LATER GM SPLIT TORQUE AUTOMATICS
The three new automatic transmissions GM introduced in 1961 for the Y-body “senior compacts” each used the split torque principle, albeit in three quite different ways. (Since we’ve previously discussed these transmissions in some detail, we’ll just look at the split torque function of each.)
DUAL-PATH TURBINE DRIVE
Buick’s two-speed Dual-Path Turbine Drive, used in the 1961–1963 Buick Special and Skylark, was probably the simplest of the three in this regard. As we’ve previously explained, Dual-Path had a single planetary gearset within the torque converter housing. The converter turbine drove the annulus of that gearset, whose planet carrier drove the output shaft. In first gear, a one-way clutch held the dual sun gears stationary. In second, a multi-disc clutch allowed the engine to drive the sun gears directly while the turbine continued to drive the annulus.
The annulus had 80 teeth, the sun gears 46 teeth, and the planets each 17 teeth, making the ratio in low gear 1 + 46/80, or 1.575:1. Therefore, the annulus received 63.4% (1 / 1.575) of input torque in second gear, with the remaining 36.6% applied to the sun gears.
Since the sun gears were driven by the engine, the demultiplication effect was considerably smaller than in Hydra-Matic — less than 37%. For example, if the engine was rotating at 2,000 rpm and there was 100 rpm of slippage in the converter, output shaft speed (discounting mechanical losses) would be about 1,937 rpm, effectively reducing hydraulic slippage from 5% at the turbine to about 1.8% at the output shaft.
The automatic used in the 1961–1963 Pontiac Tempest and Le Mans, called TempesTorque, was a variation of the two-speed Corvair Powerglide, adapting the Corvair transaxle’s oil pump driveshaft to send power from the curved driveshaft to the torque converter at the back of the transaxle. 1961–1962 editions also had a split torque top gear, a feature Powerglide didn’t share.
TempesTorque and Powerglide, like the older Ultramatic and Dynaflow transmissions, used a Ravigneaux gearset with a single annulus, two sun gears of different sizes (with different numbers of teeth), and six planets (three long, three short) on a planet carrier connected to the output shaft. As with Powerglide, TempesTorque’s driving member was the larger rear sun gear, which was driven by the torque converter turbine through the transmission main shaft.
Unlike Powerglide, which obtained direct drive by engaging a clutch to lock the smaller front sun gear to the main shaft (forcing both sun gears to rotate at the same speed), the direct drive clutch of the 1961–62 TempesTorque was splined to the transaxle input shaft, which rotated at engine speed, not turbine speed.
1961–62 TempesTorque transaxles shared the Powerglide gearset, whose annulus had 79 teeth and whose sun gears had 23 and 28 teeth respectively, giving first and reverse ratios of +/- 1.82:1. We’ll spare you the complex algebra, but in second gear, 54.9% of input torque went to the hydraulically driven rear sun gear while 45.1% went to the mechanically driven front sun gear. For example, at an engine speed of 2,000 rpm with 100 rpm of converter slippage, the carrier would rotate at 1,945.1 rpm, effectively demultiplying hydraulic slippage from 5% at the turbine to about 2.7% at the output shaft.
For 1963, TempesTorque’s final year, the direct clutch was revised so that its hub was splined to the main shaft rather than the input shaft, which eliminated the torque-splitting feature. In the 1963 transaxle, all power flowed through the input shaft to the torque converter and the main shaft in first, second, and reverse, just like Corvair Powerglide units.
Detroit Transmission Division also applied the split torque principle to the simplified third-generation Model 240 and Model 375 Hydra-Matic transmissions (sometimes called “Roto Hydra-Matic”) used in the 1961–1963 Oldsmobile F-85/Cutlass, some Holden and Opel models, and most full-size 1961–1964 Oldsmobiles and Pontiacs.
Unlike its predecessors, the third-generation Hydra-Matic was a three-speed transmission whose single dump-and-fill fluid coupling was transformed into a torque converter by the addition of a torque multiplier member (not a stator) between the impeller and turbine. There were now two interconnected planetary gearsets with interconnected planet carriers, which were in turn connected to both the torque multiplier and the output shaft. The front sun gear and rear annulus were also connected, forcing them to rotate (or not rotate) together at the same speed. A sprag clutch kept them from rotating backward in any forward gear.
Roto Hydra-Matic’s operation was broadly similar to that of its dual-coupling predecessor, relying on alternately emptying and refilling its torque converter and engaging or disengaging its multi-disc front clutch. In first, the converter was full and the front clutch released. In second, the converter was empty and the clutch released while in third, the converter was full and the front clutch engaged simultaneously.
There was no torque split in second, since with the torque converter empty, all power was transmitted mechanically through the torus cover and front clutch. However, in third, power was divided between the mechanically driven front annulus and the hydraulically driven rear sun gear. Since this meant the front annulus was turning faster than the rear sun, its rotation drove the carrier around the slower-moving sun gears, resolving the speed difference.
If we’re doing the math correctly, this demultiplied converter slippage by about 60%, depending on the specific front and rear gearsets used. Under some conditions, a small amount of torque also flowed through the converter’s torque multiplier, which always turned at the same speed as the planet carriers and output shaft.