Turbo-Hydramatic


Turbo-Hydramatic or Turbo Hydra-Matic is the registered tradename 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 geartrain, providing three forward speeds plus reverse.
The Turbo-Hydramatic or Turbo Hydra-Matic series was developed to replace both the original Hydra-Matic models and the Buick Dynaflow. 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's Super Turbine 300, and Oldsmobile's 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.
Although the Turbo Hydra-Matic name alludes to the original Hydra-Matic developed by General Motors' Cadillac division in the late 1930s, the two transmissions were not mechanically related.

Super Turbine 400 / TH400 / THM400 / THM375 / 3L80 / 3L80HD

The THM400 can be visually identified by an oil pan number four shown at . First introduced for the 1964 model year under the name "Turbo Hydra-Matic" in Cadillacs and "Super Turbine" in Buicks. The following year, application expanded to Oldsmobile and Pontiac and to some full-sized Chevrolets.
Many of the BOC THM400s produced between 1964 and 1967 were equipped with a variable-pitch stator torque converter called "Switch-Pitch" ; these are sought after by collectors and drag racers. A SP THM400 always has an external 2 prong connector, whereas a non-SP may have one or two. A Switch Pitch can be identified outside the vehicle by a narrow front pump spline.
Note: GM had also used a Switch Pitch in the 1955–1963 Buick twin-turbine Dynaflow and the 1964-1967 two-speed Super Turbine 300 used in Buick and Oldsmobile. Vehicles originally equipped with the Switch Pitch ST300 can be identified by their "Park R N D L2 L1" gearshift selector.
THM400 units had a 32 spline output shaft. A variant known as a THM375 is a THM400 built to mate to the smaller driveshaft yokes typically used for THM350 applications. They used a Chevrolet bolt-pattern case with a longer 27 spline output shaft inside a matching tailhousing cast with a "375-THM" or "TH375" identifier. Internally, the clutch packs originally had fewer friction plates. THM375s were found in some 1971-76 Buick Lesabres and Oldsmobile Delta 88s with the 5.7 liter V-8. Somewhere in the Mid-'80s Chevrolet C10 Pickups could also come equipped with a THM375. Some "Heavy Duty" THM350s were also designated THM375-B.
Another variant is the 3L80HD, often referred to as a Turbo 475. The 3L80HD has a straight-cut planetary gear set. There is no externally visible way to determine whether the transmission contains the straight-cut planetary gear set. The THM425 front wheel drive transmission shares almost all its internal parts with the THM400. Checker Motors Corporation used the Chevrolet version of the THM400 for its "A" series taxi and Marathon models until the end of production in 1982.
By 1980, the relatively heavy THM400 was being phased out of usage in passenger cars in response to demand for improved fuel economy. The THM 400 was utilized in the C- and K-series Chevrolet/GMC pickups and G-series 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.
Through the end of the '70s substantially more CBOP bellhousing THM400s were produced than any other THM400. Chevrolet bellhousing THM400s, while not rare, can be hard to find and are, as a result, usually more expensive to buy Chevrolet/GMC trucks and vans - when used with passenger cars it was usually coupled to a Mark IV engine or some high performance small blocks. The THM400 was never produced with a multicase bell housing.
Other auto manufacturers have used the THM400 and its 4L80E successor, including Ferrari ; Jaguar/Daimler ; Rolls-Royce ; the Nissan Prince Royal; AM General; and Jeep. Early Jeep THM400s used an adapter between the engine and transmission bell housing while later models had an AMC specific housing - which bolted to its inline six and V8. Though identical except for the bell housing pattern used through the '60s and ending in 1979 the THM400 was mated to the Dana model 18,20 and was the only transmission used with the Borg-Warner 1305/1339 all-wheel-drive transfer case used only in Jeeps until AMC/Jeep phased in the Chrysler Torqueflite 727 after 1979 until the FSJ platform was phased out. Additionally, the THM400 has been mated to other engines using adapter kits.
THM400 transmissions are very popular in automotive competition due to their great strength. Much of this strength comes from the use of a cast iron center support to suspend the transmission's concentric shafts that join the clutch assemblies to the gear train. The center support, which is splined to the interior of the transmission's case, also provides a robust reaction point for first gear. Since the first gear reactive force is evenly distributed around the periphery of the case, the types of mechanical failures that have plagued other competition transmissions are rare.
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.
For 1987, GM changed the nomenclature of their Turbo Hydramatic transmissions — the THM400 was renamed '3L80'. The 3L80HD was introduced in 1987 as the HD unit used in passenger trucks. In 1991, a four-speed overdrive version, the 4L80-E, replaced the THM400 in Chevrolet/GMC pickups, vans, SUVs, and commercial vehicles. The 4L80E was the first Hydramatic to incorporate electronic controls — almost all of the THM400/3L80/3L80HD's components are interchangeable.
Transmission fluid cooler line connections are found on the right-hand side of the THM400. The lower connection is the cooler feed, and the upper connection is the return. The case is tapped for either 1/4" National Pipe Straight NPS fittings,or 1/2"UNF fittings with a washer seal. 5/16" or 3/8" rigid coolant lines are generally connected via appropriate double-flared adapters.
Four-wheel drive truck applications used 3 various shorter output shafts that coupled with a female transfer case input shaft. Early transfer cases mated directly to the THM400 with a cast-iron adapter, usually a vertical oval shape. Later models used a circular style iron adapter which is generally considered the stronger of the two. The shortest was used with the NP203 transfer case.
GearRatio
12.48:1
21.48:1
31.00:1
R2.07:1

