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.
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.
Many of the BOC (Buick, Oldsmobile, Cadillac) THM400s produced between 1964 and 1967 were equipped with a variable-pitch 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.
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.
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.
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 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).
Other auto manufacturers have used the THM400 and its 4L80E successor, including Ferrari (in the 400/412); Jaguar/Daimler (in pre-1994 XJ12 and XJ-S coupes and their Daimler stable mates); Rolls-Royce (in 1965–1980 Silver Shadow and 1980-1992 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 SUVs).
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.
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] are rare.
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.
It was developed jointly by Buick and Chevrolet to replace the two-speed Super Turbine 300 and aluminum-case Powerglide transmissions.
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.
Air-cooled versions (with a baffle on the torque converter and air intakes cast into the bellhousing) of the THM350 appeared mid-1972 in the Chevrolet Vega and Nova 6.
Four-wheel drive truck applications for the THM350 used an iron adapter that mated it to the transfer case directly, similar to the THM400.
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 (THM700R4/4L60) was modified.
The THM250 was usually coupled to smaller displacement engines - the largest a third generation Chevrolet inline six found in the Nova and Camaro (1974 and 75 model year only).
It was later reintroduced in 1979 as the THM250-C in the wake of the failure-prone THM200/200C - the later 250C was further lightened with the use of a sun gear shell used with the THM350 but with 3 holes to reduce rotating mass and the low/reverse piston with 8 cutouts.
Because the THM-200 shared external dimensions and output shaft size and spline count with the THM-350, the THM-350 was often used to replace the weaker, less reliable THM-200 and THM-250C.
Starting with the 1979 model year, vehicles which had the THM-200/200C as standard equipment were optioned with the THM250-C, which is a THM-350 without the intermediate clutch pack along with an adjustable band similar to the Chevrolet Powerglide.
Also in the 1980 model year, the THM-200 received a lockup torque converter, and some internal components (primarily the low/reverse clutch drum and planetary gears) were later shared with the Turbo-Hydramatic 200-4R.
which were prone to failure in the THM200 were improved, and in the later 1980s, this transmission was used with high-power applications — primarily the Buick Grand National and the 1989 Pontiac Firebird Trans Am Indy 500 Pace cars.
With the 1985 production run, GM started installing a hardened drive shell, but continued to supply the original style until parts inventories were exhausted.
External dimensions are similar to a THM350 with a 9-inch tailhousing found in Chevrolet/GMC long wheelbase truck/vans and 1971-76 B-bodies (Bel Air, Impala, Caprice).
In 1984, the 700R4 designed for use behind Chevrolet small block V8s received a 30-spline input shaft similar to those found on TH400 transmissions and which also used a different torque converter than its 2.8 V6 and 2.2 L4 engines.
Between 1984 and 1987, internal components, from the ring gear to the oil pump housing, were updated, ending with the auxiliary valve body for 700s manufactured after October 1986.
In 1996, GM introduced a redesigned 4L60E transmission case that incorporated a bolt-on bellhousing and a six-bolt tail housing.
