HYDRA-MATIC VERSUS DYNAFLOW
We may take it as a sign of GM’s considerable wealth and resources that it entered the fifties with three distinct automatic transmissions while most of its competitors were still struggling to develop even one. Until the debut of Chrysler’s two-speed PowerFlite in 1953, the only other automaker to develop its own automatic was Packard, which had introduced its torque converter Ultramatic in 1949. (GM, not convinced that the venerable independent had the resources for such a feat, later sued Packard, claiming Ultramatic’s torque converter design infringed on Buick’s patent.) Both Studebaker and Ford turned to Borg-Warner to develop their first automatics, while most of the other independents either bought Hydra-Matic or went without.
In a later era, GM’s corporate management would undoubtedly have pushed for standardization, but in the early fifties, General Motors controlled nearly half of the largest automotive market in the world and allowing the divisions to remain independent and competitive was an affordable indulgence. Just as important was the fact that GM had invested considerable capital in development and tooling for Hydra-Matic, Dynaflow, and Powerglide, which the corporation was not about to casually discard.
Moreover, the divisions’ customers were developing strong feelings about the pros and cons of each transmission, which sometimes resembled the rivalry between competing sports franchises. Hydra-Matic fans extolled its efficiency and crispness; Dynaflow supporters proclaimed the virtues of that transmission’s seamless action, and derided Hydra-Matic’s lurching shifts. Even the chief engineers of Oldsmobile and Buick exchanged good-natured jibes about the comparative merits of “Dyna-Slush” and “Hydra-Jerk,” although the latter camp had to swallow their pride for a while in 1953, when the destruction of the Livonia Hydra-Matic plant briefly forced Olds and Cadillac to use Dynaflow. The irony is that both transmissions emerged from the the same group in the corporate Engineering Staff; browsing through GM’s transmission-related patents from this period reveals many common names and evidence of similar design methodology.
Other manufacturers quickly settled on a compromise — two- or three-speed planetary gearsets combined with a torque converter. Chevrolet went that route in 1953, reengineering Powerglide to start in low and shift automatically to high; Packard adapted Ultramatic to do the same late in the 1954 model year. Instead, GM’s engineering staff spent the next decade refining the Dynaflow and Hydra-Matic concepts.
At the same time the corporate Engineering Staff was developing the controlled-coupling Hydra-Matic, they were also hard at work refining the torque converter concept. Although Chevrolet had backed away from the pure torque converter drive approach, Buick remained firmly committed to Dynaflow and was looking for ways to reduce its inherent limitations.
The first result of these efforts was the Twin-Turbine Dynaflow, introduced for the 1953 model year. Like the first Dynaflow, its basic concepts were developed by Oliver Kelley’s corporate engineering team while the production version was overseen by Buick staff engineer Rudolf Gorsky. As the name implied, Twin-Turbine Dynaflow featured a new torque converter with two turbines, a single impeller, and a single stator. Another important new element was an additional planetary gearset, mounted within the converter housing; insofar as the converter and the transmission proper were separate entities, the gearset was part of the converter, interposed between the turbines and the transmission input shaft.
The ring gear of that planetary gearset was driven by the first turbine. The planet carrier, which was attached to the transmission main shaft, was connected via an overrunning clutch to the second turbine. The gearset’s sun gear, meanwhile, was mounted on a one-way clutch that, like the one-way clutch of the stator, only allowed the sun gear to rotate in the same direction as the impeller.
During torque multiplication, the flow of oil from the impeller would drive the first turbine, whose blading was optimized for that operating regime. Any load on the transmission main shaft would lock the planetary gearset’s sun gear against its one-way clutch, so the first turbine would drive the planet carrier (and thus the main shaft and second turbine) at reduced speed, providing a mechanical gear reduction in addition to the torque multiplication provided by the converter itself.
As the first turbine picked up speed, the second turbine would also begin to accelerate. The first turbine continued to provide all the output torque (acting through the planet gears) until the speed of the second turbine exceeded that of the planet carrier. The planetary gearset would then act as a torque-combining differential, simultaneously driven by both turbines, until the flow of oil within the converter was primarily between the impeller and the second turbine, whose blades were optimized for efficient cruising. That left the first turbine to freewheel.
Once the speed of the second turbine was nearly equal to that of the impeller, the stator would unlock, also unlocking the planetary gearset sun gear, and torque multiplication would cease. Unlike the revamped Powerglide, there were still no discrete gear changes, so the Twin-Turbine Dynaflow remained a continuously variable transmission.
This new arrangement provided greater torque multiplication — up to 2.45:1 at stall — without increasing the converter’s stall speed, which would have hurt fuel economy. Since some of the additional multiplication was now mechanical rather than hydraulic, the Twin Turbine converter also provided some relief from the early Dynaflow’s low-speed throttle lag, although really brisk takeoffs still demanded the use of emergency low. An additional one-way clutch also allowed some engine braking on the overrun, which helped to keep speed under control when descending steep grades.
