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How to Tell the Difference Between a Turbo 350 & a Turbo 400

by Cayden Conor

The turbo 350 and turbo 400 are GM transmissions that were used from 1969 through 1979 for the TH-350 and 1964 through 1990 for the TH-400. The TH-350 was a low-torque transmission that was used in heavy cars. With modification, the TH-350 could be used as a high-torque transmission in a light car. The TH-400 was a high-torque transmission that was used in heavy cars --- it was a much stronger transmission than the TH-350.

Step 1

Locate the vacuum modulator. On the TH-350, it is located on the right front of the transmission. On the TH-400, it was located on the right rear of the transmission. Also, the TH-350 had a kick down cable, while the TH-400 did not. Both were designed to be used in Chevrolet V-8 cars and light trucks.

Step 2

Look at the transmission pans on both transmission. Both transmission pans have 13 bolts, but they have a different shape. The TH-350 uses a square pan, with one corner cut off. The TH-400 uses an oblong-shaped pan with one side having a "hump" in the pan and one side is curved while the other two sides are straight. The TH-400 pan also has two circular indentations on the humped side of the pan.

Measure the transmission. The overall length of the TH-400 is 38 inches, while the overall length of the TH-350 is 33 5/8 inches, if both use the longest tail shafts available. The TH-400's body is 25 inches and the TH-350's body is 21 5/8 inches long. The tail shafts for the TH-350 are available in 6-, 9- or 12-inch lengths and the tail shafts for the TH-400 are available in 4-,9- or 13-inch lengths.

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