Hey Chris. I believe the function of the 2 piston design comes into play on a 3-2 downshift. Picture in your head being in third gear. Oil pressure on both sides of the pistons in released position. A 3-2 is demanded. Oil pressure slowly drains from the release side. The small piston having the same area on both sides starts to move to apply the band because of the drop of release side pressure on that piston. The large piston has 2 different areas going on at this time. The apply side being smaller than the release side. The difference in areas on the large piston make it delay a bit as the pressure is dropping on the release side. When the pressure has dropped enough for the large piston to start moving to apply position, a space has opened up between the outer and inner piston. The band applies, the inner piston stops moving, and the orifice controls how quickly the outer piston catches up to the inner piston as the flow has now reversed going through the orifice. This gives a softer, longer apply of the band, giving the engine a chance to catch up to the gear ratio before the band takes a firm hold.
When you switch to a single piston design, you instantly notice how quickly the band takes hold on a forced 3-2 downshift. Many times the band takes hold too quickly, not giving the engine a chance to gain rpm to match the gear ratio and you get what feels like a quick engine braking sensation as the gear ratio is now pushing the engine until the engine can get up to speed. I know what you mean about understanding the function of the 2 piston design. It took me some deep thinking to figure it out. If you just slow everything down in your head, slowly releasing and monitoring pressures, you'll begin to see how it works. Pretty ingenious design really. They should keep that function incorporated in the super servos. I believe Sonnax gives you a choice of using the inner piston or not. I always use the inner, except in race only situations.
