The torque converter is basically composed of a cover, a turbine, a pump impeller, and a stator. The cover is the outside half of the torque converter that mounts to the flexplate. The turbine is fitted into the cover and rides on a spline fit to the transmission's input shaft; during engine operation, when the turbine moves the car moves. The impeller is the half of the converter that mounts toward the transmission. Mounted inside the impeller pump is a series of longitudal fins that direct fluid around the outside diameter and into the turbine. The size of the converter and the angles of the fins all change how the converter operates. The stator is mounted between the impeller and the turbine.
Both the impeller and the turbine have vanes that direct out from the center. As the engine is operated, the pump impeller forces oil in between the blades to turn the turbine runner and transfer power from the engine to the transmission. During this fluid-pumping process, the stator serves a very important function: It multiplies torque by redirecting the oil flow into the impeller's inlet fins in the rotational direction of the engine. So that the stator cannot counter-rotate, it is mounted on a one-way clutch called a sprag. This stator/sprag quality allows an engine equipped with an automatic transmission to deliver more torque to the rear wheels than the engine actually produces. This happens when the torque converter is in its stall mode (the turbine spinning much slower than the pump) and while the vehicle is accelerating.
We can assign a value to the torque multiplication with a simple equation: Output torque input torque = torque ratio. Let's assume we have 550 lb-ft of output torque off the end of the transmission and 240 lb-ft of input torque. With our formula, we find that 550 240 = 2.29. Most torque converter ratios are in the neighborhood of 2.0 to 2.5. Think of a torque ratio as an extra lower gear in your transmission that your buddies with manual transmissions don't have.
The stator multiplies the torque the engine produces, which helps launch the vehicle. If the one-way clutch (sprag) fails, the vehicle will have a very slow launch, with poor reaction and 60-foot times. This stator features needle bearings to withstand more pressure with less internal drag than OEM-style thrust washers, which can also flake off material. | 
Fin angle plays a big role in a torque converter's stall speed, but ultimately the stall will be dictated by the vehicle and engine combination. The splined input area (center) is where the turbine mounts to the input shaft to spin the transmission. | 
While this converter is rated at 2,800-3,000 rpm, the actual stall in a vehicle may vary. Because of so many different combinations, a torque converter's stall speed does not have a preset value from the manufacturer. Instead, a unique design will produce a certain range of stall speed depending on the application (load placed on it). The load is a result of the engine torque and the vehicle's resistance to acceleration. |