Here are the basic components...
Here are the basic components in a performance torque converter. In the foreground is the Mopar starter gear/front cover. The turbine, splined for the input shaft, is at the upper right; it rides on the front cover. The stator (upper center) goes in the middle, and the impeller/rear cover assembly with the slip yoke for the transmission is at the upper left.
The next factor would be construction quality. You can buy converters from a variety of sources and in a variety of price ranges, but this is an area where generally you get what you pay for. A converter whose rebuilding process has been sloppy will result in poor performance and short life; in a high-horsepower environment, this is where you find out who did it right when the bad converter breaks. Prices are based on the level of modification for the most part, and time is money. Frank is honest to note that there are several companies capable of providing high-quality converters and components in addition to Dynamic. If you can afford the higher-priced converter, you'll be better off.
"For example, using a 9 1/2-inch unit with the more efficient positive blade adjustment angle will almost always be better than getting a 10- or 10 1/2-inch converter with negative angle," says Frank. "The smaller converter will give measurably increased performance on the track and yet be more efficient on the street. This is due to the fact that, though the converter is smaller and higher stalling, by using a positive blade angle, the fluid is directed off of the blades without having to overcome the negative angle. The smaller converter also has less reciprocal mass, meaning less energy is required to get it moving and it also won't build up heat as quickly. The positive angle adjustment is a process that involves actually bending one end of each blade upward; this is done by hand and requires skill and patience, hence the higher-end cost."
The turbine rides inside the...
The turbine rides inside the front cover. Note the amount of welding that has gone into the vanes. The positive angle bent is seen on the outside edges, allowing a smaller converter to operate more efficiently. The large steel area surrounding the input shaft splines is the anti-ballooning plate.
Another primary factor when thinking about selection is rear gearing. The higher-numerical gears obviously will give you an advantage in moving the car forward, so a given converter's stall speed will actually be less with a 4.10 gear than if it were sending power to a 3.23 gear. This is why stall numbers are more of an estimated than exact science; a car with the higher 4.10 gear will stall a converter quicker than than one which is slower to transfer power to the back tires. The 3.23 car will also put more load on the converter since it's not moving the mass as easily as the 4.10 gear, and if the converter is too loose (higher stalling), it will slip excessively in street applications.
Car weight plays a role as well, and for drag cars, aerodynamics become a factor at high speeds. Take two cars for example. One is a B-Body with a full interior, the other is an A-Body with some lightening; they have an estimated weight difference of 500 pounds. You have a driveline consisting of a 440 that makes 375 horses, a standard-ratio 7.27, and a 3.73 rear gear. To make that engine work efficiently on the street and strip, the converter will require additional stall in the lighter car, since, like the rear gear ratio, it's easier for the car to move forward. Also, since the B-Body is heavier, slippage will be greater in that application, meaning heat again becomes a real factor. A larger, more-effective-at-lower-rpm converter would be a better choice.