As we mentioned previously, the late fifties were not a particularly good time for radical or elaborate new designs. The new car market, which had boomed in 1955, slumped badly for 1956. By 1957, a national recession had buyers searching for smaller, cheaper, more economical cars.
Chevrolet general manager Ed Cole used this opportunity to push through his radical rear-engine, air-cooled Corvair, but did nothing to help GM’s mid-priced divisions, which had been hit hard by the recession. Senior corporate management responded with the X-100 project, a collaborative program to give Pontiac, Oldsmobile, and Buick their own small (or at least smaller) cars for the 1961 model year, a year after the debut of the Corvair.
Although the X-100 cars were intended to have a high degree of commonality so as to share the substantial costs of the new models, the final products ended up considerably less alike than the corporation originally hoped. The so-called “senior compacts” — the Buick Special/Skylark, Oldsmobile F-85/Cutlass, and Pontiac Tempest/Le Mans — did share the same unitized Y-body shell (an enlarged version of the Corvair body) and various minor components, but there were significant differences in their mechanical layouts and powertrains, including three completely different automatic transmissions.
Ironically, the most conceptually exotic of the trio, the Pontiac Tempest’s rear-mounted automatic transaxle, was probably the cheapest of the three to develop and tool. Dubbed “TempesTorque,” it was a variation of the Corvair’s optional Powerglide transaxle. As in the Corvair Powerglide, TempesTorque’s torque converter was at the back of the transaxle. Since the Tempest had a front-mounted engine, TempesTorque used the Corvair transmission’s front oil pump driveshaft as an input shaft, driving the torque converter impeller through the torus cover. That input shaft was also splined to the hub of the direct drive clutch, just like in a conventional RWD Powerglide. In high, input torque was therefore split approximately 45/55 between the front sun gear, which was driven by input shaft through the direct drive clutch, and the rear sun gear, which was driven by the torque converter. As with Hydra-Matic, this “split torque” layout served to reduce slippage at cruising speed in high gear.
The 1961–1962 TempesTorque had a lower converter stall ratio than did the Corvair Powerglide (2.00:1 rather than 2.60:1), but the indirect ratios were the same (+/-1.82:1 for low and reverse). As with the Corvair transmission, there was no parking pawl.
Pontiac made a variety of changes to TempesTorque for 1963, the “rope-drive” cars’ final year. Torque capacity was increased to accommodate the Tempest’s newly optional 326 cu. in. (5,340 cc) V8 engine while new planetary gears, borrowed from the latest heavy-duty Powerglide, gave indirect ratios of +/-1.765:1. A new direct drive clutch deleted the previous high-gear torque-splitting feature and the torque converter was redesigned to provide higher stall ratios (2.40:1 for four-cylinder cars, 2.20:1 for the V-8). TempesTorque and the rope-drive Tempest/Le Mans disappeared for good after the 1963 model year.
DUAL-PATH TURBINE DRIVE
The two-speed torque converter automatic transmission offered on Buick’s Y-body compacts, dubbed Dual-Path Turbine Drive, was quite different from TempesTorque and for that matter the twin-turbine transmission used in contemporary full-size Buicks. Today, the Dual-Path Turbine Drive is one of the most obscure and poorly understood of GM’s early automatics, in part because it was used only on the 1961–1963 Buick Special and Skylark. In a sense, it was Buick’s first true automatic transmission, since it was the first to actually include provision for automatic shifts between two distinct stepped ratios in Drive, something the designers had taken pains to avoid with the Dynaflow family.
The Dual-Path transmission seems to represent a merger of two distinct conceptual threads within GM’s corporate transmission group. One was the use of a split-torque clutch to provide direct drive, a concept dating back to the original Hydra-Matic (and which was essayed in somewhat simpler form around 1957 by Oliver Kelley’s colleagues Robert M. Tuck and James J. Mooney, Jr. — see U.S. Patent No. 2,929,270). The other, developed by Kelley and Gilbert K. Hause, was a three-element torque converter with a stator that could do double duty as a drive turbine. Judging by the earliest relevant patent disclosures (U.S. Patents 2,957,370 and 3,030,823), the latter was conceived as a streamlined and simplified evolution of the Twin-Turbine Dynaflow, applying some concepts from the triple-turbine transmissions to allow the deletion of Dynaflow’s separate planetary gearbox.
It appears that there was some consideration of using the Dual-Path transmission in the other X-100 cars. The patent outlining most of the major mechanical details was actually filed by John DeLorean, then the head of Pontiac’s advanced engineering section, although Hause led the development of (and patented) the hydraulic control system. Interestingly, a subsequent patent filed by Kelley and Hause described several possible rear transaxle versions, although the production Dual-Path transmission was only for front-engine/rear-drive applications. The only significant element TempesTorque ended up sharing was the torque-splitting feature, which Pontiac implemented differently.
Like the contemporary Powerglide, Dual-Path Turbine Drive had a three-element torque converter and a single planetary gearset with dual sun gears, but the mechanical similarities ended there. As in the dual-turbine Dynaflow series, the actual planetary gears were nestled in the center of the torque converter torus, with the turbine hub driving the annulus and the planet carrier driving the transmission main shaft. Unlike previous Dynaflow transmissions (and most other automatic transmissions), the impeller was mounted on the flywheel side of the torus housing, facing backward (i.e., toward the rear axle) while the turbine faced forward, toward the engine, and was connected to its hub by a series of narrow struts.
