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TH400 Performance Automatic Transmissions and Parts from PATC. PATC GM Performance Transmissions and Parts, TH400 and others. PATC builds TH400 performance transmissions in many levels. PATC is on the cutting edge of performance rear wheel drive American transmission development. The TH400 is an aluminum cased three speed automatic transmission that was produced by GM. It was introduced for the 1965 model year and made its debut in Cadillacs, and is respected for its durability and adaptability. The TH400 came in two wheel drive and four wheel drive vehicles, passenger cars and heavy duty trucks, as well as GM vehicles and those made by other manufacturers.

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.

First- 2.52

Second- 1.52

Third- 1.00


First- 2.48

Second- 1.48

Third- 1.00


Input Shaft Splines



Output Shaft Splines


21 5/8”

Length from Bellhousing to Tailshaft


Th400 Shifter

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.

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!


Related: TH350 vs 4L60E: Comparing the Differences, Th350 vs 700R4 Differences

The B&M Adjustable Vacuum Modulator (Part# 20234) comes pre-adjusted for ‘normal’ shift points, but it is fully adjustable (see below), allowing you to customize the shift points (within a range of 2-4 mph). This unit is designed for use with select applications, including GM TH400 and TH350 transmissions.

  • Allows you to tailor shift points
  • B&M Quality
  • Comes pre-adjusted
  • Fully adjustable

Please note: If you have a big camshaft and your engine does not make adequate vacuum, you will not be able to make the stock or B&M modulator work properly, usually resulting in late, hard shifts. A minimum of 12″ Hg engine vacuum is required for proper operation.

Installing the B&M Vacuum Modulator

Th400 Rebuild Kit

Fits GM TH350 & TH400 Automatic Transmissions, Part# 20234


Adjusting Shift Points

You can adjust the modulator with a small screwdriver. Turn the adjustment screw clockwise to raise shift points and counter-clockwise to lower shift points.

Get the Kit

B&M Vacuum Modulator for GM TH350/TH400 Automatic Transmissions ~ Part # 20234 on Amazon


Need a product recommendation for your vehicle or application? Give B&M a call at (866) 464-6553

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