Giving Slip the Slip: Lockup Torque Converters and Split Torque Automatic Transmissions


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.)


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.

Color diagram of 1961-1963 Buick Dual-Path Turbine Drive transmission © 2016 Aaron Severson

A schematic of Buick’s short-lived Dual-Path Turbine Drive two-speed automatic. The flywheel drove the torus cover, oil pump, and torque converter impeller (red) while the turbine drove the annulus of the single planetary gearset (blue), whose planet carrier drove the output shaft (green). There were two identical sun gears (orange), one connected to a cam-and-roller one-way clutch (fuchsia) that shared its cam with the clutch for the stator (shown here in purple). The direct clutch connected the other sun gear to the torus cover. Note that the torque converter was “backward,” with the turbine facing the engine rather than the rear axle. (author diagram)

Precisely calculating the second-gear torque split is complicated somewhat by the fact that we don’t know how many teeth the planetary gears have. We do know that the gear ratio in first, with the annulus driving and the sun gears held, was 1.58:1. (We would conjecture that the annulus had 66 teeth, the sun gears had 38, and the planets each had 14, which would make the exact gear ratio 1 + 38/66, or 1.5757:1.) That’s enough to estimate that the annulus received about 63.3–63.4% (1 / 1.58) of input torque in second gear, with the remaining 36.6–36.7% applied to the sun gears.

1961–1963 Buick Dual-Path Turbine Drive transmission diagram showing power flow in 2nd gear (high) © 2016–2017 Aaron Severson

Power flow in the Dual-Path Turbine Drive in second gear was through the torus cover to the hydraulically driven annulus and simultaneously through the mechanically driven rear sun gear of the single planetary gearset. This was not a Ravigneaux gearset — the two sun gears were identical and always rotated at the same speed even in a reduction gear. (author diagram)

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 main shaft.

Unlike Powerglide, which obtained direct drive by also connecting 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 served to connect the front sun gear to the central input shaft, which rotated at engine speed, not turbine speed.

Color diagram of 1961–1962 Pontiac TempesTorque transaxle © 2016–2017 Aaron Severson

A Ravigneaux gearset, like that used in Powerglide and TempesTorque, obtains indirect gear ratios by causing the two sun gears to rotate at different speeds. Normally, direct drive is obtained by locking the input shaft to both sun gears so that they turn at the same speed. In 1961–1962 TempesTorque units, direct drive instead locked the low sun gear to the input shaft while the input sun gear rotated with the main shaft. (author diagram)

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.

1961–1962 Pontiac TempesTorque transaxle diagram showing power flow in 2nd gear (high) © 2016–2017 Aaron Severson

In 1961–1962 TempesTorque transaxles, the direct drive second gear engages the direct clutch, allowing the input shaft (red) to drive the low sun gear (purple) at engine speed. Power also continues to flow through the input shaft to the torus cover and impeller (also red), then hydraulically to the turbine and main shaft, which drives the input sun gear (all medium blue). The planet carrier (light green) resolves the speed difference between the two sun gears and drives the differential input gear. (author diagram)

For 1963, TempesTorque’s final year, the direct clutch was revised to connect the front sun gear to the main shaft rather than the input shaft, which eliminated the torque-splitting feature.


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.

Color diagram of 1961–1964 Roto Hydra-Matic transmission © 2016–2017 Aaron Severson

The three-speed Roto Hydra-Matic was essentially a much simplified adaptation of the costlier, more complex four-speed Controlled Coupling Hydra-Matic. Perhaps its most unusual feature was the very small (8-inch/203mm) torque converter, which alternately emptied and filled in different gears. The converter’s torque multiplier — not a true stator — had no one-way clutch and was affixed directly to the carrier shaft. (author diagram)

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.

1961–1964 Roto Hydra-Matic transmission diagram showing power flow in 1st gear © 2016–2017 Aaron Severson

In first, Roto Hydra-Matic sent all power through the torque converter to the rear sun gear (medium blue). With the neutral clutch engaged, the outer race of the sprag clutch was locked to the case, so the inner race — shared by the rear annulus and front sun gear (orange) — could only turn in the direction of engine rotation. The inertia of the output shaft and the rotation of the rear sun gear would lock the sprag, driving the planet carriers and output shaft forward in reduction. There were at least three different rear gear sets for different versions of this transmission, giving geared ratios of 2.93, 2.97, or 3.03:1 in first. (author diagram)

1961–1964 Roto Hydra-Matic transmission diagram showing power flow in 2nd gear © 2016–2017 Aaron Severson

In second, the neutral clutch remained engaged, the front clutch engaged, and the torque converter was drained, sending all power through the converter’s torus cover (red) to the front annulus with no hydraulic slippage. The sprag remained locked, so the front annulus drove the planet carriers and output shaft forward at reduced speed. There were two different front gear sets for different versions of Roto Hydra-Matic, providing a second gear ratio of 1.56 for the bigger Model 375 and 1.58 for the Model 240. We assume the reason for the inconsequential variation was that the Model 375’s gear teeth were slightly wider, reducing the total number of teeth on each gear. (author diagram)

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.

1961–1964 Roto Hydra-Matic transmission diagram showing power flow in 3rd gear © 2016–2017 Aaron Severson

The torque split of Roto Hydra-Matic’s split torque third gear depended on gearing. With the original Model 375 gears (2.97 first, 1.56 second), the split was 59.9%/40.1% mechanical/hydraulic. With the original Model 240 gears (3.03 first, 1.58 second), the split became 60.6%/39.4%. With the later 2.93:1 rear gears, the split was either 57.6/47.2% or 58.9/41.1%. (author diagram)


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  1. Well Aaron . . another masterpiece. This and the GM automatic history are probably the most definitive descriptions of these technologies on the Internet, excepting pure design and engineering treatises. Well done and thank you; this must have been an enormous amount of work.

  2. I have yet to read this monumental work in depth. Whether it will add to my working knowledge is debatable, but my brain will benefit from the workout.
    Aaron is probably now the best informed person in the world regarding the history of automatic transmission development

    1. I appreciate the compliment, but I’m really not! This is a remarkably broad, convoluted, and idiosyncratic field and there’s a LOT I don’t know. For people who want a broader overview, I would recommend a book by Philip G. Gott entitled Changing Gears: The Development of the Automotive Transmission, published by the SAE as part of their Historical Series in 1991. (At this point, an updated, expanded edition wouldn’t go amiss, given all the subsequent development in CVTs and automatics with five or more speeds.)

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