To achieve different stall speeds, Autoflite Engineering uses only the methods we consider the most efficient, based on our research.
There are many ways of achieving the different stall speeds, the main one being to use smaller diameter converter shells than the original stock size. Other ways include altering the vane angles in the converter itself, and by modifications to the stator.
A variety of stators can be used for one type of converter, depending on required stall speed. Autoflite does not roll the outside edge of the turbine over, or increase the clearance between the impeller and turbine as some manufactures do.
While this does increase slippage (not stall), it restricts oil flow and coupling efficiency, creating the same effect as installing a slipping clutch in a manual car. As you know, slippage is not stall speed. A slipping converter, or clutch, will never lock-up.
An efficient converter slippage will have good stall speed and will lockup as well.