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TH400 Transmission Spotlight

th400 th-400 turbo 400

Turbo-Hydramatic or Turbo Hydra-Matic is the registered trade name for a family of automatic transmissions developed and produced by General Motors. These transmissions mate a three-element turbine torque converter to a Simpson planetary gear train, providing three forward speeds plus reverse. The TH400 is highly regarded for its supreme durability. This model has a conservative rate of 450 ft. lbs of input torque, though that can be easily upgraded with aftermarket improvements. These transmissions can come up to a maximum of 1000 torque. The TH400 transmission has available a variety of performance ratings, such as mild, heavy duty, super sport, and SS Xtreme. The higher model that is chosen, the higher rated the horsepower and torque will be.

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 including Chevrolet’s Powerglide, Buick Super Turbine 300, and Oldsmobile Jetaway. Starting in the early 1980s, the Turbo-Hydramatic was gradually supplanted by four-speed automatics, some of which continue to use the “Hydramatic” trade name. It is an immensely popular transmission in the automotive industry as well as the aftermarket. Today they are found in GM’s, Jeeps, Jaguars, Rolls-Royces, Ferraris and others.

The THM400 was the first three-speed, Simpson-geared automatic to use overrunning clutches for both first and second gear reaction, a feature that eliminated the need to coordinate the simultaneous release of a band and application of a clutch to make the 2-3 gear change. Owing to this feature, as well as the use of a large, multi-plate clutch to provide second gear reaction, the THM400 is able to withstand very high input torque and an enormous number of shifting cycles, as would be encountered in frequent stop-and-go driving. As a result, it has met with considerable success in commercial vehicle applications.

Gear Ratios of the TH400 are:

  • First Gear – 2.48:1
  • Second Gear – 1.48:1
  • Third Gear – 1.00:1
  • Reverse – 2.07:1

The 400 transmission has a main case of cast aluminum alloy with a length of 24-3/8″ long. Its aluminum case is essentially smooth. The rear mounting face of the transmission has a hex bolt pattern with ribs running forward longitudinally. The fluid pan shape is irregular (see image, left), being likened unto a distorted Texas pattern. The TH400 is the largest of the common GM auto transmissions, but still surprisingly compact in light of the immense power they can handle.

There are two significant variations of the TH400. The TH375 was a version of the transmission used from 1972-1976 in smaller displacement cars. It is identified easiest by its “375-THM” designation cast in the underside of the tail housing. The TH475 was an extra-heavy-duty version, and was found in larger trucks from 1971 on.

By 1980, the relatively heavy THM400 was being phased out of usage in passenger cars in response to demand for improved fuel economy. The THM400 was utilized in the C- and K-series (full-size) Chevrolet/GMC pickups and G-series (full-size) vans until 1990 when GM switched over to the 4L80E. Today, the United States Army HMMWV 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.