1961 Buick Skylark Dual-Path Turbine Drive transmission / 61skylark via AACA Forums
I previously talked about how the unusual 1961–1963 Buick Dual-Path Turbine Drive automatic went backwards. Here’s the rest of the story of this remarkable, short-lived transmission, including the question I’m sure you’re dying to know: Why did Buick build this thing in the first place?
Why Did the Dual-Path Turbine Drive Exist?
Before I talk more about how the Buick Dual-Path Turbine Drive transmission functioned, I should address the bigger question many readers have asked, which is, “Why did this transmission even exist? Wouldn’t it have made more sense for GM to just make Buick and Oldsmobile use the same automatic for the Y-body Special and Olds F-85?”
These are very good questions. My answer (which I admit is a bit speculative, but only a bit) is that the Dual-Path Turbine Drive probably came about in order to help to save Buick from one of the worst fates that can befall a manufacturing business: having a giant manufacturing plant that’s running way under capacity.
The original 1948 Buick Dynaflow
If you know anything about Buicks, you’re probably aware that from 1948 until 1963, they used their own automatic transmissions, a family of torque converter transmissions originally known as Dynaflow. The ideas for Dynaflow came from a GM Engineering Staff product study group, but Buick not only designed but also built the production versions at their own transmission plant in Flint, Michigan, a complex called Factory #10.
Production testing of an early Dynaflow transmission, 1948 / Joseph Geschelin of Automotive Industries
In the fifties, nearly all Buicks had Dynaflow, so during Buick’s mid-fifties heyday, their transmission plant was churning out huge numbers of transmissions: well over half a million a year in both 1955 and 1956. Buick then plowed a huge amount of money into new tooling and equipment for what was supposed to be the latest and greatest version, the all-new Flight Pitch Dynaflow (aka Triple Turbine), introduced for 1958.
Buick Factory #10, where Dynaflow transmissions were built / Gary Godin – All About Buick
Had Buick continued selling half a million full-size cars a year through the late ’50, I think there’s a good chance the Dual-Path Turbine Drive would never have been built, and Buick would have ended up using the miserable new Model 5 Hydra-Matic for its Y-body compacts, just like Oldsmobile did. Instead, Buick hit the wall: Their sales boom collapsed, and the Flight Pitch Dynaflow turned out to be a huge disaster, canceled after only two years. By 1958, full-size Buick production — and thus Buick transmission production — had fallen way below 300,000 units a year.
I don’t claim to know what Buick considered break-even level for Factory #10, but there’s no doubt that they were using only about half of its capacity by the end of 1958, and all the new equipment and tooling they’d installed to build the triple-turbine automatic and its aluminum case would soon be gathering dust. With Buick struggling to get back on its feet, it would have been foolish to buy Hydra-Matic transmissions from Detroit Transmission Division for the Special when Buick was sitting on so much unused transmission capacity.
Brochure page for the ill-fated Flight Pitch Dynaflow, offered only in 1958 and 1959 (renamed “Triple Turbine”) / Old Car Manual Project Brochure Collection
Hence, the Dual-Path Turbine Drive — made by Buick, for Buick, again based on some GM Engineering Staff concepts. Since it wasn’t built by Detroit Transmission or Chevrolet, the Dual-Path transmission didn’t need to share anything with the Hydra-Matic family or Powerglide, and since it was intended for a compact car with a 215 cu. in. (3,528 cc) engine, it shared nothing with the much bigger, heavier iron-case Turbine Drive (the transmission formerly known as Twin Turbine Dynaflow) in the full-size Buicks, which would have been way too big for the Y-body. I suspect the Dual-Path transmission also let Buick reuse some of the aluminum production facilities they’d established for the triple-turbine transmission, although I don’t think there was any meaningful parts commonality.
(Incidentally, if you read Jan Norbye and Jim Dunne’s Buick: The Classic Postwar Years and Oldsmobile: The Classic Postwar Years, they claim that the Buick Special and Olds F-85 DID use the same automatic, and assert that “Dual-Path Turbine Drive” and “Model 61-05 Hydra-Matic” were just different names for the same transmission. This could not possibly be more wrong, but Norbye and Dunne kept confidently repeating this same easily disprovable error through multiple editions of both books.)
