The THM350 is a 3 speed automatic transmission that was manufactured by General Motors. It appeared in cars and light duty trucks beginning in 1969. It is a part of the
turbo-hydramatic line of transmissions. It was designed to eliminate the outgoing
Powerglide in cars and light duty trucks. The engineers thought of it as a three speed Powerglide and the similarities are certainly there. Most people when they rebuild a muscle car or their original Powerglide fail will switch to this transmission since most common versions of it will bolt in as a direct replacement to it. It was engineered for the entire GM line, and not just the Chevrolet division. It was phased out of the from passenger vehicles in the 1984 model year, and from trucks and vans by 1986.
THM350 vs THM400
The Turbo 350 was built for lighter duty applications than it’s heavier duty counterpart. The THM400 was designed to handle more torque and horsepower, therefore it is still the preferred transmission of people looking for a factory automatic that can handle a lot of power. There is a tradeoff for all that torque capacity though, the THM400 is noticeably heavier than the THM350 and has more of a drop in power through the clutches and gears. In the aftermarket there are a lot of transmission builders like TCI and Monster transmission that sell a THM350 with all of the weak points fixed that will hang with any Turbo 400. The THM400 is also longer, so if you are going to swap it into your car, you’ll probably have to have the driveshaft shortened.
Identifying a Turbo 350
If you were to look at a Turbo 350 next to nearly any other Turbo-hydramatic transmission it would be obvious which one is which. Unfortunately, that is rarely the situation. The good news is that there are only two THM transmissions with a 13 bolt pan. The THM400 is the other one and it is noticeably larger. While they both do have 13 bolts, the Turbo 350 has a much simpler pan. Just take a look at the pan. If you have 13 bolts and 5 different corners, you can feel confident that you are looking at the Turbo 350 and not the 400.