The TH350 transmission is one of the most popular GM automatics for high-performance applications.
Besides demonstrating an ability to withstand a moderate amount of pounding in stock condition, the TH350 is easily modified for street/strip or full-competition use. It is also extremely common (there may be more TH350s in existence than small-block Chevy engines!), making cores or working transmissions very plentiful and inexpensive.
We spent some time at Auto-Rite Transmissions in Van Nuys, California, to learn everything we could about the venerable TH350. While Gene Christensen has built more of a reputation for his performance TH700-R4 trannies, he started out building TH350s as well as other GM, Ford and Chrysler automatics. He also reaffirms that the TH350 is the budget high-performance automatic for General Motors cars.
Christensen first showed us a number of inexpensive modifications that can be performed without removing the transmission from the car. Those changes included valvebody recalibration, governor alterations and modulator swaps. Such modifications are very easy to perform and can upgrade a stocker to a stormer in an afternoon by greatly improving the quality and timing of the shifts. Of course, none of the changes will fix a weak transmission.
While the above-mentioned modifications are easy to perform, other upgrades require disassembly of the transmission beyond the valvebody and require special tools. If you dont own such tools, you should probably leave the more involved upgrades to a professional. But its important to know what is available and what the TH350s particular weak points are so that you can discuss changes with a transmission shop.
With that in mind, there are a number of internal pieces that should be upgraded for applications transmitting approximately 400 lbs-ft of torque or more through the transmission. (The actual breaking point will depend on vehicle weight, gear ratio, traction and driving style.) Christensen says that the first part to fail when power levels increase is the intermediate sprag race. Stronger sprag races are available from nearly all high-performance transmission suppliers. Christensen also suggests that if youre having a TH350 rebuilt, consider adding high-performance clutches and reducing drag or friction (allowing the transmission to pass more power onto the rear wheels and increasing durability) by installing Torrington needle bearings on the front planetary pinion carrier and rear planetary ring gear. This requires a new planetary pinion carrier and ring gear that will accept the needle bearings.
Of course, a high-performance torque converter matched to the engines power curve, the cars weight and its rear gear ratio will also make a positive difference in the performance of your TH350. A good shifter is also necessary to make the most of your automatic. Christensens final comments are that a good transmission cooler and an extra-deep, finned transmission oil pan will help keep temperatures down. That will greatly improve the durability of your TH350.