Dual-Path Turbine Drive Design
The Dual-Path Turbine Drive was a two-speed automatic, like Powerglide, but it worked quite a bit differently. Where Powerglide had a Ravigneaux compound gearset, with two separate sun gears connected by long and short planet pinions, Dual-Path had only one set of planetary gears, located in the converter hub, with the ring gear directly bolted to the torque converter turbine. There was a two-piece sun gear, whose front half was fastened to the driven shaft of the multi-disc direct clutch while the rear half was connected to an overrunning clutch in the transmission case; the planet carrier drove the output shaft. Behind the torque converter were the oil pump, a bunch of different clutches, the valve body, and the governor, but all of the actual gears were within the converter housing. Here’s what it looked like in section view:
1961 Buick Dual-Path Turbine Drive
I’ve written at greater length elsewhere about the workings of this transmission, and I’ve already talked about the unusual way this transmission got reverse. Here are some highlights of how the Dual-Path Turbine Drive automatic operated in the forward gears:
In Drive, Dual-Path shifted from low to high and back by engaging or disengaging the converter clutch.
Unlike Powerglide or older Hydra-Matic transmission, Dual-Path had no brake bands: As in the later Turbo Hydra-Matic, most shifts were between an overrunning clutch and a multi-disc clutch. This made for quick, smooth shift action, with minimal “clunk” up or down, and no need for routine band adjustments. The only downside was that the overrunning clutch released when coasting, so if you needed engine braking for mountain driving, you had to manually select Low.
The converter clutch and planetary gearset were tucked into the torque converter hub.
(Some commenters have noted that the Dual-Path transmission could be driven even if it lost a lot of its fluid. As long as the forward clutch was engaged, the overrunning clutch didn’t need line pressure to operate, and while the converter clutch did, low gear in these transmissions was usable up to more than 55 mph.)
Low gear was taller (numerically lower) than most two-speeds automatics.
Unlike Powerglide, which had either a 1.82:1 or 1.765:1 low, low gear in Dual-Path Turbine Drive was just 1.58:1. This lower numerical ratio meant the engine lost fewer RPM when shifting from low to high, which was better for performance and allowed smoother shifts.
1961 Buick Skylark coupe with Dual-Path Turbine Drive / Classic Auto Mall
This ratio also meant that low gear was usable for passing or hill mountain driving up to higher speeds. With Dual-Path and a 3.08 axle, maximum full-throttle upshift was at 66 mph and kickdown was available up to 56 mph, whereas Powerglide generally wouldn’t kick down past about 45 mph.
It had a “loose” torque converter for extra pickup in low.
Dual-Path had a converter stall ratio of 2.50:1, which was about the same as the Powerglide in a six-cylinder Chevy II (and a bit lower than a Corvair Powerglide). However, the converter was loose enough that some converter multiplication was available up to almost 2,500 rpm, which gave this transmission what The Autocar called “an unusually wide ratio band” in low — thus compensating for the taller gear ratio.
When starting in Drive, the converter clutch is released and the overrunning clutches hold the stator and sun gears stationary, giving a breakaway ratio of up to 3.95:1 (1.58:1 low times 2.50:1 converter torque ratio).
For the shift to high, the converter clutch engages, allowing the engine to drive the sun gears while the converter turbine drives the ring gear and both overrunning clutches freewheel, giving a split-torque direct drive.
High gear provided a partial mechanical lockup.
In high, with the converter clutch engaged, the engine drove the sun gear directly, with no hydraulic slippage, while the torque converter continued to drive the ring gear. This meant that in high, more than one-third of the engine’s torque (36.6 percent) was transmitted mechanically through the clutch rather than through the converter, which reduced hydraulic slippage by more than 40 percent. Here’s a graph showing converter efficiency and torque ratio relative to engine speed in low and in high:
The red curves show converter efficiency, the green curves represent input speed, and the blue curves are the torque ratio, with the solid curves representing low gear (with the converter only) and the dashed curves representing the “dual-path” effect in high gear.
Dual-Path Turbine Drive didn’t provide a complete mechanical lockup like the old Borg-Warner DG automatic, but the split-torque high gear improved efficiency enough to more than make up for the loose converter. Also, the torque converter could still provide some multiplication in high gear, which the DG couldn’t.
It was air-cooled.
Air cooling wasn’t uncommon for early automatic transmissions, especially ones intended for smaller engines, and some designs were offered in both air- and water-cooled versions. (Buick actually made an air-cooled version of the later Super Turbine 300, which was used on the six-cylinder Tempest.) For the Dual-Path Turbine Drive, air cooling helped to transmission’s weight down, but it also limited its torque capacity. Dual-Path was strictly a light-duty transmission, not intended for torquier engines or heavy loads.
The Dual-Path case was designed to channel fresh air in and around the torque converter torus housing to keep it cool.
It worked pretty well for a two-speed automatic.
Car Life called the Dual-Path Turbine Drive “the best darn two-speed plus torque converter type box we’ve yet encountered” even though they admitted they had “always looked down our nose (technically speaking) at this particular type.” The Autocar also praised the transmission for its “almost indiscernible” gear changes, saying the Dual-Path “enables the car to step off smartly from rest on a 1-in-3 gradient” with “a vigorous reserve for acceleration” in top gear.
