Editing Turbo-Hydramatic (section)

=={{anchor|THM400|THM375}}Super Turbine 400 / TH400 / THM400 / THM375 / 3L80 / 3L80HD==
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[[File:General Motors Turbo-Hydramatic 400 Transmission - cadmanof50s.jpg|thumb|Turbo-Hydramatic 400 Transmission]]

The THM400 can be visually identified by an oil pan number four shown at [http://www.crankshaftcoalition.com/wiki/General_Motors_transmissions#General_Identification General Motors Transmission Pans]. 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 (automobile)|Pontiac]] and to some full-sized Chevrolets. 

Many of the BOC (Buick, Oldsmobile, Cadillac) THM400s produced between 1964 and 1967 were equipped with a variable-pitch [[stator (turbine)|stator]] [[torque converter]] called "'''Switch-Pitch'''" (SP); 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 (mostly one, except two on vehicles equipped with an internal pressure switch for spark timing retard). A Switch Pitch can be identified outside the vehicle (with the torque converter removed) 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]] (Pontiac's ST300 didn't get the SP). Vehicles originally equipped with the Switch Pitch ST300 can be identified by their "Park R N D L2 L1" gearshift selector. 

[[File:Ypsilanti Automotive Heritage Museum May 2015 056 (1963-95 Hydra-Matic 3L80 transmission).jpg|thumb|A Hydra-Matic 3L80 transmission, produced between 1963 and 1995, at the [[Ypsilanti Automotive Heritage Museum]]]]

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 Lesabre Fourth generation (1971-1976)|Buick Lesabres]] and [[Oldsmobile 88 Seventh generation (1971-1976)|Oldsmobile Delta 88]]s 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 [[Turbo-Hydramatic 425|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 (full-size) Chevrolet/GMC pickups and G-series (full-size) 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.

[[File:Hydramatic-Humvee.jpg|thumb|The underside of a Hydramatic transmission, as installed on a [[Humvee]]]]

Through the end of the '70s substantially more CBOP (Cadillac/Buick/Oldsmobile/Pontiac) 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 (they were commonly found in 3/4 ton (8500 GVW and above) Chevrolet/GMC trucks and vans (includes the P-series box vans and 1983-86 CUCVs) when RPO M40 was checked off the option list - especially when coupled to a [[Chevrolet Big-Block engine#Generation 2: Mark IV Series|454]] - usually in HD applications including the C40-C60 medium duty trucks where a bolt-on output shaft is used in place of a slip yoke) - when used with passenger cars it was usually coupled to a Mark IV engine or some high performance small blocks (e.g. the 1970 LT-1). The THM400 was never produced with a multicase [[bell housing]].

Other auto manufacturers have used the THM400 and its 4L80E successor, including [[Ferrari]] (in the [[Ferrari 400|400/412]]); [[Jaguar (car)|Jaguar]]/[[Daimler Company|Daimler]] (in pre-1994 [[Jaguar XJ|XJ12]] and [[Jaguar XJ-S|XJ-S]] coupes and their Daimler stable mates); [[Rolls-Royce Motors|Rolls-Royce]] (in 1965–1980 [[Rolls-Royce Silver Shadow|Silver Shadow]] and 1980-1992 [[Rolls-Royce Silver Spirit|Silver Spirit]] series cars, along with their [[Bentley]] stable mates); the [[Nissan Prince Royal]]; [[AM General]]; and  [[Jeep]] (usually found in the FSJ pickups and [[Jeep Wagoneer|SUVs]]). 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 [[AMC Straight-6 engine|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 [[TorqueFlite|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 (the gear train's reaction carrier is restrained from counter-rotating the engine in first gear by a [[overrunning clutch|roller clutch]] whose inner race is part of the center support). Since the first gear reactive force is evenly distributed around the periphery of the case, the types of mechanical (and some times violent) failures that have plagued other competition transmissions{{vague|date=March 2015}} are rare.

The THM400 was the first three-speed, Simpson-geared automatic to use [[overrunning clutch]]es 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' (three forward speeds, longitudinal positioning, and an arbitrary strength rating of 80, the second highest such rating assigned). The 3L80HD was introduced in 1987 as the HD unit used in passenger trucks. In 1991, a four-speed overdrive version, the [[GM 4L80-E transmission|4L80-E]], replaced the THM400 in Chevrolet/GMC pickups, vans, SUVs, and commercial vehicles. The 4L80E (and its successor 4L85E) 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.<ref>Sessions, R (1987). How to Work with and Modify the Turbo Hydra-matic 400 Transmission, {{ISBN|0-87938-267-8}}, p. 108</ref> The case is tapped for either 1/4" National Pipe Straight NPS fittings,or 1/2"[[Unified Thread Standard|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.

{| class=wikitable
|+Gear ratios
!Gear
!Ratio
|-
|1||2.48:1
|-
|2||1.48:1
|-
|3||1.00:1
|-
|R||2.07:1
|}