While the illustration above might look a lot like Chevrolet’s ubiquitous aluminum Powerglide, it’s actually a completely different transmission: the Buick-built two-speed Super Turbine 300 automatic transmission, which went into millions of Buick, Oldsmobile, and Pontiac A-body intermediates (and not a few Firebirds) from 1964 through about 1972. A Powerglide by another name? Not exactly — the Super Turbine 300 had some important differences as well as its obvious similarities.
The Persistence of Two-Speed Automatics
Although it remains a controversial point for GM fans and critics alike, the evidence suggests that from at least the late 1950s until the early 1970s, GM engineers were legitimately convinced that two-speed automatic transmissions were the best choice for smaller vehicles with smaller engines. This seems completely counterintuitive: With a smaller, less-torquey engine, wouldn’t you want more speeds rather than fewer, and a transmission with an intermediate speed suitable for highway passing? You would think so, but GM also believed there were tradeoffs with three- and four-speed automatics, including the greater size and weight of the transmission and the greater frictional losses of the more complex internal components, which smaller engines could more ill afford.
I don’t have any behind-the-scenes insights to offer here, but I’m convinced that the persistence of two-speed automatics at GM was what we’d now call a “data-driven decision,” prompted not (only) by cost considerations, but also by proving grounds test results showing acceleration times and fuel consumption that contradicted subjective impressions and conventional wisdom.
Imagine you evaluate two competing cars in the same size and price class back to back on a proving grounds test track. Car A feels decisively faster, with a transmission that readily kicks down for passing. Car B feels undergeared and often seems to be straining, and its passing gear is useless above about 45 mph. Yet, when you run the numbers, the slower-feeling Car B car turns out to be quicker. Which car is better? If you’re an engineer for the world’s biggest automaker, the answer is almost certainly going to be Car B, and if Car B is also cheaper to build, it’s likely to be a slam dunk.
There’s some real-world evidence of this (consider for example this 1966 Car Life comparison of six-cylinder full-size cars), and I’m pretty certain that this was the logic behind both the continued development of Powerglide, and also of its BOP cousin, which Buick called the Super Turbine 300, or ST-300.
1964 Pontiac GTO convertible with two-speed automatic / Mecum Auctions
(Oldsmobile, which bought this transmission from Buick, called it Jetaway, while Pontiac, which installed it in the A-body Tempest/Le Mans and later the Firebird/Trans Am, didn’t call it anything in particular. Since Buick built the transmission, and since Oldsmobile previously applied the Jetaway name to Hydra-Matic transmissions, it’s most convenient to use the Buick moniker.)
ST-300 vs. Powerglide
If you compare section views of the Super Turbine 300 and aluminum Powerglide, they don’t look much different. For 1962 to 1966 Powerglide transmissions, the giveaway is the rear oil pump, which the Super Turbine 300 didn’t have, but Powerglide deleted the rear pump for 1967, making the two transmissions look even more similar. (Separate front and real oil pumps were common on early automatic transmissions as a concession for push-starting or tow-starting a stalled car, but as that became less common, rear pumps were gradually omitted to save weight and cost. Powerglide was one of the last holdouts.)
1962–1966 aluminum Powerglide
1964–1967 Buick Super Turbine 300
The gear ratios are the same, too — 1.765:1 in low and reverse — and the gears are shifted in the same way mechanically. Both transmissions also have a vacuum modulator that adjusts line pressure based on engine load.
Where these transmissions most differ is in their controls. A non-electronic hydraulically controlled automatic needs at least two hydraulic control signals: governor pressure to measure the speed of the output shaft, and some way to measure driver torque demand so that the transmission will hold a lower gear for longer or shift down more readily for acceleration, hills, or pulling a heavy load. Powerglide does this with a throttle valve, mechanically connected to the throttle linkage. The Super Turbine 300 is controlled by the vacuum modulator, as on the Turbo Hydra-Matic.