Unlike Powerglide and Dynaflow, Dual-Path used no brake bands. Instead, it was controlled by four multi-disc clutches (the direct drive/converter clutch, reverse clutch, forward clutch, and coast clutch) and two one-way clutches (one for the stator, the other for the planetary gearset’s rear sun gear). The converter clutch was mounted in the hub of the torque converter impeller, allowing the torus cover to be locked to the planetary gearset’s front sun gear. The other five clutches occupied most of Dual-Path’s cast aluminum transmission case.
The main shaft was surrounded by four concentric sleeve shafts of varying lengths. The outermost sleeve allowed the torus cover to drive the transmission’s single oil pump, which was mounted in the front of the transmission case, like Dynaflow’s front pump. Within that shaft was a sleeve shaft connecting the converter turbine to the hub of the reverse clutch. The two innermost shafts connected the stator and rear sun gear to the inner races of their respective one-way clutches. Those clutches, which were of the cam-and-roller type, shared a common cam, which was connected to the forward clutch and coast clutch. The forward clutch, like the neutral clutch of the Controlled Coupling Hydra-Matic or Triple Turbine transmissions, allowed both one-way clutches to be selectively neutralized (i.e., allowed to turn freely without locking) by disconnecting the cam from the transmission case. The coast clutch, meanwhile, allowed the cam to be locked to the inner race of the rear sun gear so that all three would turn together on the sun gear sleeve shaft.
All this sounds very complex, but Dual-Path’s operation was reasonably straightforward. Selecting either D (Drive) or L (Low) on the selector (which had a PNDLR pattern) would engage the forward clutch. This enabled the one-way clutches for the stator and the rear sun gear, preventing them from turning backward. The hub of the torque converter turbine then drove the annulus of the planetary gearset, with the stator providing additional torque multiplication in the customary fashion; stall ratio was variously quoted at 2.40:1 or 2.50:1. Reverse torque on the planet carrier locked the rear sun gear’s one-way clutch, putting the planetary gearset in first gear and providing a mechanical gear reduction of 1.58:1.
Selecting L would engage the coast clutch as well as the forward clutch. This allowed the stator to function normally, but prevented the rear sun gear from turning in either direction, ensuring that the planetary gearset would remain in reduction on the overrun. Dual-Path’s hydraulic controls included no provision for automatically disengaging the coast clutch, so in Low, the transmission could not shift out of first gear.
With the selector in D, however, the hydraulic control system would shift automatically between first and second. Upshifts were executed by engaging the converter clutch, which established a mechanical connection between the torus cover and the front sun gear. That drove both sun gears forward, which caused the rear sun gear’s one-way clutch to automatically unlock so that the planetary gearset was no longer in reduction. Engine torque was then split 36.6/63.4 between the front sun gear (which turned at impeller/engine speed) and the annulus (which turned at turbine speed), reducing converter slippage. Downshifts were executed by simply releasing the converter clutch, which caused the rear sun gear to automatically re-lock and put the transmission back in first. Shift points were determined by a combination of throttle setting and car speed. Dual-Path was the first Buick automatic to be equipped with a centrifugal governor, previous Dynaflow and Turbine Drive control units having had no need to measure road speed.
Dual-Path obtained reverse in much the same way as the earlier triple-turbine automatics, although the arrangement was slightly simpler because Dual-Path had only one turbine and one planetary gearset. Moving the selector to R released the forward clutch and engaged both the reverse and coast clutches. The reverse clutch then held the turbine stationary, which caused the turbine and stator to effectively swap roles. With the forward clutch disengaged, releasing the one-way clutch race from the case, reaction torque on the stator caused the stator, its sleeve shaft, and the clutch race to spin backward. Since the coast clutch was also engaged, the reverse rotation of the clutch cam drove the rear sun gear backward. The annulus, which was held in place along with the turbine, then acted as a reaction member, allowing the sun gear to drive the planet carrier in reverse reduction.
The point of this unusual arrangement was to minimize the number of components, keeping the transmission as compact and as light as possible. With its air-cooled aluminum case, Dual-Path Turbine Drive was one of the lightest automatic transmissions ever developed by a U.S. automaker, weighing only 95 lb (43 kg) with fluid. That was a bit less than the Buick Special’s standard three-speed Warner Gear T-85 manual transmission and less than half as much as the full-size Turbine Drive or four-speed Hydra-Matic. The transmission tunnel intruded into cabin space only slightly more than that of the rope-drive Tempest, with its rear transaxle.
From a performance standpoint, a two-speed automatic linked to an assortment of modestly powered V8 and V6 engines doesn’t sound promising, but contemporary testers found that cars equipped with Dual-Path were unexpectedly spry. Although Dual-Path’s first gear was quite tall — it was only slightly shorter than second gear in the T-85 three-speed manual or, for that matter, third gear in the Controlled Coupling Hydra-Matic — the torque converter provided a starting ratio comparable to first gear in the four-speed Hydra-Matic. With a 3.08:1 axle, standard on automatic Specials, first could be held to 63–64 mph (101–103 km/h), so Low was useful in mountain driving that would be uncomfortably buzzy with many contemporary two-speeds. In all, performance was really not bad, if still somewhat inferior to the available manual transmissions. The closeness of the ratios and the use of one-way clutches rather than brake bands also made Dual-Path’s shifts impressively smooth.
One minor sacrifice was the capacity for push-starting, something allowed by most earlier GM earlier automatics. Hause’s patent (U.S. No. 3,108,493) included an auxiliary oil pump to be used solely for that purpose, mounted at the rear of the transmission just ahead of the governor. However, the production transmission had only one pump, presumably in the interests of minimizing cost and weight.