The impeller is connected to the engine. The higher the engine revs, the faster the impeller spins. It has shaped vanes that fling transmission fluid away from its center. Higher rpm impart more speed, and therefore more energy, to the fluid.
The turbine is connected to the transmission and ultimately the drive wheels. It catches the fluid coming off the impeller. That moving fluid spins the turbine, which in turn spins the output shaft and ultimately the drive wheels.
The turbine’s shape draws in transmission fluid from its outer diameter and forces it back toward the impeller along both components’ centerlines. As a result, the transmission fluid is constantly flowing from impeller to turbine along their outer edges, and from turbine to impeller near their centers.
It’s possible to achieve thrust with just these two components, but the inherent slushiness of a fluid coupling would make this setup inefficient and slow to react. The engine and impeller would always be spinning quite a bit faster than the turbine. That’s where the stator comes in.
This fanlike disc has vanes that reroute the transmission fluid’s return trip to the impeller, essentially sending it back along the centerline of the spinning assembly. After being diverted by the stator, the fluid hits the impeller blades with great force. This creates a hydraulic boost for the engine, causing what’s known as torque multiplication, and it’s most effective when the impeller and turbine are spinning at vastly different speeds–like when you hit the gas from a stop, for example.
Modern automatics also have a physical clutch plate that engages when the impeller and turbine are moving at nearly identical speeds. This negates parasitic loss while the car is cruising under minimal load, helping fuel economy.
One more element of torque converter magic is the stall speed. This is the point at which the automatic transmission goes from transferring minimal engine thrust to the wheel to dumping quite a bit of torque into the driveline. On a street car, the stall speed is generally set a bit higher than idle so the car transitions from stop to go without delay.
Drag racers, though, often play with special setups that allow for very high-rpm stall speeds. They allow the engine to spin up to its maximum torque point before that energy gets released through the torque converter to the drive wheels. A high stall speed is the equivalent of revving a manual-transmission car before dumping the clutch, but automatics benefit from even more pronounced torque multiplication through the stator once the wheels start spinning.
Automatic transmissions get a lot of flak outside of drag racing for their lack of responsiveness or direct control compared to a manual or DCT gearbox. However, there’s no denying their durability even under tremendous torque conditions–it’s hard to snap fluid, after all. And thanks to the torque multiplication effect, an automatic can make the most out of a high-powered launch on sticky tires.