The ST-300 modulator incorporates a spring-loaded rubber diaphragm that’s exposed to manifold vacuum and can move back and forth in response to changes in intake manifold, with an additional evacuated bellows that corrects the movement of the diaphragm for differences in atmospheric pressure. Here are two contemporary Buick diagrams illustrating the modulator arrangement:
In the bottom diagram, “Pm” stands for “manifold pressure” while “Pa” stands for “atmospheric pressure.” Atmospheric pressure acts against a spring-loaded aneroid bellows (whose internal pressure is zero) to compensate for variations in atmospheric pressure, such as at high altitude.
The modulator valve also adjusts the transmission line pressure based on manifold vacuum, and there’s a separate modulator limit valve (not shown above) that keeps the full-throttle shift points consistent, independent of manifold pressure.
So, you get earlier, gentler shifts in gentler driving, but higher-rpm shifts and firmer brake/clutch engagement when you’re driving hard, with both tailored to the engine’s actual intake manifold absolute pressure. Besides that, a vacuum modulator doesn’t need to be adjusted like a mechanical throttle valve linkage does, and it compensates automatically for altitude. I think these vacuum controls were more expensive than a mechanical linkage, but that was partially offset by simpler assembly — not a bad tradeoff.
Switch-Pitch Stator
Buick was once very big on switch-pitch torque converter stators, which they’d added to Dynaflow in the ’50s to improve passing response. The blades of the stator could actually be cranked between two different positions, making it possible to combine the characteristics of a “loose” and “tight” converter in single unit. The switch-pitch feature provided two different stall ratios, depending on whether the blades were at high or low angle.
Buick hadn’t used a variable-pitch stator on the lightweight Dual Path Turbine Drive used on the Y-body Buick Special from 1961–1963, but they adopted it for some early versions of the Super Turbine 300. (Pontiac never used the switch-pitch converter — I assume they didn’t want to pay for it — but Buick and Oldsmobile did from 1964 to 1967.) Here are the torque converter ratios:
| Stator Blade Angle | V-6 | V-8 |
|---|---|---|
| Low Angle | 2.75 | 2.50 |
| High Angle | 1.98 | 1.82 |
On older Dynaflow and Twin Turbine transmission, stator position was changed using a hydraulic servo controlled by the accelerator linkage, but for the Super Turbine 300 version, Buick decided to use an solenoid-controlled valve instead. The reason for switching to electric control was to eliminate the mechanical linkage, and also to make it possible to have the stator blades switch twice: The stator blades would stay in “high” position at idle, switch to “low” position for acceleration, and then switch back to “high” for cruising. Using the high angle at idle reduced the “creep” you normally get when you release he brake with the transmission in gear.
Stator blade control valve and solenoid for Super Turbine 300 transmissions with a variable-pitch stator
The switch-pitch feature gave you a kind of half-speed step between the transmission’s low and high gears, so the switch-pitch ST-300 is sometimes described as a “2.5-speed” transmission. The stator pitch would switch back to “low” angle if you opened the throttle more than about two-thirds, even if the transmission didn’t or couldn’t kick down from high to low gear, giving a little extra dig for passing.
Both Buick and Oldsmobile dropped this feature for 1968 and went to fixed-pitch stators like Pontiac. (There’s an article about the change in the July 1968 Motor Trend (pages 94–96).)
1964 Buick Skylark V-6 with two-speed Super Turbine 300 automatic / Mecum Auctions
Built by Buick
The other important and obvious difference between Powerglide and Super Turbine 300 was that Powerglide was built by Chevrolet, while the ST-300 was built by Buick. Buick had been building its own automatics since 1948, so their transmission plant had a pretty substantial capacity. Since most full-size Buicks were switching to Turbo Hydra-Matic for 1964, Buick could retool their own plant to build the new two-speed for the A-body Special/Skylark (and some B-body LeSabres and smaller-engine Oldsmobile 88s) and recoup the cost by also selling automatics to Oldsmobile and Pontiac.
