The overdrive transmission was Detroit's secret weapon in battling fuel economy regulations in the '80s, but it soon became a means for hot rodders to have their cake and eat it too. Initially, Detroit merged the new overdrive transmissions with lock-up torque converters and 3.08:1 final-drive ratios for maximum thriftiness, but just a few years later, the overdrive automatic became an integral part of Detroit's musclecar renaissance, allowing the return of factory-issued 3.42 and 3.73 gears without upsetting CAFE standards. Car crafters were soon installing these same overdrives in their older street machines to permit leisurely cruising on the highway while still running deep gears.
The first overdrives were mechanically controlled, like the two- and three-speed automatics they replaced. But in the '90s, Detroit made the switch to computer-controlled transmissions that rely on electronics to determine shift timing and integrity. Of course, this requires an electronic processor, and most of the vehicles that were originally equipped with these transmissions contained the required electronics in the same computer that controlled the engine. In the case of General Motors, the popular TH700-R4 four-speed automatic evolved into the 4L60E. The change in the name is mostly due to a change in GM's transmission nomenclature, as a 4L60 is actually basically the same as a TH700-R4; it's the "E" part-denoting electronic controls-that makes all the difference. The TH200-4R didn't make the transition to the electronic age, but on the heavy-duty side, GM introduced the 4L80E, an overdrive version of the tough TH400.
From the factory, the 4L60E saw use in the Camaro/Firebird twins, the Caprice/Impala line, and the Corvette, as well as in light-duty pick-ups and SUVs. The 4L80E has been limited to heavier truck applications, like 31/44-ton pickups, Suburbans, and the like. The good news for owners of these vehicles is that now, instead of having to delve into the transmission itself in order to alter the timing of the shifts and increase their authority, the transmission can be reprogrammed to achieve the same results. However, the bad news was the average Joe had no way of doing this on his own, at least initially. Fortunately, the aftermarket came to the rescue, offering several means of plugging into an electronically controlled vehicle to achieve more performance-oriented shifting. Most of these are more involved than the old chip swap methods of reprogramming, since factory electronics are significantly more sophisticated. We've rounded up the current crop of transmission tuners for the GM electronic automatics and listed the features of each for your perusal. Now, there are no more excuses for mushy, premature shifts.
Jet PerformanceAlthough known for late-model performance enhancements, Jet offers a wide variety of hardware upgrades for a broad spectrum of vehicles. Included in its palate of services are complete automatic transmission packages (overdrive and non-OD), overdrive transmission swap kits (for non-OD vehicles), carburetor refurbishment and power tuning, ignition enhancements, and fuel-injection upgrades. There are actually several items listed in Jet's catalog to improve the performance of GM's electronic overdrives, but we'll focus on the Power Shift module.
What it is: Jet Performance Power ShiftWhat it does: Provides the ability to select between four different settings for progressively increased shift firmness. The first setting is stock, which is followed by Stage I, II, and III, with the third stage providing the hardest shifts.How to install/use it: The Power Shift unit simply connects inline with the transmission's original wiring harness. It's a plug-in deal, so no splicing is required, and it can be removed at any time to return the vehicle to stock. The unit is controlled via a dash-mounted panel, with indicators to show which stage is engaged. Changing modes is simply a matter of touching a button.