What is a torque converter besides a big round thing used with automatic transmissions that holds a lot of tranny fluid and makes a mess if you spill it? I wasn't sure either; maybe this is due to the fact that I've only owned one or two vehicles out of 15 or 20 that were missing the third pedal and never had any problems with torque converters. I just haven't ever had to cross the converter bridge-until now.
I was unsure of where to start so I called the experts at TCI, B&M, Gear Star, and Phoenix Transmissions to get the straight scoop. Stanley Poff at TCI had this easy analogy to put the torque converter into laymen's terms: "One of the best explanations of this is the two-fan explanation. Set two fans up facing each other and turn one on. As the air starts to move through the fan blades of the fan with no power, the blades start to turn. This is exactly what happens in a torque converter. You can also take a piece of cardboard and redirect the airflow and speed the nonpowered fan blades up or down. We do this in the converter by constructing stators with more or less blades, or by redirecting fluid flow based off of 2 1/2-degree increments of angle in the blades of the stator." Sounds simple enough, huh? There's obviously more science and engineering that goes into a converter, but at least we have a starting point. Let's take a look at the four basic parts that make up a torque converter.
Impeller Pump
We've probably all heard someone refer to the pump in an automatic, which is actually a very simple piece compared to what you may conjure up in your mind. The impeller pump is the outside half (housing) of the converter on the transmission side of the center weld line.
Inside the impeller pump are a series of fins that centrifugally drive the fluid toward its outside diameter and into the turbine. The size of the torque converter (and pump) and the number and shape of the fins all affect the characteristics of the converter. If long torque converter life is an objective, it is extremely important that the fins of the impeller pump are adequately reinforced against fatigue and the outside housing does not distort under stress.
Cover
The cover (aka front) is the outside half of the housing toward the engine side from the center weld line where the cover is welded to the impeller pump.
The cover serves to attach the converter to the flywheel, which is bolted to the engine's crankshaft, and is the other half of what holds in tranny fluid. This cover doesn't serve a performance purpose per se, but it is important that the cover remain rigid under torsional and thrust stress incurred by the tremendous hydraulic pressure generated inside the converter.
Turbine
From the inside, the turbine looks remarkably like the impeller pump. The turbine spins within the confines of the cover and transmits power to the rest of the drivetrain via its splined hub, which the input shaft of the transmission fits into. When the turbine moves, the rear wheels move, whereas when the engine moves, the impeller pump moves with it since it's welded to the cover, which is again attached to the flywheel.
Stator
The stator, which slides onto the input shaft of the transmission and is between the turbine and impeller pump, changes fluid flow between the turbine and pump, in turn multiplying engine torque. The stator looks simple, but the design, size, as well as the number of and angle of its fins dictate how it multiplies engine torque. A torque converter without a stator would not be capable of multiplying torque and would be called a fluid coupler.
 |  From the outside, the impeller...  From the outside, the impeller pump looks like, well, a torque converter. |  Before the impeller pump and...  Before the impeller pump and cover are welded together, this is how a torque converter stacks up. |
 Clockwise from the top: the...  Clockwise from the top: the cover, the impeller pump, the stator, and the turbine. |  But inside is a whole 'nuther...  But inside is a whole 'nuther ball game. All the fins, their placement, and angles mixed with the turbine and a good dose of centrifugal force are what get the transmission fluid flowing. |  This shows the inside view...  This shows the inside view of the cover in a B&M Turbo Holeshot 2,400 converter that has a lockup for freeway cruising... |