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Dynaflow was the trademark name for a type of automatic transmission developed and built by General Motors' Buick Motor Division from late 1947 to mid-1963. The Dynaflow, which was introduced for the 1948 model year only as an option on Roadmaster models, received some severe early testing in the M18 Hellcat tank destroyer, which were built in Buick's Flint, Michigan assembly plant during World War II. It was also used in the 1951 Le Sabre concept car.
The Dynaflow initially used a five-element torque converter, with two turbines and two stators, as well as a planetary gearset that provided two forward speeds plus reverse. In normal driving, Dynaflow started in high gear (direct drive), relying on the converter's 2.1:1 torque multiplication to accelerate the vehicle. Low gear, obtained via the planetary gearset, could be manually engaged and held up to approximately 60 mph (97 km/h), improving acceleration. The transmission was incapable of automatic shifting, requiring the driver to move the shift lever from low to drive to cause an upshift. Buicks equipped with the Dynaflow transmissions were unique among American automobiles of the time in that the driver or his/her passengers would not detect the tell-tale interruption in acceleration that resulted when other automatic transmissions of the time shifted through their gears. Acceleration through a Dynaflow was one smooth (if inefficient and slow) experience. It was because of this slow acceleration that the Dynaflow transmission was nicknamed "Dynaslush."
The Dynaflow was an inherently inefficient design due to its sole reliance on the torque converter in normal driving. Exacerbating the situation was the dual stator arrangement, which wasted more power than the simpler three element converters used with other automatic transmissions, such as Chrysler's TorqueFlite. The multiple stators increased turbulence in the converter, even when operating in the coupling phase.
In 1953, Buick redesigned the Dynaflow, calling it the "Twin Turbine Dynaflow". The converter now incorporated two turbines and a planetary gear set, with a single stator. The first turbine was linked to the ring gear and the second to the planets, which gave a 2.5:1 torque multiplication which was now partly mechanical. This resulted in better efficiency, especially at highway speeds, and a higher level of performance and no penalty regarding the trademark smoothness. Buick also incorporated a variable-pitch stator in 1955 for greater flexibility. While these changes improved the transmission's overall performance and efficiency, the Dynaflow still was no match for other designs that utilized three element converters with automatic shifting.
A final version appeared in 1958 incorporating three turbines that Buick named the "Flight Pitch Dynaflow". Buick made this transmission standard on its top of the line Roadmaster 75 and Limited models and optional on all others. This version was further refined for 1959 and renamed the "Triple Turbine" but was offered only as an option on all models (Buick dropped the "Dynaflow" name after 1958). This unit was similar to the Twin Turbine, but bore some operational differences, amongst them a variable pitch stator that increased converter's torque multiplication to 3:1. The stator element of the torque converter has two blade positions, controlled by the driver via the accelerator pedal to offer a 'passing gear' and extra response at any speed from heavy throttle application. In normal driving the stator blades are arranged at 'cruise' angle, with improved efficiency and response at light throttle. Opening the throttle changes the angle of the stator vanes hydraulically to 'performance angle', which permits the converter to achieve stall about 1000 rpm higher than in 'cruise'. In this situation oil is redirected to strike the next-lowest drive turbine, which effectively lowers the drive ratio, and allows engine speed to flare to a speed where output is greatest. A few identifying features: the older Twin Turbine model was fitted with a rear pump, which meant the vehicle could be push-started (considered desirable at the time). Also, the Twin Turbine would allow engagement of low gear up to 40 mph (64 km/h), and had a shift quadrant that read P-N-D-L-R. In contrast, the Triple Turbine unit did not have a rear pump, and could not be push started. It would allow engagement of low gear up to 45 mph (72 km/h), and had a shift quadrant that read P-R-N-D-G (where "G" stood for "grade retard"). The grade retard feature was not designed as a low or forward acceleration gear and was meant to be used only on long declines to generate a degree of engine braking. The Triple Turbine was cancelled after 1959 due to technical problems and poor sales with only the Twin Turbine being produced until 1963.
In the late 1950s the Buick division of GM collaborated with Darby Buick of Sarasota, Florida to investigate potential marine uses of the Dynaflow transmission. The test boat was a 21 foot Correct Craft. The engine was a 364 CID Buick with a four barrel Rochester carburetor of nominal 300 gross HP. While the boat could attain speeds of about 60 MPH, which was considerable in the era, the transmission suffered from several problems. First, the torque in reverse was excessive, although this could have been alleviated with different gear ratios. Second, the role of "Park" in a car necessitated a stopped drivetrain. Without the locked-in-place tires of a car, it was difficult to put the transmission into the equivalent of "Park."
In 1964, the Dynaflow was discontinued in favor of the more efficient Super Turbine 300 two-speed and Super Turbine 400 three-speed transmissions, Super Turbine 400 being Buick's trade name for the Turbo-Hydramatic. One feature of the Dynaflow, the variable-pitch torque converter stator, colloquially the "Switch-Pitch", lived on in some versions of the Turbo-Hydramatic (Super Turbine 400) fitted to full-size Buicks, full-size Oldsmobiles and Cadillacs built from 1964–1967, as well as the Buick Super Turbine 300 and Oldsmobile Jetaway.
The early 1950s Cadillacs were normally equipped with Hydramatic transmissions. In 1953 the General Motors Hydramatic Plant burned to the ground, leaving Cadillac without a source of transmissions. Buick Dynaflow transmissions were hastily adapted to Cadillac mount points, and some 19,000 1953 Model 62 Cadillacs, and some 28,000 Cadillacs of all models, were equipped with Dynaflow transmissions. Several thousand 1953 Oldsmobiles were also equipped with Dynaflow.