These are high-gear accumulator...
These are high-gear accumulator pistons. The stocker at left has two O-ring grooves, while the CPT version sports four. Stock accumulator pistons can leak some, allowing high-gear slip, so CPT cures that with the taller, four-seal piston to assure line pressure.
Next, the Overdrive (Fourth gear) is achieved by applying a band around the direct drum (Third gear). This band is narrow at 1.5 inches wide, and it too often fails by slipping.
Furthermore, the valvebody is set up to supply 30 percent less line pressure to Third and Fourth (OD) gears. This was Ford's misguided attempt to increase fuel mileage via reduced power losses to drive the transmission's pump-miniscule mileage was gained at the expense of numerous Third and OD failures due to low clamping pressure.
If you fix those conditions, you'll find the AOD's two-piece input shaft is not quite the stuff of legend. Given just a little extra power and half sticky tires, the stock AOD input shaft will instantly snap at the first full-throttle shift. In addition, a stock AOD will not shift into Overdrive at full throttle (and if you try, you'll smoke the direct clutch pack), normal shifts are a bit squishy, and the shift pattern is undesirable.
Drilling some holes and substituting...
Drilling some holes and substituting some springs in the valvebody cures the AOD's typical lazy factory shift and allows full-throttle upshifts into Fourth-or OD as it's more commonly called.
Late in the game as we are these days-Ricky's coupe is 10 years old and has 125,000 miles on it-a few maintenance items are also important. As with all automatics, heat is the enemy, and the constantly moving fluid and slipping, power-on shifts lead to plenty of heat. Seals dry up and need replacement, the clutches are typically worn, and the stock oil cooler is woefully inadequate. The Throttle Valve (TV) cable is also prone to stretching around the 50,000-mile mark, so this is now a standard replacement item.
From a performance standpoint, the torque converter is also in need of upgrading. The stock 12-inch unit is most often replaced by a 10-inch converter with a touch more stall speed, so converter construction and integrity come into play.
Addressing these issues is relatively straightforward, with the typical trade-offs and costs associated with hot-rod parts.
The direct drive (Third gear) clutches and steels can be increased by recutting the drum's snap-ring groove or using a different drum, which is what California Performance Transmission does. This allows seven clutches and steels, which are also of considerably higher-grade material.
CPT builds its torque converters...
CPT builds its torque converters beginning with numerous stock-core units. These are cut open, and new impellers are built using custom-made, hand-set, furnace-brazed vanes as seen here. Once the insides are done, the housing halves are rejoined and welded.
CPT also has its favorite...
CPT also has its favorite stator. This too is an OEM part, but CPT whittles on it to tailor it to the application. The large, deep vanes can accelerate more fluid more quickly than the smaller, shallower-pitch vanes of the stock AOD converter.
Removing and installing the...
Removing and installing the AOD is textbook work. The driveshaft is removed, and all the usual disconnections are made. These include the speedometer cable, the fill/dipstick tube, the TV cable, the starter motor, the crossmember, and so on. The AOD has no drain plug, and CPT drain's the transmission on the bench after removal. You may want to temporarily drop the tranny's pan to get the lion's share out.