The Powerglide Transmission
GM’s Powerglide transmission
was first introduced in late 1949, and during its life it was continually being improved. The first model had an exotic five-element torque-converter section with twin pump turbines and twin stators. These had different blade angles to give a healthy stall torque multiplication ratio of 2.2 to 1 and a broad torque range at higher speeds (since the twin pumps and stators would start to freewheel at different road speeds).
GM engineers were so satisfied with the converter, in fact, that they didn't see any need for any auxiliary gear torque multiplication in DRIVE range. You just took off on the converter and that was it. Needless to say, acceleration left something to be desired. Zero-60-mph times were considerably slower than the same engine mated to a manual box, and the high slip losses at low speeds took quite a toll in fuel consumption. It was during this period that automatic transmissions gained their reputation for poor performance.
To get the best performance from these original powerglide units, you needed to start off in LOW range, where there was a 1.82-to-1 planetary gear step-up, then shift to DRIVE at 20 or 30 mph. In 1952
Chevrolet manufactured their 1 millionth Powerglide transmission. For the 1953
US GM models, Chevrolet engineers made important changes. They went to a simple three-element converter (single pump turbines and stators) and incorporated the 1.82-to-1 gear step-up in DRIVE range. Thus you would start off in LOW range, and the transmission would automatically up-shift to HIGH at some speed between 12 and 45 mph, depending on throttle opening.
This helped acceleration and fuel economy a lot in DRIVE range and made the Powerglide's performance more competitive with some of the newer automatics being manufactured by other car makers. But even back then, the handwriting was on the wall. GM's four-speed Hydra-Matic was in its heyday. This unit gave very quick breakaway acceleration, and your kick-down gear for passing (actually third gear) would carry you to 65 or 70 mph before the final shift into HIGH.
Hydra-Matic was being adopted by several companies outside of GM (Nash, Hudson, Kaiser, Lincoln). Meanwhile, the Ford-O-Matic was being improved. This had three forward speeds from the start, but on the early versions, Ford used only the top two ratios in DRIVE range. You started in second and shifted to high. It was no trick for Ford to rework the hydraulic system to start off in low in DRIVE range and run through all three gears. This gave them just as big a margin of bread-and-butter road performance over Chevrolets as they had before - when Ford started in second and Chevy in high.
The benefits of three forward speeds with a torque converter were becoming obvious during the 1950s, the torque converter alone may have provided a torque multiplication ratio of over 2 to 1 with brakes
locked and a wide-open throttle. But you didn't use full throttle when starting off in normal driving. And this fluid torque multiplication in the converter faded out quite quickly as the car picked up speed. You didn't have much left when you reached 10 or 15 mph. This was where you needed that extra low gear for really responsive acceleration on the street. There was a big gap in the acceleration curve between 15 and 35 mph with a two-speed converter. And you had another gap at the top end.
With two speeds, you generally needed to use a low gear ratio around 1.8 to 1 for adequate breakaway. But this meant you had to up-shift to HIGH at only 55 or 60 mph at the most - to keep from over-revving the engine. This was right in the middle of your passing range on the highway. It was the worst place to have the engine up-shift, with the inevitable bog in acceleration that followed when the engine rpm dropped back. The only answer was a three-speed gearbox with a closer ratio between second and high. This averaged around 1.45 to 1 on three-speeds of the early 1960s, and allowed a maximum speed of anywhere from 65 to 75 mph before the up-shift. This gave a much better and safer highway passing range.
Improving the Powerglide
But the GM engineers continued to buck the industry trend for years. Instead they continued to improve the old two-speed Powerglide - switching to an aluminium die-cast housing, improving the gearing, beefing it up, changing the ratio to 1.76, developing a better torque converter, hydraulic system, seals, and controls. The Powerglide of the mid 1960s was undoubtedly the most inexpensive automatic transmission (to manufacture) in the industry. Also, it was probably near the top of the list in reliability, long life, and ease of servicing. It remained a brutally practical transmission. And there was a minimum of service headaches – which was not such a bad thing for Holdens here in Australia that were fitted with the unit.
But by 1966
the writing was on the wall, and the two- speed Powerglide was on the way out. Rumour had it that the change was supposed to have been made in 1965
except for a conflict over design among the divisions. Many GM officials wanted Chevrolet to use the new three-speed torque converter being made by the Hydra-Matic Division (which was then being used by U.S. GM divisions including Buick, Cadillac, Olds, and Pontiac). This was in line with Ed Cole's top-level policy to standardize on more mechanical components throughout the corporation. The idea was to build up production on the Turbo Hydra-Matic until there would sufficient quantity to also equip the Chevrolet division.
But Chevrolet officials were none too happy about making the switch. Many Chev exec’s were quoted as saying that Chevrolet were capable of building their own three-speed converter more inexpensively than they could buy it from the Hydra-Matic Division. And they were convinced they could build it more simply without sacrificing performance, reliability, or durability. At this time in the U.S. car industry the GM divisions were jealous of their rights to do things their own way in most technical areas, and this was a perfect example.
But it was not to be, and in 1969
, the three-speed Turbo Hydramatic 350 (THM350) was introduced as a light-duty companion to the Turbo-hydramatic 400, and made available on virtually all Chevrolet cars and trucks with six-cylinder or small and medium-sized V8 engines, as well as intermediate sized cars of other GM divisions.