1964 Oldsmobile F-85 convertible with two-speed Jetaway automatic / Mecum Auctions
This undoubtedly made more financial sense than Chevrolet having to add yet more Powerglide capacity to supply the B-O-P cars, and it gave Buick an opportunity to build what they thought was a better transmission, albeit still along similar lines. (This was more or less the same reason GM divisions usually got to build their own engines: Design was cheap compared to tooling and production capacity, so if the divisions had to establish separate manufacturing facilities anyway, producing a different design in each was not much more expensive than everyone using the same design.)
1964 Pontiac GTO convertible with two-speed automatic and Tri-Power engine / Mecum Auctions
I think what finally persuaded GM to kick the two-speed habit in the seventies was that it was becoming too big of a competitive disadvantage. Given their checkered record with some of the later light-duty Turbo Hydra-Matic derivatives, though, the engineers who insisted on clinging for so long to the older but sturdier two-speeds might have had a point after all.
Related Reading
Powerglide: A GM Greatest Hit or Deadly Sin? (by Paul N)
The ST-300 was also used in small block powered full size cars like the Olds Jetstar 88. This was a long-tail version due to the longer wheelbase in those B-body cars. And the ST-300 was dropped after the 1969 model year when the TH350 was released.
Yes, I mentioned the LeSabre originally, but it was also available on lower-end B-body Oldsmobiles. I added that to the text.
My Grandmothers 72 Corolla had a Toyo glide 2 speed auto. I suspect your right on as far as GM testing of two speed autos. Despite them feeling odd they did do what was asked of them and my grandmothers car never felt scary on an on ramp like some 3 speed manual trucks I have driven. The torque converters seemed to work awful hard thou.
Having recently graduated college, I acquired my grandparents former ’66 Tempest, equipped with the base OHC-6 and ST-300. I remember riding in the car as a kid (I was responsible for the plastic seat covers, as I frequently got carsick), and overhearing Grandpa comment on how the car would creep up to 90 MPH without you knowing it…
Prior to the car coming into my possession, I had driven it infrequently and came to like the “whoosh” feel of the ST-300. It compares somewhat to driving a modern CVT-equipped vehicle, albeit with one almost-unnoticable gear change.
By the time I acquired the Tempest, the OHC-6 top end had gummed itself up and was inop. I swapped in a SBC/THM300 drivetrain and had me a nice sleeper until I sold it about a year later. The cam carrier from the OHC-6 hangs in my shop today.
The Powerglide/CVT comparison is interesting. I’ve only ever driven one of each as far as I know. The E-CVT in our Prius was a good match for that car’s hybrid power plant, and the Powerglide in my school friend’s ‘67 Chevelle was a good match for its 283 V8.
Some thoughts on 2-speed performance. . .
The SBC V8 Chevys I recall from the ’60s didn’t seem to suffer as much as 2 gear powertrain combos. SBC Chevy liked to rev. Combined with standard axle ratios, most of the cars I recall could hit 60 before making the Great Leap Forward into high gear. Since 0-60 was a common acceleration measure, lack of a shift point up to that speed penalized SBC Chevys less than most other combos.
I don’t know if all the BOP engines normally paired with the ST300 rev’d as willingly as the SBC. A ’65 Pontiac 389 I once owned certainly didn’t rev as willingly. It couldn’t make it to 60 before the Great Leap Forward struck.
Until the modern Ford 289, I can’t recall a single Ford engine as willing to rev as the SBC. Powerflite ended auto use after ’61. Chrysler’s higher reving LA engine was introduced in ’64, so the two were never married.
GM was one of the world’s most profitable companies for several decades. And this was despite the fact that, for most of that time, the various divisions of GM were somewhat independent of each other in matters such as this. I can’t imagine what might have been if the divisions would have shared more design and production in this era.
