Turbo-Hydramatic 400

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The Novak Guide to the GM TH400 Automatic Transmission

This article is mostly aimed at Jeep & other 4x4 owners but is a good read.

The TH400 is an automatic shift, three-speed, longitudinally positioned transmission. It is regarded to be a supremely durable and legendary. The transmission is conservatively rated at 450 ft. lbs. of input torque. Aftermarket building techniques take it well past that figure.

The Turbo 400 was introduced in 1964 in Buick and Cadillac cars and in Chevrolet and Oldsmobile cars the following year. An innovative variable pitch stator was available in the 1965-1967 cars that could essentially vary the characteristics of the torque converter.

The TH400 made its way into heavier duty GM trucks in the 1970's in 2wd and 4wd configurations. The 400 was not just a GM exclusive transmission. It is an immensely popular transmission in the automotive industry as well as the aftermarket. They are found in GM's, Jeeps, Jaguars, Rolls-Royces, Ferraris and others.

As with other GM transmissions, GM changed the naming scheme of the TH400 in 1990 and it was then referred to as the 3L80 (3-speeds, longitudinally positioned, 8000 lbs. GVW). An overdrive version of it was introduced as the 4L80-E. This electronically controlled transmission remains in production and wide use in heavy-duty GM and military applications.

The 400 transmission has a main case of cast aluminum alloy with a length of 24-3/8" long. Its aluminum case is essentially smooth. The rear mounting face of the transmission has a hex bolt pattern with ribs running forward longitudinally. The fluid pan shape is irregular being likened unto a distorted Texas pattern. The TH400 is the largest of the common GM auto transmissions, but is still surprisingly compact in light of the immense power they can handle.

There are two significant variations of the TH400. The TH375 was a version of the transmission used from 1972-1976 in smaller displacement cars. It is identified easiest by its "375-THM" designation cast in the underside of the tail housing. The TH475 was an extra-heavy-duty version, and was found in larger trucks from 1971 on.

If you are trying to determine the TH400 from another in a vehicle, a fast way to tell is to look at the kick down mechanism. The TH400 uses an electrical slide switch, which is controlled by the throttle linkage. The TH350 uses a mechanical cable kick-down mechanism that is attached to the throttle linkage.
An interesting variable pitch stator feature of the Turbo 400 was available in 1965-1967 Buick, Olds and Cadillacs. These are identified by the two-prong plug on the case. While 1970-1974 models also had this plug, it is for the transmission controlled spark system.

Transfer Case Adaptability
This transmission makes an excellent conversion transmission due to its adaptability into most Jeeps longer than CJ5s. Both 2wd and 4wd versions of the Turbo 400 can be used equally well, and there are no inherent advantages to either one once you have installed our adapter assembly. It is of interest that the 1976-1979 AMC case, while more or less similar from the collar of the case, back, is tilted about four degrees. This was presumably for transfer case clearance purposes. Novak's #134 & #141 adapter designs compensate for this clocking to re-level the adapter and mount assembly.

2wd transmissions feature conical shaped tail-housings and an output yoke, which are replaced with a typically shorter 4wd style output shaft (included with our adapter assemblies) of varying lengths and spline counts, depending on the application. The Hydra-Matic can be adapted to the popular Jeep (and many IH) transfer cases, including the:

Dana 18, 1941-1971
Dana 20, 1962-1979
Dana 300, 1980-1986
New Process Jeep 208, 219 & 229
New Process 207, 231, 241OR, 242 & 249

Essentially all factory GM 4wd applications available with an OEM configured TH400 have adapters and transfer cases that are prohibitively long for a Jeep, and transfer cases whose sizes and gearing fall short of desirable for most Jeep applications.

Engine Compatibility and Adaptability

The front face TH400 is natively compatible with either the Chevy 90 degree “Small Block” & “Big Block” patterned engines, including the V6, V8, I6 & Iron Duke I4 (the latter mentioned only for accuracy and perhaps some comedy).

These differences only affect the front bell of the case, and all Turbo 400 cases are largely similar from that point back.

Buick / Oldsmobile / Pontiac / Cadillac
Buick engines and TH400's are natively compatible when choosing a Buick V8 or V6 version of the transmission. All Buick, Olds, Pontiac and Cadillac TH400's share the same engine pattern, commonly referred to as the "BOPC" pattern.

AMC/Mopar Jeep
The first Turbo 400s found in Jeeps had a factory adapter plate, while later AMC versions had a dedicated AMC style case.

The Chevy 400 can be made compatible with AMC I6 & V8 engines. See our Kit #437AMC for details. This conversion can help make for an improved Jeep power train over 727, 999, AW4 and other, lesser transmissions.

A fantastically designed and built transmission, the Turbo 400 is broadly popular for Jeep conversions. They are very serviceable, build-able and their strength puts them into the top tiers of Jeep sports and performance applications.

Original article located at:

For more history & development of the TH400 try this article by Wikipedia.






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