THM350

The Turbo Hydra-matic 350 was first used in 1969 model cars. It was developed jointly by Buick and Chevrolet to replace the two-speed Super Turbine 300 and aluminum-case Powerglide transmissions. So, although it carries the Turbo Hydra-matic name, the Hydra-matic Division of General Motors had little, if anything, to do with its design. The 350 and its 250, 250C, 350C and 375B derivatives have been manufactured by Buick in its Flint, Michigan plant, and by Chevrolet in Toledo and Parma, Ohio and Windsor, Ontario.
The THM350 was also regarded as a 'three speed Powerglide' and during its development, was generally called this. Although it uses the same torque converter as the THM400 it has a familial resemblance to the 1962-73 aluminum Powerglide from Chevrolet and was largely derived from the Chevrolet design. An important difference in the THM350 compared to the THM400 is that there is no fixed center support midway through the geartrain; this difference in layout would have permitted the THM350 to be adapted to the Corvair where the drive and driven ends are the same, but this feature was not exploited. Air-cooled versions of the THM350 appeared mid-1972 in the Chevrolet Vega and Nova 6.
One THM350 weak point was excessive end-play between the pump and center support and resulting wobble of the direct clutch drum due to both the end play and use of a relatively narrow bushing in the drum. This weak point can be addressed by using an extra thrust washer between the planetary gear and direct clutch to remove the end play and using a wider aftermarket bushing in the direct clutch drum. Another weak point is the relatively thin center support and the lightweight matching splines in the case. This weakness can be addressed by using an inexpensive aftermarket case saver kit.
Four-wheel drive truck applications for the THM350 used an iron adapter that mated it to the transfer case directly, similar to the THM400. The THM350 adapter was cast iron and used a sliding sleeve to couple the transmission output shaft to the transfer case input shaft with a steel coupler sleeve that was splined to accept both shafts and couple them together. An internal snap ring inside the coupler sleeve controlled the sleeve's position on the shafts, with circular seals in the adapter sealing the transmission from the transfer case.
For the 1981 model year, a lock-up torque converter was introduced which coincided with the new EMC control of most GM cars; this version is the THM350-C, which was phased out in 1984 in GM passenger cars for the 700R4. Chevrolet/GMC trucks and vans used the THM350-C until 1986. The lock-up torque converter was unpopular with transmission builders. B&M Racing once marketed a conversion kit for THM350-Cs during the early 1980s until the advent of high stall lock-up torque converters when its overdrive counterpart was modified. The standard TH350 is still very popular in drag racing.