ALL ABOUT GM AUTOMATIC TRANSMISSIONS
By Ron Ceridono
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 converter: 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 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.
band 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.
valve body
The valve body 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.
valve body improvement kits
Most stock automatics are designed to be seamless; that is the shifts are smooth to the point of being hard to detect; this 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 the results in a smooth shift, there is a certain amount of slippage that takes place in the process, which wears the friction surfaces.
Valve body 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 valve body 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.
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 use.
level 2
Gear Star’s most popular offering, these transmissions are also rebuilt with all new internal parts. In addition, the torque capacity of the 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 drag strip use and are equipped with high torque, 600-plus horsepower engines. Additional modifications include three different 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 custom classic truck use.
powerglide
First produced with cast-iron cases until 1962 and then from 1963 up, aluminum. In the 21st century these transmissions have two primary applications—restorations of original vehicles and racing. The early versions are often redone for Tri-Five Chevy restored (Gear Star routinely rebuilds Powerglides and other vintage transmissions, so call for more information) and there is practically an entire industry devoted to building Powerglides for drag racing.
hydramatic
The original Hydramatic was a tough but complex four-speed automatic transmission used in a variety of GM vehicles into the 60’s, including Chevy and GMC pickups, as well as other makes, such as Lincoln. These transmissions were so tough that they became some of the first automatics to find favor in drag racing (the B&M Hydo-Stick was based on the Hydramatic). The Hydramatic was replaced by the much less complicated and more sophisticated Turbo-Hydramatic. As an aside, occasionally vehicles that should have been equipped with Hydramatics are found with a different transmission. A fire (Editor’s note: The 1953 General Motors Hydramatic transmission plant fire in Livonia, Michigan, is to date the biggest business interruption insurance loss in U>S> history. I thought you guys should know that.) Destroyed GM’s Hydramatic plant so Oldsmobile’s and Cadillac’s used Buick’s Dynaflow and Pontiacs used Chevrolet’s Powerglide.
turbo 350
These small. Compact, and affordable three-speed transmissions were the most popular custom classic truck transmission before the availability of overdrive automatics. Still a viable choice, Zack has seen a resurgence of interest, and they seem to be making a comeback. The Turbo 350 is a great buy at just a little more than a grand, and comes complete with converter, TV cable, dipstick and tube, dust cover, cooler, and fluid.
turbo400
Physically larger and able to handle more horsepower than the 350, the Turbo 400 isn’t as efficient, but it is tough. These three speed transmissions are still an excellent choice for high horsepower applications, where overdrive isn’t necessary.
200-4r
According to Zack, this underrated transmission is the best four-speed overdrive automatic that GM has come up with. Because of its compact proportions, many people assume it’s not particularly strong; however, they will handle 800 horsepower or more with the proper modifications. Extremely efficient, thanks to a unique bellhousing bolt pattern; it will bolt to any GM engine.
770-r4
While this is one of the most popular of the overdrive automatics, the 700-R4 does have a reputation for having a difficult-to-adjust throttle valve cable. As improper adjustment can cause erratic shifting, Gear Star has developed the Fall Safe Maximum Pressure Valve Body System, which reverts to high pressure in the event of a misadjusted or damaged throttle pressure cable. A vacuum lockup system is also used that denies lockup to the torque converter under aggressive throttle application.
4l60
The latest version of the 700-R4, the 4L60 features a more sophisticated valve body, but is otherwise the same (including gear ratios).
4l60e
A more refined version of the 700-R4/4L60, the E-series transmissions use an electronically controlled valve body. The TV cable is eliminated, but a standalone computer is required.
4l65e
There are two models of the 4L65E transmission; the early version bolts to the LT series of blocks, and the later version uses a longer input shaft and is used with LS engines.
4l70e
Found behind the LS7 engine in the new GTO, Trailblazer SS, and SSRs, this newer version of the 4L65E uses larger front and rear five-pinion planetaries.
4l80e
The big brute of automotive automatics, this transmission is larger than the 700-R4/4L60 series and is capable of handling well over 1,000 horsepower when properly modified.
As you can see, there are many, many choices when it comes to GM automatic transmissions and the modifications that can be made to them. Of course if you’re still having trouble deciding, give Zack a call and be prepared to provide as much information as you can, including the approximate weight of the vehicle, horsepower, rear gear ratio, rear tire diameter, and its intended use. You can look forward to lots of trouble-free miles in your custom classic truck with the right transmission and the appropriate modifications for the applications, and you won’t have to shift for yourself.
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