Automatic Transmissions - Shifting Priorities

There was a time when automatic transmissions had all kinds of unique names-Cruise-O-Matic, Hydra-Matic, Ultramatic, and the list goes on. But while the names were different, the basic operation was about the same and they shared the same basic parts.

Today automatics are more often identified with a number rather than a name, and there are a number of numbers from which to choose when it comes to picking one for a street rod. But while it may seem there is a confusing array available, like most choices that have to be made, the more informed you are the easier it is to make a decision. To that end, we're going to examine the basic parts of an automatic, as well as the common modifications that are made, and take a look at the GM transmissions that are commonly used.

Parts Are Parts
While automotive transmissions may have two, three, four, or more gears, computer controls and a variety of bells and whistles, the basic internal parts remain the same-there are just more of them. Let's take a look at them.

Torque Converter
This is what connects the engine to the transmission. Not only does it allow the engine to keep running while the car is stationary, but it can also provide torque multiplication when accelerating from a standstill. For a simplistic explanation of how a torque converter works, picture what would happen if two electric fans were facing each other and one was turned on. If you say the other one would begin to turn, you've got the idea. In a torque converter, both fans are in a container: One is connected to the engine and the other to the transmission, and oil is involved rather than air.

As might be expected, there is some slippage inherent with a torque converter, so many now have a lockup device that hooks the transmission directly to the engine for increased mileage. There is also another reason. There is an increased load on the torque converter with the higher ratio provided by an overdrive Fourth gear, and some slippage takes place as a result; that slippage creates heat and that's the enemy of an automatic transmission. Slippage is eliminated, and so is the additional heat, by locking the converter with a hydraulically applied internal clutch.

Planetary Gears
As gears go, planetaries can do it all, and they are at the core of an automatic's operation. Made up of three elements-a sun, ring, and planet pinion gears-they can provide forward or reverse rotation, a speed increase, constant speed, or a speed reduction.

Three things are necessary to make planetary gears operate: an input (power from the engine), an output (power going out), and a reactor (one of the elements is held stationary). The gear ratio and the direction of travel depend on which element is performing each function. Most transmissions have more than one planetary gear set to provide a variety of gear ratios.

Bands and Clutches
Bands and clutches hold the reactors stationary. An automatic transmission goes into gear by holding one part of a planetary gear set 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.

Valvebody
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 car 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 what damages transmissions. Of course the perfect solution is a lockup converter with increased stall speed-that's the best of both worlds.

Shift Kits
Most stock automatics 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 release as the other begins to apply with a certain amount 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.

Shift 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. 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 shift kit theme is the hopped-up computer. It can and will do the same thing as a conventional shift 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 gear sets, 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:

Level 1
Described by Zack as one step above stock, these transmissions are completely remanufactured using improved Alto frictions and a mild shift kit. They are capable of reliably handling 300-plus horsepower and are perfect for daily drivers.

Level 2
Gear Star's number one seller, these transmissions are also rebuilt with all-new internal parts. In addition, the torque capacity of the clutch packs is increased by more than 70 percent, and a more sophisticated shift kit is used for a firm, positive shift. Horsepower capacity is 400-plus.

Level 3
Equipped with all hardened and stress-relieved hard parts-including shafts, drums, and planetaries-these transmissions are for rods that see street and part-time dragstrip use and are equipped with high-torque 600-plus-horsepower engines. Additional modifications include three different shift kits, expanded capacity clutch drums with Alto Red Eagle frictions, and Kolene steels.

Level 4
The King Kong of transmissions, these are built to withstand the horsepower a blown big-block can put out. Also popular behind Hemis and big-block Fords, these transmissions have all the Level 3 modifications plus a more aggressive shift kit.

An Overview Of Gm Transmissions
GM has produced a substantial number of automatic transmissions; these are the most popular for street rod use: