and turbine, which keeps turbulence down and directs the fluid; the fluid recirculates through the outer and inner sides of the assembly.

Centered between the impeller and the turbine is a smaller piece called the reactor, or stator, which serves two purposes. First, its angled vanes reverse and direct the fluid flow coming from the turbine back to the impeller, but more importantly, the stator actually increases the torque. Inside the stator is a one-way clutch unit that consists of a splined inner race, an outer race fit into the stator, and a sprag, or roller clutch, separating the two. The inner race is spline-fit to a fixed support on the transmission's front pump and cannot turn; the stator rotates in just one direction because of the clutch rollers.

How does it work?
At idle, the impeller begins spinning fluid into the turbine, which, in turn, pushes it back through the stator, which is locked by the roller sprag. Once it reaches a certain fluid pressure level, the stationary stator vanes actually create vortex flow, slingshooting fluid at an ever-faster rate back into the impeller. In fact, the impeller efficiency is almost doubled as this centrifugally charged fluid is pushed into it, multiplying torque and spinning the turbine harder. All of this happens instantly as the engine rpm begins to increase, when you hit the throttle. Stall occurs until the turbine reacts to this torque multiplication and catches up to the impeller. Once the turbine and impeller are close to the same speed, the sprag will unlock and let the impeller, stator, and turbine turn together and work as a single transfer unit.

Now, what about stall speed? This is the speed at which the turbine overcomes its resistance and turns the input shaft. As the impeller spins, the turbine begins rotating but must overcome the resistance of the driveline. As the stator continues to accelerate the fluid, the impeller finally creates enough pressure to get the turbine/ input shaft to rotate. Stall speed is the engine rpm level at which this event occurs.