TH350/400 Shift Point Adjustment
The 3 speed Turbohydramatic 350 and its big brother the TH400 were some of GM's most widely used automatic transmissions from the mid 60's through the early 80's. The TH350 was mainly factory installed with small blocks and the TH400 with big blocks and high performance and heavier duty vehicles. They also find their way into many Chevelles and other GM classics if not originally equipped as such, as well as street rods, race cars, etc. They are inexpensive, reliable, and easily modified for performance and racing use. If you have a vehicle equipped with one of these and the automatic shift points (engine rpm/vehicle speed) in Drive range seem too high or too low, it usually can be rectified without even pulling the trans pan. If your trans is not original to the vehicle it's currently in, or the rear end gear ratio has been changed, the trans governor is probably not calibrated properly to the rear gear ratio. The governor is a pressure valve that contains centrifugal weights and springs much like a distributor and is geared to the output shaft of the trans. If the rear end ratio is changed, the governor spins faster or slower in relation to vehicle speed than originally intended. The speed of the governor causes its weights to move out or in against springs and move a valve that alters fluid pressure in the trans, which in turn affects the shift points. Fortunately, there are spring and weight kits available to fine tune the shift points. They are available from performance trans shops such as TCI (www.tciauto.com) and others.
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