One of the most common components in today's classic trucks is an overdrive automatic transmission, and one of the most popular of the breed is the General Motors 700-R4. Although the 700-R4 is a good transmission and can be modified to withstand huge amounts of horsepower, reliability is sometimes an issue, but it's often not the transmission's fault.
Although it seems strange, many 700-R4 failures are the result of the carburetor in use; let us explain. All automatic transmissions use hydraulic pressure to apply friction components, either bands or clutches, to shift into each gear. The 700-R4 (as well as the 200-4R and the 4L60) uses a cable (called the throttle valve, or TV cable) connected to the throttle linkage to establish that pressure. While the stock GM induction components (carburetors or fuel injection) have the correct levers, those on many aftermarket carburetors don't have the proper geometry. As a result, shifting can be erratic, and at part throttle the hydraulic pressure may be low enough that the transmission slips, which results in overheating that can cause severe damage and ultimately complete failure.
As production cars have become more sophisticated, it's no surprise that the latest transmissions don't use a TV cable and are now computer-controlled. And it's also no surprise that the same level of sophistication has come to the aftermarket. Electronically controlled transmissions such as the 4L60E and the 4L65E can easily be adapted to street rods with the use of a standalone computer. But while these transmissions benefit from modern electronic technology, they can still benefit from some old-fashioned hands-on hopping up.
While the current crop of electronic overdrive automatics don't have the issues associated with TV cables, they do have a reputation for being unable to handle lots of horsepower. But according to Zack Farah of Gear Star Transmissions, that shortcoming can be resolved.
The process of strengthening these transmissions starts with disassembly and a complete cleaning and inspection of the case and any of the parts that will be reused. But where the increase in strength comes from can be found in the long list of special parts Gear Star installs. To understand why some of them are necessary, a basic understanding of transmission operation may be helpful.