1962 Buick Skylark convertible with Dual-Path Turbine Drive / The Wayne Shaw Collection via Mecum Auctions
In a three-way comparison of the Special, rope-drive Tempest, and F-85 in December 1961 (previously presented on CC), the Car Life editors again remarked on “the considerable superiority of Buick’s 2-speed ‘Dual Path’ transmission,” declaring, “with only 2 speeds it is as good as any 3-speed we’ve tried.”
Why It Was Dropped
Despite the transmission’s advantages, no division other than Buick ever used the Dual-Path Turbine Drive, and it was never offered on any production models besides the 1961–1963 Y-body Buick Special and Skylark. Buick dropped the Dual-Path transmission after 1963 in favor of the heavier, more conventional Super Turbine 300 two-speed automatic.
1963 Buick Skylark convertible with Dual-Path Turbine Drive / GAA Classic Cars
Why? There were four principal reasons for dropping the Dual-Path unit for 1964 and later cars:
It didn’t have enough torque capacity.
For 1964, Buick replaced the aluminum 215 engine with a bigger iron-block 300 cu. in. (4,923 cc) V-8, which had about 30 percent more torque than its all-aluminum predecessor. Buick engineer Charles S. Chapman (later the managing director of Holden) admitted in a 1964 SAE technical paper that the Dual-Path unit simply couldn’t handle the larger engine’s greater torque.
Buick 300 cu. in. (4,923 cc) “Wildcat 310” V-8 / Barrett-Jackson
It had little growth potential.
A Ravigneaux gearset two-speed automatic like Powerglide or Super Turbine 300 is easy to beef up, and that’s even easier on the assembly line, where it’s possible to create medium- and heavy-duty versions of the same transmission that still share many common parts. The Dual-Path transmission’s split-torque high gear, air cooling, and reliance on cam-and-roller overrunning clutches meant that increasing the input torque capacity would have involved a major redesign.
It was probably expensive to build.
A Super Turbine 300 or aluminum Powerglide needed only two clutches (direct and reverse) and one brake band. The Dual-Path transmission needed four multi-disc clutches plus two overrunning clutches. The math isn’t hard to figure.
It never sold as well as Buick wanted.
The Y-body Special helped to raise Buick’s total volume back over 400,000 units a year by 1962, but its sales were disappointing, so production of the Dual-Path transmission never topped about 150,000 units per model year. I’m pretty sure it was enough to get the Buick transmission plant operating in the black again, but it wasn’t a runaway hit, and none of the other GM divisions had any use for the little transmission. (Dual-Path Turbine Drive might have made sense for Holden, Opel, and Vauxhall, but they were all saddled instead with the Model 5 Hydra-Matic, which for a while seemed set to become the standard GM light-duty automatic.) This was where the Super Turbine 300 made a lot more sense: It was something Buick could sell to Oldsmobile and Pontiac as well as producing it for their own use.
1964 Olds Cutlass with two-speed Jetaway (Super Turbine 300) automatic / Mecum Auctions
At the end of the day, production expediency almost always trumps clever design or elegant engineering — for GM as for any other manufacturer.
Further Reading
CC Tech: 1961–1963 Buick Dual-Path Turbine Drive Transmission – Forward Thinking About Going Backwards (by me)
Curbside Classic: 1962 Buick Special – A Truly Special Buick (by J P Cavanaugh)
Vintage Car Life Comparison: 1962 Pontiac Tempest 4, Buick Special V6, Olds F-85 V8 – Decisions, Decisions (by Paul N)
Dynaflow, Turboglide, Roto Hydra-Matic, and Other Early GM Automatics (at Ate Up With Motor)
Giving Slip the Slip: Lockup Torque Converters and Split Torque Automatic Transmissions (at Ate Up With Motor)
GM’s many efforts in automatic transmissions has fascinated me for a long time. I have never inhabited Buick-world, so the various versions of Dynaflows have always mushed together in my head. Thanks for the dive on this one, which helps me keep at least a few of them straight. It really sounds like the Buick 215 and this transmission may have been the best combination of all the Y body cars. It has been my perception that more of the Buicks survive than Olds or Pontiac versions, and this could be the reason.
I will never fully understand where the lines were between what GM corporate could/would dictate to the individual divisions and where the autonomy began. The line also seemed to move around a lot, depending on what year it was, and I suppose also who was in charge at any given point.
Thanks for the detailed article .
-Nate
What a fascinating time in GM engineering. As a Buick fan, I appreciate the time you put into these articles, Aaron.
Logical explanation as to why it was built. There’s few things worse than unused production capacity sitting around. My fascination with these little units has only grown.