While GM had a first-rate modern 3-speed automatic in the form of Turbo Hydra-Matic 400, a lot of late ’60s GM cars still had 2-speed automatics, which the divisions insisted provided adequate performance at lower cost. It wasn’t until 1969 that Chevrolet and Buick joined forces to create the excellent TH350, the medium-duty 3-speed automatic the corporation had needed for at least five years. This 1969 Skylark Custom sport coupe is one of the first Buick intermediates to get the new automatic.
I’ve said this before, but it bears repeating: many perplexing automotive product decisions make a lot more sense if you look at where and how things were actually produced. One of the surest ways to lose money in manufacturing is to have factories that are underutilized or sitting idle, and the need to make fuller use of that capacity can lead to some peculiar decisions.
Another vital point is that before the 1980s restructuring, GM divisions had quite a bit of their own manufacturing capacity. The car divisions had their own engine plants, and Buick and Chevrolet also built their own automatic transmissions. Neither division used the older fluid coupling Hydra-Matic transmissions, and while they later bought Turbo Hydra-Matic (TH400) from Hydra-Matic Division, they still had big investments in their own transmission production facilities.
So, Chevrolet continued to use the 2-speed aluminum Powerglide in very large numbers, while Buick converted its transmission plant, which had previously built Turbine Drive and Dual Path Turbine Drive, to manufacture the Powerglide-like 2-speed Super Turbine 300. Buick used that transmission in its midsize and some full-size models, and also sold the transmission to Pontiac and Oldsmobile, which used it in their A-body intermediates, the B-body Oldsmobile Jetstar 88 and Delmont 88, and some versions of the Pontiac Firebird.
GM engineers and executives offered lots of rationalizations for their continued fixation on 2-speed automatics. The 2-speeds had fewer frictional losses than most 3-speeds, so they consumed less power and theoretically returned slightly better fuel economy. An aluminum-case 2-speed also weighed less and took up less space than a 3-speed automatic. Of course, the bigger incentive was that the 2-speeds were cheaper to make and used existing tooling, which was better for the divisions’ bottom line.
However, GM wasn’t the only corporation making cars in the U.S., and by the late ’60s, they were the last 2-speed holdout. Chrysler had phased out its 2-speed PowerFlite by 1962, and by the 1965 model year, Ford had the 3-speed C4 to replace the miserable 2-speed Fordomatic. AMC had only used 3-speed Borg-Warner automatics for years. Nonetheless, if you bought a new GM car, even a full-size Oldsmobile or Buick, you might still get a 2-speed. Chevrolet was such a juggernaut that it didn’t do them any visible commercial harm, but the 2-speed hardly gave the mid-priced divisions a competitive edge.
The divisions slowly expanded the availability of Turbo Hydra-Matic, making TH400 available with the big-engine intermediates and some full-size cars with smaller engines. Behind a Chevrolet 327 or Buick 340, the 3-speed automatic offered much better flexibility, especially with the switch-pitch torque converter used in some applications (dropped after 1967). However, TH400 was arguably overkill with smaller engines, which didn’t need the greater torque capacity, and it cost more than the 2-speed, both to build and to buy.
By the start of the 1968 model year, Chevrolet and Buick had concluded that they needed a better compromise, and belatedly came up with a new 3-speed transmission, sometimes called the CBC 350, for “Chevrolet-Buick Combined.” Although it would be marketed under the Turbo Hydra-Matic name, as the TH350 (or TH-350), the involvement of Hydra-Matic Division was initially minimal; it was something the two car divisions developed to build themselves.
The new TH350 transmission was conceptually a lot like the TH400, with a Simpson gearset (two interconnected planetary gearsets with a common sun gear, taking input through one ring gear and outputting through the other, with equal-size planet gears) operated by a series of multi-disc and sprag clutches. All TH350 applications initially shared an 11.75-inch torque converter with a stall ratio of 2.10 to 1; the TH350 didn’t use the variable-pitch stator that had been fitted to some earlier TH400 and Super Turbine 300 transmissions.
Despite the broad-strokes similarities, the TH350 arranged its various elements differently than the TH400 did. As Chevrolet engineer John Mahoney explained to Motor Trend in 1968, “This transmission is supported from the front and back while the Hydra-Matic has a bulkhead in the center.” The TH350 also had only one overrun band (for intermediate) rather than two. As the “350” designation implied, it theoretically had about 20 percent less input torque capacity than the TH400 (although the TH350 could be and later was beefed up to be almost as strong), but it weighed about 23 pounds less and was a little bit shorter.
