The Hy-Drive  semi-automatic transmission was a very late 1953 model year introduction and carried over intact to 1954.  There was no way for a dealer to convert a manual transmission model to the "Hy-Drive" option because essentially under the sheet metal, it was a different car. The engine block was  different, and the transmission required substantial changes to the body in the cowl area. The clutch was different with substantial changes to the linkage that engaged it. The drive line was not as long, and used a longer, heavier tail shaft to handle the additional torque. The gear ratios were also different. The speedometer cable was much longer. The radiator was larger to handle the extra heat from the transmission. The carburetor used a dashpot to keep from stalling when quick lifts off the accelerator were made, and the floor pans were different. Got one of the 75,000 Plymouths that had this transmission? Good luck!


The Powerflite automatic transmission, officially introduced in late 1954, is an  example of the best of Chrysler engineering. The engineers had apparently done their homework before creating this two-speed planetary gearbox with  torque converter. It started in low, unlike the Fordomatic, shifting into high at eleven miles per hour. It could also be manually held in low, and the car could be push started without any damage to the transmission. The quality of the Powerflite design was carried  over into the design of its three-speed successor, the famous Torqueflite, arguably the best automatic transmission in the world.


The Powerflite's one drawback was the lack of a Park position or a lock-up in Reverse. This made a very strong, effective  parking brake a necessity. Consequently, the parking brake was designed as  an internal-expanding unit mounted on the transmission tail shaft. It was virtually a small version of a drum brake designed to immobilize the output shaft of the transmission, and was similar to  the  brake on the manual transmission. Although difficult to work on, the unit functioned well and allowed the driver to avoid hearing things like "Oh my God! Where's the car?" 


Not much credit is given to the Powerflite, most likely because it was overshadowed by the Torqueflite.  The Powerflite, however, continued to be offered throughout the line many years after the introduction of the Torqueflite, finally ending its run  in 1961. In actuality it was a simple transmission built around a planetary gear set; it was very strong, handling any engine that Chrysler built,  utilizing far fewer parts than its competition.

In one of its strangest applications, the Los Angeles Police equipped its 1960 Plymouth Savoy fleet with a 318 cubic inch V-8 and the Powerflite transmission. It was unusual because the vastly superior Torqueflite had been around for three full years, and was king of the fleet buyer's specifications. In a rather strange requirement, LAPD specified that the transmission had to share the oil with the engine. It was an odd demand that lasted several years. Only Chrysler was able to meet the specification, and without any changes to the transmission itself. Chevrolet's Powerglide and  Ford's  Fordamatic both  failed within a few hundred miles with 20 weight engine grade oil in their reservoirs.  L.A.P.D  also purchased 1961 Dodge Darts equipped the same way as the 1960 Plymouths. No reported extensive transmission problems occurred. Personally, I prefer to run mine on milk.

In 1955, the Powerflite automatic was given an unusual dash-mounted lever. In 1956, the pushbutton automatic was introduced along with the Torqueflite automatic, continuing experimentation with new ways to change gears. The next major change would be the AutoStick in the 1990s. They also briefly considered having the driver lean out the window and pull on a piece of wire connected directly to the transmission valve body selector lever, but this was considered too primitive.


The 6 cylinder and V8 transmissions in 1955 are all the same except the clutches. 
The PowerFlite transmission is air-cooled, as is the torque converter. The absence of a vacuum modulator enhances the simplicity of this unit. On the V8 car, a full throttle upshift occurs at about 60 mph. Kick-down will occur all the way up to 50 mph. 

One of the idiosyncrasies of the design is a very high (260 lb/sq.in.) fluid pressure when the transmission is in reverse. The life of the PowerFlite can be extended by first shifting to drive (foot on the brake) to cut the engine idle speed and then go into reverse.  During a rebuild, transmission life can also be extended by putting  the kick down cushion spring in the trash can instead of in the PowerFlite. This spring softens the shift by momentarily holding the transmission in low and drive during kick-down, and really puts a strain on the internal components.

The  PowerFlite also came with the four pinion rear axle. The four pinion unit was the stronger of the two offered in 1955. In 1956 the four pinion axles were high performance options. 3.54 was a standard rear gear with the PowerFlite, 3.73  an optional ratio. Manual transmission sixes got 3.90, 4.30 with overdrive; the V8 counterparts received 3.73, and 4.10 with overdrive. These were the two pinion units and did not appear to be as strong as the four pinion units.

Chrysler wrote this  about the Powerflite in 1957:  "PowerFlite no-clutch automatic transmission has a unique record for delivering care-free service. The design has been right, and right from the start. PowerFlite also has the advantage of push-button control...within easy reach of the driver, but away from any other hands (particularly from young fingers) ... To start, press button N (Neutral), turn the ignition key, press button D (Drive), step on the accelerator to go - touch the brake to stop - gears will change silently, automatically, to meet the driving need. ... The L button holds the transmission in lower gear if you want the safety of engine braking for a long, steep grade. The pushbuttons are illuminated for night driving. Automatic lockout is a safeguard in case you accidentally pressed the Reverse button when the car is moving forward above 10 mph."




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