OEM manufacturers did not come to the current state of the art in transmission and drivetrain technology in a single leap. The evolution began with overdrives, often without the lockup feature, meaning driving continuously through the converter stall just going down the road. With the addition of the lockup feature, the disconnected feeling was eliminated, along with the excessive heat and lost energy generated in the slipping converter. Still, these early transmissions were problematic, with a hydraulic control mechanism relying on unstable balances of hydraulic fluid pressure, spring tension in the various valve body circuits, and mechanical linkages to command the transmissions functions. Hydraulic controls just lack the precision to reliably and accurately control the transmission, a problem that was eventually overcome through the use of modern electronic controls.

A modern electronically controlled, lockup, overdrive four-speed transmission is simply at a whole different level compared to an old Torqueflite three speed. It doesn't take much proof to come to this conclusion beyond getting out of a modern vehicle and then hitting the open road in your vintage Mopar. Upgrading to a modern transmission is obviously a worthy goal, but the application presents some difficulties. Chrysler's own electronically controlled four-speed automatics do work well in later model trucks, but swapping one into an older muscle car is more than a little problematic.

The Chrysler 46RE (and the earlier hydraulically controlled A-518) transmission was an evolutionary design, created as a modification of the same old A-727 automatic transmission used since the early '60s. The overdrive portion was stuffed onto the back of the existing planetary architecture in the tailhousing, resulting in a functional four speed, but one that is too bulky to fit in the older passenger car chassis. With these Chrysler four-speed automatics, the bulbous overdrive tailhousing interferes directly with the existing torsion bar crossmember. Attempting this swap will require a major reworking of the suspension crossmember and floor pan. Due to this fit issue, the swap to a late Chrysler automatic has never become popular.

The Keisler A-41
Given the problems with the Chrysler automatic four-speed swap, most Mopar enthusiasts were simply out of luck when looking for a truly modern automatic transmission upgrade. Keisler Engineering, a company that has made a name for itself in the Mopar community with their overdrive manual conversions, set out to finally provide a solution. The answer was found based upon the GM 4L60E, a state-of-the-art electronically operated four-speed automatic. This transmission has an integral overdrive gear, so the fit problem associated with the 46RE/A-518's oversize tailhousing is not an issue. The real beauty is that the transmission features a detachable bolt-on bellhousing. Keisler designed and cast specific OEM-quality bellhousings for Chrysler applications, making the transmission bolt on as though it was designed for the Mopar engines. This was the genesis of the Keisler A-41.

The Keisler bellhousing allows the transmission to mate to the engine, but there are many other aspects of such an installation that needed to be addressed before this retrofit could become a reality. Keisler's approach is to accomplish a "perfect-fit" conversion, complete and ready to go. Among the many special components developed are a unique flexplate for the Mopar engine with an integral starter gear, a universal Throttle Position Sensor arrangement, a revision to the tailhousing to accept a mechanical speedometer drive, and of course all the required hardware, and the crossmember mount.