The Three Stages Of Torque Converter Operation
Now that we have a basic idea of what's inside a torque converter and their basic functions, let's look at what a torque converter is doing while in use.
Stall
The first of the three stages is stall, and just what is stall, you ask? I didn't know myself and it has always been something that's baffled me (I know I'm not alone). I again turned to Stanley Poff at TCI, who had this to say: "Stall speed is the rpm that a given torque converter (impeller pump) has to spin in order for it to overcome a given amount of load and begin moving the turbine (and in turn the wheels. -Ed). When referring to 'how much stall will I get from this torque converter?', it means how fast (rpm) the torque converter must spin to generate enough fluid force on the turbine to overcome the resting inertia of the vehicle at wide open throttle. Load originates from two places; the first is from the torque imparted on the torque converter by the engine via the crankshaft. Second is from inertia, the resistance of the vehicle to acceleration, which places a load on the torque converter through the drivetrain. This can be thought of as how difficult the drivetrain is to rotate with the vehicle at rest, and is affected by car weight, amount of gear reduction and tire size, ability of the tires to stay adhered to the ground, and the stiffness of the chassis." Boiling what Stanley had to say, we could look at stall as when the trans is in gear, but the brakes stop the vehicle. Under stall conditions and increasing rpm to the converter's stall rating enables the torque converter to multiply the engine torque to its full potential-as shown on the dyno-until the torque exceeds the brake's holding power and the tires start to spin.
Since most V-8 engines make peak torque numbers at lower rpm, that is the area where you want the maximum benefits from stall, so the converter can multiply the engine torque to the max and put that power to the tires.
Generally, a stock converter will have a stall speed of 1,800-2,000 rpm. When you add a mild cam, carb, and so on, or anything that's going to increase the torque of the engine, you want the stall speed to be just above it, not even or below, to reap the full benefits of torque multiplication. People have the misconception that the transmission will slip when rpm drops below the stall speed, but that's not the case. You can have a high stall converter on the street as long as your drivetrain is built to suit and vice versa. Stall also only occurs under full acceleration and not part throttle.
You may hear someone refer to flash stall, which is basically the rpm level the engine reaches when accelerating from a dead stop to when the vehicle begins to move. Of course, there are many options when it comes to picking a stall speed; this is where you need to get on the phone with the manufacturer and discuss your needs with them. You need to be prepared to give them your engine, transmission, rearend, and tire specs as well as vehicle weight if possible and your intended use, so they can get you the right converter. Weight actually has a big effect on converters. Greg Ducato at Phoenix Transmission Products says: "A 2,500-rpm stall speed converter in a T-bucket will probably stall around 1,800 rpm, but put that same converter in a pickup and that'll go up to around 2,800 rpm."
When talking to Gear Star Inc.'s Zack Farah about stall, he had this to offer: "The misconception of stall torque ratio (torque multiplication) is that more must be better. This is not always the case. High stall torque ratio applications are typically for industrial equipment or engines with limited low-rpm engine torque. With high stall torque ratio converters, there are important trade-offs. What you take at one end, you give up on the other. Typically, a torque converter with a very high stall torque ratio, such as 2.0-2.5, will be much less efficient above its rated stall speed. There is a sacrifice in higher rpm efficiency to achieve high stall torque ratios. That lower efficiency translates into less horsepower transmitted to the tires over an rpm range." This reinforces the need to get what you are actually going to use and not the "biggest and baddest."
 ...A lockup mechanism is basically...  ...A lockup mechanism is basically a little clutch that when engaged at freeway cruising speed keeps the pump and turbine spinning together to avoid slipping, which would be a waste of power for cruising. A lockup can improve fuel efficiency as much as 65 percent on the open road! |  It is the stator that makes...  It is the stator that makes a torque converter a torque multiplier and not just a fluid coupler (which has no torque multiplication capabilities). Couplers were used in the very early automatic transmissions like the original Hydramatic, according to Kenji Takahashi at B&M, the primary company responsible for turning the early Hydramatic into the infamous Hydro-Stick and making automatics competitive for drag racing back in the day. In essence, the stator enables the transmission to put out more torque than the engine is producing, which is obviously a good thing! |  I drove up to B&M in Chatsworth,...  I drove up to B&M in Chatsworth, CA, to see their torque converter dyno in action. For many years this was one of the very few torque converter dynos in the country, and has served many, including some of the Big Three... |
 ...The readouts on the left...  ...The readouts on the left show the dyno results. The engine's torque coming into the converter is 234 lb-ft (B&M's baseline), and under stall at just over 3,000 rpm, the torque converter multiplied the engine torque by almost double (1.979 times, to be fairly precise)! This torque multiplication efficient is why you want a good torque converter in your automatic transmission. |  On the outside of the pump...  On the outside of the pump housing of the same B&M racing converter is what's known in the industry as a balloon plate. This is an extra steel plate welded to the housing to double the thickness of the area to prevent the converter from ballooning and possibly exploding during high-pressure situations in racing. |  Here's a stock impeller pump...  Here's a stock impeller pump on the top and its fins-loose fins, that is-... |