Topic: GM THM 350 Transmission Questions

After our first race with an automatic transmission, the THM 350 in our race car seems like it's always in gear. In my truck, my other truck, and my daughter's car (our only automatic vehicles), when you get off the gas it feels like it coasts after a few seconds, or stopped at a light you're not fighting the engine with the brakes. In the race car, when you put it in gear, it chirps the tires, and you have to keep a lot of pressure on the brakes at a light.

That said, it is nice to be able to effectively engine brake, although downshifts can be harsh in mid-turn. It downshifted mid-kink at CMP and it was downright unsettling, although that is manageable.

So is something broken, or is this just how autos work when racing? Also, if something is broken, then is it a problem or can I just keep running it like that without damaging it further? I do like the ability to engine brake, as our brakes are not the greatest.

Everybody grab your brooms, it's shenanigans!

Re: GM THM 350 Transmission Questions

That sounds like maybe the unlock feature isn't unlocking (note, not an expert in automatics).

That guy

Re: GM THM 350 Transmission Questions

Infiniti 4 sp auto has little engine braking, saying that, we have dirt tracked with a thm350 but it was so long ago I forget if it behaved this way.
Saying this
Aside from the engine brake effect, is your idle set too high?

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Re: GM THM 350 Transmission Questions

Is it possible that it is a TH350-C?  The C version had a lockup converter that needs to be wired properly or retrofitted with a non-lockup converter.

Re: GM THM 350 Transmission Questions

I've never raced a TH350, and it's been about 20 years since I've driven mine, but I never had an issue like that unless it was like a kickdown thing or the return spring on the carb.  It's an electric choke Edelbrock.  Not sure that helps at all, but those are all the pennies I have on this issue. 

Outside of that, how was racing with that transmission?

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6 (edited by rmcdaniels 2018-12-27 09:34 AM)

Re: GM THM 350 Transmission Questions

Idle sounds fine, although tach is ancient, British, and for a different engine, so not sure it is accurate.
Nothing electric on it, came out of a '79 Buick.
Kickdown is a cable from the throttle linkage, and that seems to work fine.
Racing it is fine, actually pretty easy, one less thing to think about.

Everybody grab your brooms, it's shenanigans!

Re: GM THM 350 Transmission Questions

Transmission guy here. 30 years at GM. I know THM350s like the back of my hand.
1. High idle, most probable.
2. converter too tight. less probable.
3. stuck roller clutch. least probable.

Your kickdown cable is misadjusted if it is shifting mid turn.
Holding the brakes is a grand safety issue in the paddock. Lower the idle.
Manny