Brake Stall vs. Flash Stall:

Flash stall is the maximum your engine's torque can stall a torque converter. In essence flash stall and full stall are nearly identical. If you had a transbrake, you could find full stall by putting your foot to the floor and reading your tach. For argument sake, let's say we're testing a 3500 stall Yank ST 3500. If you had a transbrake, you would see around 3500 rpms. If your motor was at idle and then you suddenly floored the throttle, you might see slightly more (maybe 100 rpm more) stall for a half second as the momentum of the motor's internals "flashed" the converter a small bit above its true stall rating.

Brake stall, on the other hand is a very subjective thing. For most, it's the highest stall you can achieve before your tires spin. This varies greatly based on many factors: Traction, gearing, brake clamping force, and engine torque. With a ST3500, I may only be able to get 2200rpm "brake stall" on the street with street tires...any higher rpm and the motor torque would overwhelm the tires. But if I was at the track with racing slicks on the starting line, I might be able to get 3200 brake stall before the motor torque overwhelmed the tires. See...brake stall is very subjective.

Yank rates their converters based on their intended application. A ST 3500 will stall 3500 rpms in a stock LS1. If you had a 422 and wanted a ST 3500, the converter you received would still stall 3500 because it would be built around the torque of a 422, not a stock displacement LS1. Yank checks the stall of their converters and their competitors by using either a trasmission dyno or a "tranny tricker" in the vehicle tested. With the tranny tricker, you can place the vehicle in 2nd or 3rd gear and stab the throttle to the floor...making it easy to read both flash stall and full stall.

How a 12" converter can have the same stall speed as a 9.5" converter:

Two different sized converters can have the same stall speed, but the efficiencies will vary greatly. A converter pump will tend to have a higher efficiency when its blades have a positive angle to them. The positive angle feeds the most amount of fluid to the turbine. The more fluid you feed the turbine, the harder it pushes on it. The harder the turbine is pushed, the more torque is transferred to the transmission.

For a 12" converter that normally stalls at 1600, to be converted to a 2600 stall, most converter builders will bend the pump blades back to a negative angle to feed less fluid to the turbine. This means the pump will have to turn more rpms to force the turbine with the same amount of fluid as before. As you can imagine, the efficiency starts to drop off rapidly as you bend the blades more and more negative. You will lose rwhp.

A 9.5" converter stalls higher because it generates less fluid by virtue of it's smaller size. It takes more stall to achieve the same amount of hydraulic force of a larger 12" converter. Good thing about a 9.5" converter is that you can achieve very high efficiency in higher stall applications because the pump blades still maintain a very forward pitch to them (positive angle). So in essence, by bending the blades negative on a 12" converter, you are turning it into a heavy, inefficient, higher stall converter in comparison to a smaller diameter converter. This is the very reason why Yank Converters uses a special pump and stator combination to achieve high efficiency along with high stall in its Stealth line of 12" converters.