GMs first "shiftless" automatic transmission, the Powerglide transmission, was introduced in 1950. Since that time, GM has expanded from the original two-speed transmission to three-speed automatics, then four-speed automatic over-drive units to the six-speed units used in the latest vehicles.
Early Powerglide transmissions were two-speed units that were made of cast iron. In the early 1960s, GM redesigned the cases by manufacturing them from cast aluminum, and incorporated a removable oil pan. Produced until the early 1970s, Powerglides had either a 1.76- or 1.82-to-1 first-gear ratio and a 1-to-1 final-drive ratio.
The Turbo-Hydramatic 350 three-speed automatic transmission was developed by GM in 1969 as a lower-performance version of the heavy-duty TH400. With a first gearing of 2.52-to-1, it provided better low-speed acceleration than the Powerglide. Second gear had a 1.52-to-1 ratio and the final drive ratio was the same as the Powerglide: 1-to-1.
Powerglide transmissions are very popular in high-horsepower drag racing applications. The higher first gear ratio helps aid traction but reduces initial acceleration slightly. In modified form, they are capable of handling far more power than the TH350. Both transmissions are dimensionally similar, and are interchangeable. The larger heavy-duty TH400 is not, but is capable of handling much more power than the TH350.
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