Is This Conversion Worth The Trouble?

    JJ: I just roasted second gear in my TH350. Besides having to change the yoke on the driveline, what else will I have to do to mount a TH400 in my car. Are they the same length, so I won't have to get a longer driveline or move my crossmember?

    JD: Before going to all the trouble of converting to a TH400, consider the TH350 instead. The TH350 can be built to withstand a lot of torque. A transmission builder here in Los Angeles has had great success with TH350's; he has built TH350's that have lived behind nitroused big blocks in 8-second full-bodied racecars. These are not throw away transmissions, they live a long time in these cars. The TH350 can be built into a very bullet-proof unit.

    W_: A TH400 is longer than a TH350. This means that you will have to have the drive shaft shortened and rebalanced. The "kick down" on a TH400 is done electronically and not with a "kick-down" cable as on a TH350. This means that you have to mount a switch at the carburetor base. There are several aftermarket brackets for doing this. The rear transmission mount gets moved rearward. I believe that the original mount will work. I'm assuming that a different torque converter will have to be purchased. There will have to be "clearancing" done to the floor pan for the larger size of the housing.

    DS: The slip yoke for a TH400 (3L80) is different from a TH350 which has 27 spines. The Powerglide has 32 splines. The driveshaft should be trimmed 1", because the TH400 is 1/2" longer than the TH350 or Powerglide. If you are using a parts house transmission mount, then the mount will fit the TH400, because the holes are slotted. You can find an aftermarket kickdown for a TH400 from B&M, or you can locate the parts from the GM parts counter. There is a bracket that bolts to the accelerator, which mounts the kickdown switch. The crossmember will have to be moved 6" to the rear, and there is a possibility of drilling additional holes in the frame. The 1967 Chevelles have additional holes in the frame, because the TH400 was offered as an option.

    Archivist: Tom Wilson

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