Like the TH400 and Super Turbine 300, the TH350 used a vacuum modulator to vary both shift points and line pressure based on engine torque demand, which was signaled by intake manifold absolute pressure. (The modulator incorporated a spring-loaded anerobic (evacuated) bellows that automatically compensated for variations in atmospheric pressure.) GM had patented some of this control scheme in 1964, but, as with the Turbo Hydra-Matic name, that intellectual property belonged to the corporation rather than the individual divisions, so there was no legal problem with Chevrolet and Buick adopting them.
I’m not going to try explaining the full operation of the TH350 transmission, but if you’re interested in a detailed technical description, I recommend Ron Sessions’ 1985 book Turbo Hydra-Matic 350 Handbook, which explains just about everything you might conceivably need to know.
Although the TH350 didn’t officially become available until the 1969 model year, it was fitted to about 2,000 late 1968 Chevrolet cars — including Chevy II, Chevelle, and Camaro as well as some full-size models — on a COPO (Central Office Production Order) basis for evaluation. Many of these cars were retained by the company for staff use, but some were sold to fleet users like taxi companies, and some of the TH350-equipped ’68s did eventually end up in public hands. I don’t know if Buick had a similar 1968 test fleet, but it seems likely.
Chevrolet anticipated a demand for at least 1.2 million of the new transmissions a year; they would eventually convert three former Powerglide production lines to build TH350, one in Windsor, Ontario, the others in Parma and Toledo, Ohio. Buick planned to entirely convert its Flint transmission plant by 1970, but the changeover was gradual, so various 1969 models still offered the 2-speed Super Turbine 300. Oldsmobile and Pontiac also used the new TH350 for some models, but they bought it from either Buick or Chevrolet rather than building it themselves.
Because Buick was selling a good chunk of its initial TH350 production to other divisions, the new transmission wasn’t universally available on 1969 Buick cars, even ones that could have used it. TH350 was optional on the Skylark Custom, GS-350, California GS, and the Sportwagon with the 350 cid V-8. It was not yet available on the Special Deluxe (except California GS), base Skylark, or LeSabre (although the LeSabre could still be ordered with the four-barrel 350 and TH400 by specifying the “400” High Performance Group). The automatic GS-400 and 400 cid Sportwagon continued to offer the TH400 instead, as did the full-size Wildcat and Electra 225.
On the models where it was available, the TH350 listed for $205.92, $21.12 more than the 2-speed Super Turbine 300. It was a bargain: The 3-speed automatic’s 2.52 low gear gave much better off-the-line snap even with the 2.56 axle ratio Buick now often specified on cars with automatic. The 1.52 2nd gear also provided a gear for highway passing or mountain driving (up to about 80 mph, depending on axle ratio), something the smaller-engine GM intermediates had needed for years.
I don’t have a breakdown of how many 1969 Buicks had the TH350, but the division built 83,826 cars that could have had it, including 35,639 Skylark Custom sport coupes like this one.
In 1969, the Skylark Custom was the plushest version of the Buick A-body intermediate line, offering a choice of expanded vinyl or Kinross cloth-and-vinyl upholstery, foam seat cushions, and fancier door trims.
I think the primary audience for the Custom was single people, many of them working women, who wanted Buick posh in an easier-to-park package.
Neither Consumer Reports nor Car Life was terribly enthusiastic about the 1969 Skylark. It wasn’t especially roomy even for four, its brakes were mediocre unless you ordered front discs (which few people did), and its fuel economy was unimpressive. Car Life said it had “a lovely ride,” but a total lack of enthusiast-type handling response and very little grip. (Interestingly, 205R14 radial tires were a factory option on the 1969 Skylark, although the test car didn’t have them, and I don’t think they were common.)
The CL car had the regular fuel 350-2 engine, also fitted to the white car pictured here, but linked to the 2-speed Super Turbine 300, which gave 0 to 60 mph in 11.6 seconds. I’d expect the TH350 to cut something close to a second off that time and allow much more confident highway passing — like I said, a real bargain for $21.12. Consumer Reports recommended it.
For 1970, the Super Turbine 300 was gone, leaving the TH350 as the sole automatic choice for most smaller-engine Buick, Oldsmobile, and Pontiac models. Chevrolet was slower to let go of Powerglide. Their representatives told Motor Trend in 1968 that they didn’t see the 3-speed automatic replacing Powerglide “in the foreseeable future,” even though they expected TH350 to outsell Powerglide by at least 2 to 1. “Market conditions can change, but we have felt there is a place for an economical 2-speed automatic transmission,” they said. I expect the bigger consideration was that Chevrolet wanted to slow-roll the tooling conversion, which was inevitably expensive. Powerglide stuck around on some models through 1973.