When I was a kid really starting to understand cars in the 1970s, I was shocked to grasp that you could get a 350 V-8 from Chevrolet, Pontiac, Oldsmobile, and Buick, and they were all *entirely different* engines. It didn’t make sense to me then, and it barely does now.
The GM divisions were originally just different car manufacturers collected/bought by Durant. Any kind of meaningful centralization didn’t start until the Sloan era, and it was of course mainly just the bodies, as he realized that body design and building was very expensive and there were efficiencies to be had by sharing body shells.
Back then, many car manufacturers depended on outside suppliers for their bodies; even Ford bought a huge number of bodies from the Dodge Brothers. An automobile back then was seen as two very different basic elements: the chassis, including the drive train, and the body, which often came from various suppliers.
This tradition just extended into the post war era at GM, each division building its chassis/drive train, and using bodies by Fisher. And the reason it went on so long, especially with engines, is that each division had invested a huge amount of money in their engine production facilities, including the massive and very expensive transfer line machines that bored and machined the blocks. These machines and transfer lines were built specifically for the unique engine bore spacing for each division’s engines, and couldn’t be changed.
So there was no incentive to change that and go to one V8 engine, as the capital cost of doing so would have been enormous. Only in the more modern era, when the demands of fuel efficiency and emission regs became very significant, did it make sense to start breaking this tradition. Since Chevy had a massive capability to build its V8s, and cheaper than any other division, and its own V8 demand decreased, it became obvious to start using it in other divisions. Same with the Buick V6, the Pontiac Iron Duke four, etc..
As with many/most things at GM, it actually was cheaper to just keep the status quo than to make big changes like this.
Another relevant point was that if the divisions used components supplied by another division, they had to BUY them at a markup, just as they would any outside supplier. So, for high-volume products like engines, the lower unit cost of making them in-house could offset the tooling costs.
The corollary to that was that where the divisions already had existing factories, it was important to utilize them as fully as possible. Manufacturing plants have tremendous overhead costs, and leaving them idle or under-utilized is kind of like setting money on fire.
This was obviously a big consideration in the ST-300 and later TH350. If we assume that in the early ’60s, 95 percent of full-size Buicks and 90 percent of Y-body Buicks had automatic (I have more precise figures somewhere, but this is close enough to make the point), and Buick built all its automatics in-house, they had capacity of at least 350,000 to 400,000 transmissions per year. That’s a HUGE volume for something as complicated as an automatic transmission, and so it was vitally important that that plant have something to build if it wasn’t going to be building Dynaflow/Twin Turbine or Dual-Path transmissions anymore.
Aaron’s gift for making complex items seem easier to understand has always been appreciated my me.
Looking forward to more of his articles here!
+1
Powerglide had a vacuum modulator, too. To this day they are available for repair and for retrofit with aftermarket adjustable versions.
I remember the one on my 1967 Camaro liking to slip off a short piece of rubber tubing, the flexible link connecting it to rigid steel tubing.
Yes, Powerglide had a vacuum modulator (which it says in the text), but it only varied line pressure, not shift points. The modulator in the ST-300 (and TH400) did both.
I race a 65 Buick Gran Sport with a 401 Nailhead/Switch pitch ST300/3.42 rear in the Pure Stock Muscle Car Drag Racing series (PSMCDR) The car has run a best of 13.85 @100 mph. I have raced this car for 15 years in this configuration and am quite a fan of the ST300, especially when paired with the torque of the Nailhead. As tested in 1965, the writer observed ” the Transmission is strong where the engine is weak, and vice versa” and I’ve found that to be true. I have a built ST400 ready to go, but this car thrives on the low parasitic drag of the 300, and the variable stall really covers well for the high first gear. The transmission itself is bulletproof, this build has an easy 1000 WOT passes on it and drives like it was just put together. This build does use race duty clutches, steels and bands from the Powerglide, I was surprised there was any parts crossover. And I thought in non racing situations I’d miss the three gear configuration but honest I never notice. And you should see a passenger’s face when it holds at 5500 rpm for what seems like an eternity before the 1-2 shift. Great article on a misunderstood and unjustly maligned transmission.
but this car thrives on the low parasitic drag of the 300
This is a commonly underappreciated fact about the ST-300 and PG. Their parasitic losses are significantly lower, which helps offset the additional gear.