The Twin Turbine Dynaflow was a definite improvement over the original, but 2.45:1 was still marginal for the prodigious curb weight of contemporary Buicks. To address that limitation, in 1955, the twin-turbine Dynaflow gained a new variable-pitch stator. Like the variable-pitch propellers used on some aircraft, the stator blades could switch from a high angle, for greater torque multiplication, to a low angle, for greater efficiency at cruising speeds. Flooring the throttle would flip the stator blades back to the high position with an effect analogous to a kickdown downshift in a stepped-gear transmission. The following year, Dynaflow added a second, fixed-blade stator, increasing the converter’s maximum torque multiplication to 3.5:1. Low could now be held until just past 60 mph (say, 100 km/h), allowing magazine reviewers to record some rather racy 0-60 mph (0-97 km/h) times.
Although the Twin Turbine Dynaflow was finally approaching the performance and flexibility of contemporary stepped-gear automatics, that was not enough for Transmission Development Group chief Oliver Kelley. Kelley’s team was hard at work on the ultimate torque converter transmission: the triple turbine.
CONTROLLED COUPLING HYDRA-MATIC
In 1952, engineers P.J. Rhoads and Kenneth Gage of the Detroit Transmission Division began work on the second-generation Hydra-Matic, a project that ultimately cost some $35 million. The new transmission, which went on sale for the 1956 model year, was known by a variety of trade names — Oldsmobile called it Jetaway Hydra-Matic while Pontiac christened it Strato-Flight and AMC dubbed it Flashaway — although all were functionally identical. The patent application, filed in November 1953 by engineer Walter Herndon (who had been part of Earl Thompson’s original Hydra-Matic development team), called it a “controlled coupling” transmission.
The controlled-coupling transmission was a thorough redesign of the original Hydra-Matic. Like its predecessor, it had three planetary gearsets, giving four forward speeds plus reverse. A fluid coupling took the place of a conventional clutch; the engine drove the coupling’s impeller through the front planetary gearset, to reduce creep at idle. There were two oil pumps and an external oil cooler to prevent overheating. With a cast iron case (except the front torus cover, which was aluminum), the complete transmission was fairly heavy, at 240 lb (109 kg), and quite bulky.
Most of the changes to the new Hydra-Matic were designed to smooth its notoriously jerky shift quality. The first major change was replacing the troublesome brake bands with sprag (one-way) clutches for both the front and rear gearsets. (A rear band was retained, but it was used only in Low range.) The second and most dramatic change was the addition of a second, smaller fluid coupling between the front and rear planetary gearsets.
The new coupling, which was about 25% smaller than the main coupling, replaced the original Hydra-Matic’s front clutch pack. The coupling’s impeller was driven by the ring gear of the front planetary gearset, while the turbine was connected to the front planetary’s sun gear. Since a fluid coupling can’t be mechanically disengaged, it incorporated valves that would allow the oil in the torus housing to be drained or refilled in only 0.4 seconds. In first and third, the coupling was empty and thus effectively disengaged. In second and fourth, it was full, driving the sun gear of the front planetary at the same speed as the ring gear and putting the front gearset in direct drive.
These changes made Hydra-Matic substantially smoother than its predecessor. The 1-2 shift, which was achieved by filling the front coupling, was almost imperceptible. The 2-3 shift, which still involved the engagement of the rear clutch pack, could still elicit a modest thud, but it was far less pronounced than before. (When the new transmission was first shown to the press, engineers admitted they would have preferred to replace the rear clutch pack with a fluid coupling as well, but cost and space considerations had ruled out that possibility.) The redesign also eliminated the need for periodic band adjustments, which most owners had neglected anyway.
Like the earlier Dual-Range version, the controlled-coupling Hydra-Matic sought to provide a gear for every occasion. It retained the Dual-Range H-M’s second (D3 or Super) Drive range, which would keep the transmission from shifting to fourth gear at speeds under about 75 mph (120 km/h); Low range kept the transmission in second up to about 40 mph (64 km/h). The new Hydra-Matic also offered both part-throttle and full-throttle kickdowns for passing.
Owners soon discovered that the new Hydra-Matic was not quite as efficient as its predecessor was, nor was it as durable. Aggressive driving could destroy the sprag clutches, operation in extreme temperatures could be erratic, and the aluminum torus cover was prone to hairline cracks. Nonetheless, most buyers considered the new Hydra-Matic a welcome improvement.
Thanks to its daunting production costs, the new Hydra-Matic found fewer users than its predecessor did. It was used by Cadillac, Oldsmobile, and Pontiac and was sold to AMC for use in some 1956-1957 Nash and Hudson models, but GMC and Chevrolet trucks stayed with the earlier Dual-Range transmission until the early sixties, as did Rolls-Royce, which built the Dual-Range H-M under license.
Over the next few years, GM made many minor modifications to the dual-coupling transmission, most aimed at improving its durability and cold-weather performance. The most noticeable change came in the 1960 model year, when the case was redesigned to reduce its considerable bulk. None of these revisions altered the controlled-coupling Hydra-Matic’s basic operation, nor did they make it any cheaper to build. It remained one of the most expensive transmissions of this era and only its ample production volume kept the price within reason.