In the ’70s, the TH350 became a GM mainstay, used in very large numbers in more than half a dozen variants. Ron Sessions puts total production at something in excess of 32 million transmissions through 1984. The TH350 has a good reputation, and gave solid service in a wide range of applications; later light-duty Turbo Hydra-Matic variants like the TH200 had a much more checkered career.
Related Reading
CC Tech: The Buick Super Turbine 300 Automatic Transmission – Not a Powerglide (by me)
CC Tech: Inside Buick’s Turbine Drive (Twin Turbine) Automatic Transmission (by me)
CC Tech: More on the Buick Dual-Path Turbine Drive Transmission – Asking “Why” As Well As “How” (by me)
Curbside Classic: 1969 Buick Skylark Custom – No, It’s Not A Chevelle… (by Tom Klockau)
Curbside Classic: 1969 Buick Skylark Custom Convertible – Let Her Cry, For She’s A Lady (by Joseph Dennis)
Curbside Classic: 1969 Buick Special DeLuxe Wagon – Not So Very Special In 1969 (by Paul N)
1969 Buick Sportwagon 400 – The Soccer Mom Supercar (by me)
Aaron, as always very informative article. I was vaguely aware that GM was late in widespread use of 3 speeds automatics, but not that it took so many years compared to the other big companies.
Curiusly, it’s predecessor Powerglide seems to be quite popular with drag racers – I was at an event recently and several cars mounted it
The Powerglide appeals to drag racers with high-horsepower cars because the tall first gear can help with traction, and as Aaron mentioned, the two-speed has “fewer frictional losses” than a three-speed. Finally, it’s one less shift, so less time is wasted (albeit it minimal). Finally, a Powerglide can be a very strong transmission.
I appreciate the reply. I am no expert in drag racing but traction indeed appeared to be a big problem for some cars, they had obvious wheel spin well after launching
Oh, yeah, hardcore drag racers LOVE Powerglide. Its power consumption is low (by some estimates, less than half of a TH400 and roughly one-third of a Ford C6), it’s easy to beef up, and there are torque converters available with a whole range of stall speeds and torque ratios to let you tailor launch characteristics pretty much however you want.
Thankyou for the reply. I had no idea that there was such a difference in efficiency!
You can even get CNC machined billet cases for the PG these days
My 409 car came with a PG but I lost no time getting a TH350, one of the easiest swaps there is. For racers the modern PG hardware out there is insane. Have a 3500 HP drag car? No problem, off the shelf! https://www.atiracing.com/products/trans/pg/index.htm
The Powerglide still had less parasitic hp loss compared to the THM350, some website sources list 18hp vs 36hp, while the THM400 is listed as 44 hp. C6-55-60hp. Thus, still relevant into the early 70’s for lower powered 6-cyl cars like the Chevy Nova, and others. On mid-size and large V8 powered GM cars, in the day,
seeking beter acceleration were often buying a B&M Holeshot Torque Converter with a higher stall speed for their Powerglides if they were not converting it to a THM350.
Having worked in the trans business for almost 40 years bringing the thm 350 forward was a genius move for gm leaving power glides for the purpose I believe they were best intended for now we see 9 speed transmissions which have put greater economy in the forefront of consumers reliability
The ’69 Chev Townsman wagon my dad had with the 2bbl 327 had the PG. Wouldn’t surprise me if the TH350 wasn’t even available with the 327. I have that book by Ron Sessions leftover from my trans rebuilding days (great paying off season work when in the boat repair biz) his book is great.
The 327 was available with the TH350 in 1969, although just about every Chevrolet engine up to the L48 350 was ALSO still available with Powerglide at that point.
I always heard that GM did not want to pay the patent royalties to Simpson, don’t know if that’s true. Simpson worked for Henry Ford but had health issues and moved out west to live out his life but he ended up living a long time and filing patents on planetary gearsets.
The Buick suffered the most in styling by far compared to it’s GM stablemates until they cleaned up the 1970.
An amazing transformation I remember as vividly today as then was a transmission change-out of my cousin’s 1968 Chevrolet Camaro in the early 80s. It had a Powerglide and the base V8: 327 2 barrel carburetor, single exhaust and 210 gross HP. Its performance was adequate, not in the least spirited.
