A lot of people claim that he TH350 has less drivetrain loss than the TH400, which means that all things being equal, the TH350 is going to get more power to the rear wheels from an engine with equal power. This is due to more rotating weight existing in the larger TH400 case. Here is a writeup on ET loss from TH350 to TH400. Turbo 400 Performance Racing Transmission Parts by FTI Performance. FTI Offers a wide variety of TH400 Parts including racing transmisison pumps, valve bodies, gear sets and deep transmission pans.
One of the largest questions amongst Chevy guys is TH350 vs. TH400. Which one is better. Even though they have been out of production for decades, there is still a strong demand for them. Their simplicity and durability make them attractive to people in the LS swap community. They are certainly are both great transmissions. They share the same bolt housing patterns, and often came behind the same engine types. Whereas the TH350 would be found behind a lower performance small-block 350 Chevelle, the TH400 would be the transmission that came with the 454 SS. The TH400 also found itself in many heavy duty trucks and vans.
Input Shaft Splines
Output Shaft Splines
Length from Bellhousing to Tailshaft
The TH350 is certainly smaller than the TH400 and can usually fit into nearly any rear wheel drive Chevrolet passenger car with no problem. Tunnel modification can come into play on some of the smaller cars. The TH350 uses a mechanical kick down switch, whereas the TH400 uses an electric one. The electric kick down is much easier to set up, espescially if you are doing an LS Swap,
They do both have an aluminum case, and three forward gears. Although the gear ratios are nominally different. Neither transmission has any sort of computer control to it, nor did they use locking converters.
A lot of people claim that he TH350 has less drivetrain loss than the TH400, which means that all things being equal, the TH350 is going to get more power to the rear wheels from an engine with equal power. This is due to more rotating weight existing in the larger TH400 case. Here is a writeup on ET loss from TH350 to TH400.
Th400 Torque Converter
They also don’t weight the same amount. The Turbo 350 weights 150pounds while a Turbo 400 weighs almost 170.
If you are going to buy a stock transmission and know it’ll fit in your vehicle, without a doubt the TH400 is the way to go. It’s super durable and will provide a long life of useful service with no modification from stock, even in higher horsepower situations.
If you are buying an aftermarket transmission from someone like TCI or Monster transmission, I’d say go with the TH350. These folks have had decades to get all the flaws and weakness out of it. You’ll have less mass in the drivetrain and a lighter transmission.
Thank you for reading!
Th400 Trans Brake
Th400 Transmission Cooler Line Routing
Related: TH350 vs 4L60E: Comparing the Differences, Th350 vs 700R4 Differences