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 transmission 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 carburetor 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 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.
Automatics use planetary gearsets to establish the ratios for each shift. Planetary gears are unique in that they can provide forward or reverse, a gear reduction, direct drive, or overdrive, all that changes is which element is the input, which is the output, and which is held stationary. With multiple planetary gearsets (which most transmissions have), a variety of gear ratios are available. If nothing is held stationary, no power is transmitted that’s what happens in neutral.
When an automatic shifts, one combination of elements is released and a different combination is held. That’s what the clutch discs and the band so—they hold elements of the planetary gearsets. What’s important to understand is this. When a transmission slips, it means the clutches and bands are not holding, thereby creating added transmission heat, which can damage those components. In the worst-case scenario, the clutches and bands won’t hold in at all and that particular gear doesn’t work at all.
To withstand increased horsepower and the demands the increase places on the clutches and band, Gear Star replaces all the friction materials with Alto Red Eagle Racing products. In addition, hardened seals, extreme-duty springs and sprags, along with special hardened shafts and drums are installed.
Another factor that figures prominently in automatic transmission performance is the hydraulic pressure that applies the clutches and the band. That’s why Gear Star uses increased-capacity servos and special pumps. But it takes more than increased internal pressure to make an automatic bulletproof, where and when that pressure is applied is critical. To that end, reprogrammed valvebodies—the hydraulic brains of the transmission—are installed.
Of course, electronically controlled transmissions require a computer, and Gear Star uses one of the best—the Compushift Transmission Controller by HGM. It comes with factory-set programming, all the necessary wiring harnesses, and a throttle position sensor. Shift firmness and the speed at which they occur can be adjusted to suit individual preferences. In addition, paddle, manual, and automatic shift modes are built in, as well as converter lockup controls.
One of the most unique component Gear Star uses is their 10-inch lockup torque converter. It combines all the performance advantages of higher-than-stock stall speed with the mileage benefits of the lockup feature. To understand why these converters are unique, we’ll look at each of the functions. Let’s start with stall speed. Simply put, stall speed is 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 can create heat, which is what shortens transmission life. On the other end, the lockup feature eliminates all slippage at cruising speeds (usually only in third and fourth gear), which results in reduced rpm and increased mileage. These converters really do provide the best of both worlds.
GM’s 4L60E and 4L65E offer all the advantages of an overdrive automatic with none of the disadvantages inherent in the early designs. Indeed, these may be the perfect automatic transmissions for many classic trucks, and with the special components Gear Star puts inside of them, handling huge amounts of horsepower with unparalleled reliability is no longer an issue. Here’s a look at how Gear Star does it.