A year after acquiring the car we replaced the PG with a THM 350, and added factory dual exhausts. Following this, the car went from lethargic off the line to a car that literally LEAPT, and pulled HARD, well until limited by the tiny 2bbl carburetor ( it looked physically smaller than the ubiquitous Rochester 2GC ) and tame camshaft. I was truly impressed.
It would be interesting to know how much of that improvement came from the THM-350 and how much from the dual exhausts. Duals typically add some 25-30 real (net) hp, which is a significant jump. But the combination undoubtedly made a very palpable change. Just needed a 4-barrel carb to complete the makeover!
I’d say it was both: It pulled stronger through the mid-rev range, say 4000 rpm, but the initial slam of acceleration ( and I do mean SLAM ) from a stop or very slow speed suggests it was the shorter 1st gear of the THM 350 compared to the PG.
Perhaps it could be that the rear axle ratio was short enough to compensate for the taller PG 1st gear, but combined with the shorter T350 it just amplified it. A few times it actually chirped the tires on an automatic 1-2 upshift. Blew my mind.
He replaced that car with a 1969 RS Camaro, 307 inch engine and a THM 350, and while it was sportier looking, it was a slug in performance compared to the older car.
Aaron, you have once again taught me something new. For the life of me, I would have never guessed that the THM350 was a Chevrolet and Buick design, not Hydramatic. I had always assumed that the THM350 was made by them. The THM350 was a good and durable unit. In taxi use, they lasted pretty much exactly 200,000 km. That is in pretty rough conditions.
A family friend had a 1969 Skylark. As a kid, I was deposited in the tiny, black hole back seat, not that Dad’s 1970 Pontiac Strato Chief was any better.
I had always heard that the TH350 was Powerglide-based. Maybe not exactly true, but closer than a light duty TH400. Chevrolet and Buick seemed to share engineering philosophy more than other divisions, in-line OHV engines, torque tube drive, torque converter automatics, coil spring rear suspensions.
My mother’s first car with an automatic transmission was a 1973 Chevy Monte Carlo with the TH350. No more stick shifts for her after that!
I realize that the looks of the 1968-69 Buick Skylark coupes are polarizing, but I find them a guilty pleasure to the point that my fantasy garage of 10 classic cars would include one.
Aaron I love your articles! This piece on trannies refers to various strengths and weaknesses among their number, and crucially raises the question of idle capacity. Speaking as a mechanical engineer and a socialist, in the case of shortcomings of performance and reliability, the place to discover them is in product development. The competition to get to market (and associated secrecy) works against full testing and development, the solution to which is a unified democratically-directed nationalized auto industry in place of irrational competition. Of course industry-wide, the question of duplicate capacity and absence of interchangeability among the trannies and their parts and tooling, not to mention duplicated labor at every level up through senior engineering, is colossal. Seemingly three or four openly-designed tranny types would have served the nation in the era under discussion. Finally, I never knew that the Powerglide had a following at the drags! At the tranny shop where I stabbed ’em in ’73, including the cast iron ones, we sneered at Powderglides!
The duplication of labor was less than you might assume, given the enormous production volumes of the time. This was part of the reason GM divisions mostly had their own engines: They needed most engines in huge numbers, which was going to require multiple plants and multiple assembly lines regardless. For a corporation the size of General Motors, the facilities and tooling costs were always significantly greater than design or engineering costs, and even where it might have notionally made sense to consolidate around a common design, doing that at scale would have been a truly enormous expense. There were other factors, but that was a big one.
It’ll always be stolen body in the trunk car from the 1980 movie 9 to 5 to me.
Your dash pictures showing the shift quadrants are not correct. Any that has L2 L1 has the TH400 in it. The TH350 did not have those shift ‘options’.
I’m not sure where you’re getting that, but it’s wrong. Buick and Chevrolet versions of the TH350 with column shift had a P-R-N-D-L2-L1 pattern. Oldsmobile and Pontiac column shifts had P-R-N-D-S-L. This is shown in the AMA specifications and the shop manuals.
