TH350 vs. TH400 Transmissions: What are the Differences? – Jalopnik

Long before automatic transmissions like the 4L60E acquired such a bad reputation, General Motors entrusted the Turbo Hydra-Matic family to handle the shifting duties in its legendary line of muscle cars. The first automatic transmissions used in cars were primarily two-speed, such as GM’s popular Powerglide. However, the automatics of the TH350 and TH400 raised the bar with an extra forward gear. Although the two Turbo Hydra-Matic automatic transmissions shared GM origins and three-speed operation, there are some key differences in intended purpose, component design, and overall reliability.

Despite the higher number in the name, the TH400 was released in 1964, five years before the TH350. The TH400 is also heavier and has stronger components such as a 32-spline output shaft, while the TH350 uses a smaller 27-spline version. In stock form, the TH400 is designed to handle torque inputs of up to 450 pound-feet, significantly more than the TH350’s 350 pound-feet. Overall, the TH400 was destined for durability to withstand the abuse of heavy trucks and high-performance vehicles.

But eventually the TH400’s rugged construction became a liability and the lightweight TH350 became popular in the muscle car production of the era. Today, it is still a good choice for daily drivers with mediocre engine performance, especially when compactness and fuel economy are prioritized over durability.

The history of General Motors Turbo Hydra-Matic automatic transmissions goes all the way back to a time when whale oil helped cars survive before modern lubricants took their place. They were derived from the Oldsmobile-designed four-speed Hydramatic of 1939. The modifications of the Hydramatic for use in World War II military vehicles improved the design and made it more complex, and the lessons learned led to improved designs that were passed on to the TH400 and TH350.

The TH350 design served General Motors well from its debut in the late 1960s, but was eventually replaced by the 700R4 in 1982. The TH400 survived for a while and was used in heavy duty applications until the 4L80E replaced it in 1991. The newer transmission designs featured overdrive, and the 4L80E in particular also included electronic controls. Over time, the mechanically controlled 700R4 was replaced in the early 1990s by the 4L60E, another transmission with electronic controls.