The Turbo Hydra-Matic 400 transmission made its first appearance in 1964 Cadillac models. It has been given a conservative input torque rating of 450 lb-ft, although its application in many high performance vehicles suggest that the transmission can handle much more abuse. It is slightly larger and marginally stronger than the TH350, as it was designed for use behind General Motor's various big block V-8 engines. In trucks, the TH400, or "Turbo 400" as it is often referred to, the transmission was used until 1990 when it was replaced by an overdrive evolution of itself, the 4L80E. The name changed in 1987 to the 3L80, which was essentially the result of General Motor's new drivetrain naming convention. The "3" stands for three speed, the "L" for longitudinal (mounted), and the "80" for a GVWR of 8,000 lbs.
The transmission, like other automatics, relies on the Simpson planetary gearset, which revolutionized the operation of the automatic transmission. As previously stated, the transmission is popular in high performance applications and can be affordably built to manage a tremendous amount of power and is praised for its high strength to weight to ratio.
TH400 Transmission Ratios
| 1st | 2nd | 3rd | R |
| 2.48 : 1 | 1.48 : 1 | 1.00 : 1 | 2.07 : 1 |
TH400 Transmission Specs
| Transmission | Turbo Hydra-Matic 400 (TH400) | |
| Type | 3 speed automatic transmission | |
| Manufacturer | General Motors | |
| Applications | 1964 - 1990 GMC, Chevrolet cars and trucks, as well as vehicles from other divisions of General Motors 1982 - 1990 C/K series pickups (6.2L diesel, 3/4 and 1 ton models) |
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| Case Material | Aluminum | |
| Case Length | 24 3/8" | |
| Input Shaft | 32 spline | |
| Max Input Torque | ~ 450 lb-ft | |
| Weight | ~ 150 lbs w/ ATF | |
| ATF Type/Spec | DEXRON automatic transmission fluid | |
| Identification | Integrated 1 piece bellhousing |  |