1969 Oldsmobile A-body AMA specifications (p. 22): https://over-drive-magazine.com/wp-content/uploads/2024/01/1969-Oldsmobile-F85-Cutlass-Cutlass-Supreme-AMA-1-34.pdf
1969 Buick A-body AMA specifications (p. 17): https://over-drive-magazine.com/wp-content/uploads/2024/02/1969-Buick-Special-Skylark-V-8-AMA-1-26.pdf
1969 Chevrolet A-body AMA specifications (p. 21): https://over-drive-magazine.com/wp-content/uploads/2024/02/1969-CHEVROLET_Chevelle-and-El-Camino-1-33.pdf
I would assume that Olds and Pontiac bought all of their TH350’s from Buick, that went behind V-8s (*at least initially), for the same reason they used a version of the ST300, their V-8 engines shared the BOP bell-housing pattern. Which of course goes back to that plant utilization rate, Chevy sold enough on their own to keep their transmission factories running at full tilt. Meanwhile Buick, Olds and Pontiac couldn’t each keep a plant at capacity so Buick’s plant supplied all 3. It wouldn’t have made sense for the Chevrolet plant to have tooling set up to do the machining of the BOP pattern cases, (*well at least initially).
The first energy crisis of course changed all that. Full size Chevy sales fell and meanwhile a lot of people decided the mid sized Cutlass was the car to have and took the Impala’s best seller title. So we are back to plant utilization, Olds simply didn’t have the engine plant capacity to meet demand while Chevrolet suddenly found themselves with underutilized plants, both engine and transmission. Of course we all know the main part of the solution they took, Chevrolet sold engines to Olds to keep both division’s their plants humming and set themselves up for future litigation.
*However that presented a bit of a problem as the Chevy engines were to supplement and not replace the Olds engines. That meant they would have needed to purchase and keep both Chevy and BOP pattern TH350s on the line. Chevy of course had excess transmission capacity too so a 3rd TH350 case, the Universal that fit both patterns was introduced. This was made simpler by the fact that both patterns share the location of the lower 4 bolt holes and the alignment dowels. At that point I expect that Olds^ bought all their transmissions from Chevrolet, likely including those that went behind the Buick V-6s.
^ Based on working on cars from that era back in the day the Olds from that era seemed to use the universal case exclusively, while Buicks and Pontiacs seemed to continue for a least a while with the BOP pattern cases. Chevorlets could be found with both, so I assume they at least initially just switched 1 of the lines to the universal pattern.
On a modern day note plant utilization was a major consideration in Ford’s upcoming universal platrom and it’s “up to 8 body styles” They realized that a lot of those body styles will be too low volume to justify an assembly line on their own and it ihard to predict just how big a new niche will be. So they have plans to add models until they reach a good utilization rate.
That same thinking is what gave us the Bronco Sport and Maverick. Only problem is they sold better or miuch better than predicted and they never got to the 3 model intended for that line, the still born generation of the Transit Connect.
Yes, I agree that’s likely. Buick’s transmission plant capacity was really quite large — it was not Chevrolet large, of course, but from 1948 through 1963, they were building all their own automatics. In the ’50s, they were selling over 500,000 cars a year for a while, 97 or 98 percent with automatic, so it’s not hard to do the math. In the early ’60s, their full-size volume was still way off, so they also made Dual Path for the Y-body. Once Buick decided to start buying TH400 from Hydra-Matic Division, the big question was undoubtedly, “What are we going to do with all this transmission capacity?”
So, they were clearly capable of building way more automatic transmissions than they needed for the A-body and LeSabre, and once their plant was fully converted to TH350, they probably had enough capacity to supply most if not all of what Pontiac and Olds needed at that point.
Thanks Aaron, this piece answers a few lingering questions I have had on the TH350. As a fan of GM’s ’67-’72 light truck lines, it’s interesting to look at how the TH 350 was phased into the truck line. The 1969 Chevy truck salesman’s review booklet mentions ‘a new 3 speed automatic’ along with the continuing Powerglide and Turbo Hydra-Matic (400) automatic transmissions. While the new TH350 was not used in trucks with the 396 V-8 it seems it was used in most any other truck, including C-30 1 tons with 350 engines. The new for 1969 K5 Blazer was the first GM 4X4 light truck with an automatic transmission (aside from a handful of late-50’s GMC’s with 4 speed Hydra-Matics) and by 1970 automatics were a regular option in all 4X4 light trucks. In all instances the transmission used was the TH350 and it wasn’t until much later that the Turbo 400 went into any GM 4X4’s. This was a little surprising as Jeep had long used the Turbo 400 in many of their 4X4’s. Incidentally the old Powerglide was available in 1/2 and 3/4 ton 4X2 trucks through 1971. I have seen a few of these later Powerglide trucks and in most instances the trucks were oddball optioned 6 cylinder models.