Fin Angle & Stall Speed
The converter on the left has a fin angle that is what is referred to as "negative," meaning negative as viewed from center. The converter to the right has a bowl that has a "positive" fin angle. The positive fin angle produces a lower stall speed. Converter fins are measured in degrees of fin angle for proper identification. Some are stamped with a number from the factory, and these can be used as identification of a bowl design or fin angle. The more negative a fin angle is, the less efficient a converter will be, especially at lower rpm. While it is possible to solely use fin angle to tune the stall speed, it's preferable to get things into the ballpark by selecting the proper converter shell diameter first, then fine-tune stall speed through fin angle, as Phoenix does.