Acceleration
The acceleration stage is, well, when the vehicle is accelerating. Unfortunately the torque multiplication will drop during this stage. The vehicle will begin moving, but there is a big difference in turbine and impeller pump speeds. This is why drag racers bring up the engine rpm to the optimum stall speed, launch the car, and then increase the engine speed to take advantage of the engine's peak horsepower from there on out.
Under normal driving conditions, your automatic will shift during the optimum stall speed to take advantage of all the "extra" torque made by the transmission.
Coupling
The coupling stage is when the turbine is within about 90 percent of the pump speed. At this point, if your torque converter has a lockup clutch, it would engage and basically fix the turbine and pump together via a small clutch mechanism between the cover and turbine. Under lockup there is no slipping between the two halves of the converter and efficiency inside the converter as well as fuel efficiency goes way up-as much as 65 percent, according to some manufacturers. With today's crazy gas prices, you might wanna upgrade to an overdrive transmission with a lockup converter if you haven't already!
What Else To Look For
Besides stall speed, there are many other factors to consider when buying a torque converter. Most of this has to do with the quality of parts used to build it and efforts taken to make a converter work within and withstand high torque and horsepower applications. Most nonstock converters can be considered performance pieces, and will most likely feature improved construction like furnace-brazed fins in the pump and turbine. An OEM converter has fins that are basically held in place by bending the ends over. If you physically grab one of these fins it will have some movement to it. A quality aftermarket piece should at least have had its fins furnace-brazed in place, which is the first step in the right direction. This will help reduce failure and greatly increases strength since each fin now acts as a gusset in the housing. The next step for increased strength is to actually TIG-weld the fins as much as possible in addition to furnace-brazing.
Another area of concern is to look for improved internal parts like thrust bearings, hubs, and lockup components. While it is common practice to refurbish and improve OE parts such as turbines, covers, and stators, you should be buying a converter that has had its other internal parts upgraded. After all this, look for balloon plates and even billet steel covers on high-performance converters where high pressure and increased heat and rpm are common. This brings us to one last thing to think about-heat. Heat (fluid temperature) is the number-one killer of an automatic transmission and should be taken seriously, especially when you start upgrading your torque converter and/or transmission, but that discussion will be for another month.
Education and help from the pros will be your biggest help in buying the right torque converter, so give them a call and see what your options are!
 ...and a B&M piece with furnace-brazed...  ...and a B&M piece with furnace-brazed fins. Notice anything different? Buying a converter with furnace-brazed fins is the first step to a high-quality converter. |  This turbine is out of B&M's...  This turbine is out of B&M's 8-inch race converter with a 5,100 stall speed. Notice all the TIG welding on the fins. When you start getting into high-performance converters, it's work like this that you'll be paying for. |  Gear Star manufactures its...  Gear Star manufactures its top-of-the-line Yank Racing converters using a CNC-machined billet cover, high-end Torrington bearings... |
 ...and hardened input and...  ...and hardened input and turbine hubs to help ensure quality and strength. |  | |