TH350 Vs. TH400 Transmissions: What're The Differences?

Long before automatic transmissions like the 4L60E earned such bad reputations, General Motors entrusted the Turbo Hydra-Matic family to handle shifting duties in its legendary lineup of muscle cars. The first automatic transmissions used in cars were primarily two-speed affairs like GM's popular Powerglide. However, the TH350 and TH400 automatics upped the ante with an extra forward gear. While the two Turbo Hydra-Matic automatic transmissions shared GM origins and three-speed operation, there are some important differences in their intended purpose, component design, and overall reliability.

Despite the higher number in its name, the TH400 came out in 1964, five years before the TH350. The TH400 is more heavy-duty, too, featuring stronger components like 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 up to 450 pound-feet, significantly more than the 350 pound-feet rating of the TH350. Overall, the TH400 was destined with durability in mind to endure the abuse of serving in heavy-duty trucks and high-performance vehicles.

Ultimately, though, the heavy-duty build of the TH400 became a disadvantage, and the lightweight TH350 became popular in production muscle cars of the era. Today, it's still a good choice for daily drivers with moderate engine performance, especially when compactness and fuel economy are prioritized over durability.

The history of the 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 its place. They were derived from the Oldsmobile-designed four-speed Hydramatic dating back to 1939. The Hydramatic's adaptations for use in WWII military vehicles improved its design while making it more complex, and the lessons learned from it led to improved designs handed down to the TH400 and TH350.

The TH350 design served General Motors well from its late 1960s debut, but it was ultimately replaced by the 700R4 in 1982. The TH400 held on for a while longer, seeing use 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. In time, the mechanically-controlled 700R4 was replaced by the 4L60E, another transmission with electronic controls, in the early 1990s.