Th350 Center Support Weakness

390707193Can you see the crack? this tranny placed 3rd after a 2 1/2 hr feature run in a demo derby, one of the most extreme things to ever put a Transmissions through. snow plow trucks are very similar as well as drag racing.  the TH350 was designed not to shift to low 1 at high rpm and/or road speed. This was done through the use of both a manual low control valve and an internal shift control valve that is part of the 1-2 shift valve in the valve body.

The shift control valve basically has line psi to the back side of it with the modulator valve unhooked. to engage reverse, oil flows through this valve to the rear holding clutch pack and does not need to move….giving you reverse. to get manual low 1(which engages part of the rear clutch pack to help hold low 1 in addition to the center support) it must shuttle this valve to expose the passage to flow oil to the rear holding clutch pack.

so what am i getting at here? in demo derby and drag racing, the rear low 1 clutchpack in a nutshell is not allowed to move because the tranny believes it is hauling ass down the road with the modulator valve unhooked. leaving the center support and rear sprag to bear the entire load of holding member for 1st gear. however, unlike the th400 line, the th350 and it’s later counterpart the 700R4 underwent many different variations of valve body calibration and control for different vehicles. therefore this is not a golden rule.

So, as a rule of thumb for the Th350, during a rebuild i always recommend retro-fitting in a center support assembly out of a later model 700R4/4L60 transmission, even on driver transmissions. they are a larger sprag and can take more abuse. also always run your modulator valve hooked up. if you are doing any kind of performance for street. strip an aftermarket valve body kit or shift calibration kit is recommended as well. for demo derby….just go get a th400 or run a 3 speed. yes th350’s can run and last for demo derby…..i have had guys swear by 700R4 trannys too for derby. don’t do it. the results are inconsistent.

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