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Turbo-Hydramatic is the registered tradename of a family of automatic transmissions developed and produced by General Motors. These transmissions mate a three element torque converter to a Simpson planetary geartrain, providing three forward speeds plus reverse. more...

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The Turbo-Hydramatic (THM) series was developed to replace both the original Hydra-Matic models and the Buick Dynaflow. In its original incarnation as the Turbo-Hydramatic 400 it was first used in the 1964 model year in Cadillacs. The Buick version, which followed shortly thereafter, was known as the Super-Turbine 400. By 1973 THM units had replaced all of GM's other automatic transmissions (e.g. Chevrolet Powerglide, Buick Super Turbine 300, Oldsmobile Jetaway, etc.). From the early 1980s onward, it was progressively replaced by later four-speed and five-speed automatics (which incorporate an overdrive gear), some of which continue to use the "Hydramatic" trade name.

Although the Turbo-Hydramatic name is related to that of the first fully automatic transmission, Hydramatic, developed by General Motors Oldsmobile division in the late 1930s, the two transmissions were not mechanically related.

Super Turbine 400 / THM400 / 3L80 / 3L80HD

The THM400 (or the Turbo 400, which it is called by drag racers and car enthusiasts) is a 3 speed automatic transmission that was first introduced at the beginning of the 1964 model year in Cadillacs and Buicks (under the name Super Turbine 400). The following year, use expanded to Oldsmobile and Pontiac and to some full-sized Chevrolets (usually coupled to the Mark IV big-block V8). It can be identified by an oil pan in the shape of what some would call similar to that of the state of Texas.

Many of the Buick, Cadillac, and Oldsmobile TH400s produced between 1965-67 were equipped with a torque converter that incorporated the Switch-Pitch variable-pitch stator, which is sought after by collectors and drag racers. These can be identified outside the vehicle by a noticeably narrower front pump spline. Externally there are no differences from the non-SP TH400.

THM400s were not the only Switch-Pitch units used in GM vehicles - the Super Turbine 300 (ST300 or 300THM) had a similar setup as well as Buick's 1955-1963 twin turbine Dynaflow.

By 1980, usage in GM passenger cars was on a decline because of the 1973 Arab Oil Embargo, when the Hydramatic 200-4R and 700R4 overdrives were phased in. The Presidential Limo (a modified 1984 Cadillac Fleetwood Brougham) used during the Reagan Administration was the last known GM passenger vehicle to use the THM400, alongside the C and K series (full size) Chevrolet/GMC pickups and G-series (full size) vans. Today, the U.S. Army HUMVEE is the only vehicle using the THM400. The civilian Hummer H1 originally had the 3L80s, but the current model has had a 4L80E since the mid-1990s.

Read more at Wikipedia.org