The Turbo Hydramatic 400 automatic transmission (TH400) -- developed by General Motors' Hydramatic Division -- is a heavy-duty transmission intended for engines that produced high levels of torque. It continues to be widely used in severe-duty applications.
Originally designed in the 1960s to replace the Powerglide, Super Turbine 300 and Jetaway transmissions, it has even been used in older full-size Jeeps and Jaguars. It was commonly used in GM cars and trucks well into the 1990s.
The Turbo 400 is based on the patented Simpson gearset design -- a heavy-duty three-speed automatic transmission geartrain -- which GM acquired licensing rights to in 1964. GM engineers also incorporated over-running clutches with multiple friction plates and designed the case and internal structure to handle increased power levels.
In standard form, the Turbo 400 was able to withstand the output of GM's large-displacement, high-performance engines of the late 1960s and early 1970s. Modified with aftermarket components, it is commonly able to withstand horsepower levels in excess of 700, and sometimes as much as 900 horsepower or more with extreme-duty parts.
