GM Automatic Transmissions - Shifting Priorities

How times have changed-when most of the vintage trucks we know and love were new they were likely equipped with "three on the tree" or "four on the floor" manual transmissions. As automatics became more common in the early 50's, trucks wore badges like Fordomatic or Hydramatic, but that was then and this is now. Today, most of us want to put a more modern transmission under the floorboards of our custom classic ride, particularly if the rest of the running gear is being updated.

Modern automatics are more often identified with a number rather than a name, and there are a number of numbers from which to choose. And while the array of offerings may seem confusing, 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, some just have more than others.

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. As the one under power began to turn and move air, the other one would also begin to turn. If you can visualize that, you've got the basic idea of how a torque converter works. 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 used rather than air.

As might be expected, there is some slippage inherent with a torque converter, so many overdrive automatics now have converters with a hydraulically applied internal clutch that hooks the transmission directly to the engine for increased efficiency. While better fuel mileage is often considered to be the advantage of a lockup converter there is another, often overlooked, purpose. The higher ratio provided by an overdrive Fourth gear can put an additional load on a conventional converter causing excessive slippage even in light-throttle, cruise conditions. That slippage creates heat and heat's the enemy of an automatic transmission. When used with an overdrive automatic a lockup style will lower engine speed in cruise conditions and lengthen the transmission's life. Overdrive automatics and lockup converters are made for one another.

Planetary Gears
As gears go, planetaries can do it all. 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.