I recently read a vintage review of a car just like yours, and they took it to the strip and were very impressed with the combination and the results. I’ll post it soon.
LOL yeah less parasitic losses, we had a straight piece of road locally where cars got tried out at full throttle it was also on a 5% grade up hill the first ever car to accelerate from the speed limit to 100mph up that had a powerglide box in it, the same guy tried twice first in his dads 283 Impala 2 speed fail, second try was in a 3.3 Vauxhall Velox which did it easily.
Any automatic trans car was a rarity until the 70s nobody wanted them as the extra cost was unbelievable, It had to be ordered from the factory then imported built up which added freight and tariffs UK to NZ takes weeks and the salty atmosphere on ships usually meant you got a ready made rust bucket so it was harder to sell used, this was in the days when cars did not depreciate due to supply issues.
It’s been pointed out that the 2-speed transmission in the Buick Special gave better performance than the 3-speed in the Oldsmobile F-85. Does this indicate that the 3-speed was a mediocre design, the 2-speed was a good one, or a bit of both?
Both. The earlier Dual-Path Turbine Drive had a lower numerical first gear (1.58:1), with the converter giving a ratio of up to 2.5:1 at all, so you still had plenty of breakaway torque multiplication and could stay in low up to around 60 mph. In Drive, the shift from low to high was going from an overrunning clutch to a multi-disc clutch, so it was both quick and pretty seamless, and in high the split-torque arrangement gave you a partial mechanical lockup so the loose converter didn’t hurt cruising efficiency.
The F-85 had the miserable light-duty Roto Hydra-Matic, which had a huge ratio gap between first and second and was hampered by its reliance on the dump-and-fill coupling: Shifts involved coordinating a band, a multi-disc clutch, and first emptying and then refilling the fluid coupling, so it was comparatively very slow. (Also, Detroit Transmission Division came up with some exciting new developments in transmission seal technology, which combined with the high operating pressures meant a penchant for enthusiastic oil leakage, never an endearing trait.)
The Super Turbine 300 effectively replaced both of those transmissions until the medium-duty TH350 arrived for 1969.
Your articles at AUWM and this one have greatly expanded my knowledge on automatics, although there are moments in your technical descriptions when I struggle to follow every detail despite the clarity of your words and graphics.
I really need to update my PG post you linked to, as having done a number of Vintage Reviews of PG or ST-300 equipped cars has made me appreciate them even better. They are often maligned, but usually by those that simply don’t fully appreciate their qualities and strengths.
Speaking of strengths, one thing has become very obvious: Ford’s 2-speed Fordomatic that came out in 1960 was a significantly more inferior device than the PG. That comes across over and over in objective results and commentary in vintage reviews.
I’ve read several vintage reviews where the ST-300 was called a “2.5 speed automatic” due to the switch pitch TC.
The 350 turbo was a God send compared to the 2 speed.
Mom owned a 1961 Buick Special with the Aluminum V8 and a two-speed automatic transmission. It wasn’t a Powerglide, and according to this article, the ST300 debuted in 1964, so it wasn’t an ST300. Then, what was the A/T in Mom’s Special? Inquiring minds want to know!
The 1961–1963 Special had the Dual-Path Turbine Drive, which had no relationship whatever to the later Super Turbine 300 except for having two speeds. There’ll be more on the Dual-Path transmission soon.
Great article! This was a decent, reliable and often forgotten transmission. Our family had a ’66 leSabre with it. No issues. I still have to remind people that their GTO’s and Tempests, and F-85s did not come with a Powerglide….they came with the ST-300.
i commented, it immediately went.