Bands and Clutches
Bands and clutches hold the reactors stationary. An automatic transmission goes into gear by holding one part of a planetary gearset stationary with a band or a clutch that is applied by hydraulic pressure. One reactor is released and another applied when the transmission shifts gears; the transmission is in Neutral if no reactor is applied. If you've ever been in a car and the transmission felt like it was slipping, that's exactly what was happening-the band or clutch pack wasn't holding the reactor stationary and it was slipping and not transferring full power. In an extreme case, the clutches, or band, don't hold at all, and one or more gears (and in some cases, every gear) stops working as a result.

The valvebody is the hydraulic "brain" of the transmission; it controls the shifting of gears by controlling which reactor is applied and when. Some transmissions use hydraulic pressure from a governor, throttle valve, or vacuum modulator to determine shift points, while many contemporary versions use computer-controlled electromechanical servos.

Transmission Modifications
Let's start with the torque converter. In many cases, the stall speed of the converter is increased. Simply put, stall speed is the rpm that the engine will reach with the transmission in gear, the brakes applied, and the throttle held wide open. The higher rpm simply allows the engine to produce more power, which will launch the truck harder from a standstill. The downside is that higher stall speed converters will slip more in "normal" use. That can create excessive heat, which is the cause of what damages automatic transmissions. Of course, the perfect solution is a lockup converter with increased stall speed-that's the best of both worlds.

Valvebody Improvement Kits
Most stock automatic are designed to be seamless; that is the shifts are smooth to the point of being hard to detect; this is done by timing the release and application of the various reactors. One may begin to apply as the other begins to release so there is a split second of overlap. While this results in a smooth shift, there is a certain amount of slippage that takes place in the process, which wears the friction surfaces.

Valvebody improvement kits generally do two things: They change the timing of the release and application of the reactors, which results in a firmer, more noticeable shift and may also lengthen the life of the transmission. In addition the hydraulic pressure to apply the reactors is often increased too, which means the clutches and bands have more holding power, thus increasing the torque capacity of the transmission.

With many transmissions today using computer controls, a variation on the valvebody improvement theme is the hopped-up computer. It can and will do the same thing as a conventional kit, but changes can often be made without dropping the trans pan.

Friction Surfaces
A common method to increase an automatic transmission's torque-carrying capacity is to improve the friction surfaces that hold the reactors. This may be done by using improved materials, more clutch plates, wider bands, or a combination of them all.

Hard Parts
Just as with a manual transmission, an automatic has mechanical components that are susceptible to damage when overstressed. As the horsepower applied to it is increased, it is often necessary to increase the strength of an automatic transmission's shafts, clutch drums, planetary gearsets, sprags, and other internal parts.

Tailoring A Transmission To Your Needs
Depending on the customer's needs, Zack Farah of Gear Star Performance Transmissions normally recommends one of the following levels of performance enhancement.