Th400 vs. Th350 Transmission – In-depth Comparison

The Th400 and Th350 transmission are two GM transmissions that have been making waves since the ’60s. Hence, an excellent fit for both old and new cars. Comparing the Th400 vs. Th350 is like putting two champions in a wrestling ring to fight. However, while these two iconic transmissions differ in capabilities and features, translating to their uniqueness, they share few commonalities.

So in this article, I will explain both transmissions and their unique features and place the Th400 vs. Th350 side by side to compare. Just so if you are torn between which to choose between them, this guide will help you single out your choice.

difference between th350 and th400

Th400 Transmission Explained

The Th400 is a three-speed automatic transmission introduced by General Motors in 1964 to replace the ST300 2-speed automatic transmission. And it was primarily used by all GM applications in the ’60s and ’70s.

However, while it was used mainly by GM vehicles, it was also fitted to other cars. It was featured in cars like Ferrari, jeep, Roll Royce, Jaguars, etc. Even GMC and Chevrolet used it in their trucks until the ’90s, even after it was renamed —3L80.

The Th400 was the stronger version of the light-duty Th350 and was particularly fitted to big applications with lots of torques. The Th400 transmission was built so strong that GM had to reduce the quality of subsequent versions because they lasted too long and still performed. They can last more than 250k miles which, of course, was a terrible business for GM.

While the turbo 400 transmission is originally built heavy, you can reduce its weight if you choose its parts carefully. Technically, the lighter it is, the more efficient. Again, while the Th400 is the largest widespread transmission from GM, its design is very compact.

The Th400 transmission gears came in the following gear ratios; first gear:2.48:1, reverse:2.08:1, and third gear 1:1. It also featured three tail shaft lengths and a 32 spline driveshaft. The Th400 was placed longitudinally by the engine, so it provides the best power ratio and last longer for cars with rear-wheel drives.

It has a torque rating of 450 ft. lb though aftermarket versions can be modified to withstand more torque. Talking of horsepower, how much horsepower does a TH400 have? A stock Th400 can handle between 400-450 hp but can be tweaked to handle as high as even 1000hp.

The Th400 transmission was built with iron and aluminum, with a bell housing integrated into it. Here is a more detailed explanation of how to identify a turbo 400 transmission. Particularly, its case was made with cast aluminum alloy, which accounts for its strength and durability.

The case is also extremely smooth and about 24–3/8 inches long. Without fluid, the Th400 weighs 135 pounds, but higher when fluid is added. The ribs and rear mounting point of the transmission also featured a hex bolt pattern that advanced longitudinally. Its fluid pan is also irregular in shape.

GM’s Th400 came in different versions during its production years. The first versions were launched in 1964 and used in just the Cadillac and Buick. The second generation was produced in 1965 but with an additional variable pitch torque converter. This version was featured in Chevrolet and Oldsmobile and was only used from 1965-1967.

Cars with this variable pitch stator or torque converter can be spotted with the two-prong plug on the casing. The later version, however, featured a fixed pitch stator. Nonetheless, all Th400s ever built by GM featured a big black can-type vacuum modulator placed at the passenger’s flank of the transmission near the dipstick tube.

The Th400 transmissions were available for use in both 4wd and 2wd GM’s heavy-duty vehicles in 1970. However, as far as other vehicles are concerned, the Th400 was first used as an OEM by the Jeep CJ series between 1973 and 1979. It was later fitted to the Buick 350 in the Wagoneers, Jeep J-series pickups, and SJ-series Cherokees.

The Dana 20 or Borg Warner Quadra-Trac transfer case also featured it as OEM. The Th400 was also fixed behind the Chevrolet and GMC 4WD trucks, though with a new transfer case till 1991.

The turbo 400 transmissions were made in such large quantities that their availability rate was high. Reports show that they are still very common with driven cars today and even quickly found in your local scrap junk yards.

Read Also: 4L60E transmission—All you need to know

th400 transmission
Image Credit:

Th350 Transmission Explained

Introduced in 1969, the Th350 transmission is a three-speed, automatically controlled transmission used to replace GM’s Powerglide automatic transmission-2-speed transmission.

Buick and Chevy jointly developed the Th350. Hence, also called the CBC 350 or Chevrolet-Buick combined transmission.

Because of its ability, compactness, and versatility, the trans soon became famous and preferred by many. Typically, while small, the transmissions delivered so much power and were fitted in a wide range of small vehicles. What about horsepower? How much HP can a built TH350 handle? A stock Th350 can handle between 315hp and 400hp, though with a few tweaks; it can handle more.

The Th350 has the following gear ratios: 1st:2.52:1, 2nd:1.52:1, 3rd:1.0:1 with a reverse gear of 2.07:1. The Th350 casing was built with a cast aluminum alloy and came as a single unit with bell housing used on its body.

The turbo 350 transmission is short and light, making it utilize less power to spin.

So comparing the turbo 350 vs. turbo 400 power loss, the th400 uses more power during operation than the turbo 350.

It featured a 27 spline driveshaft with each pan in transmission able to take up to 4 quarts of oil. However, it will require more depending on the size of the torque converter. For example, with a 9.5″ torque converter, the th350 can take up to 10 quarts.

GM’s turbo 350 transmission received a few upgrades during its production years. One of which is the addition of the air baffle and air intake to its torque converter in 1972, which helps keep the transmission cool. However, while it keeps the transmission cool, it’s also a source through which dirt enters the inner components. Hence, you need to keep up with regular cleaning of these components.

In late 1979, Gm introduced another version of the th350s, which featured a lock-up torque converter. However, production of this version was stopped in 1984 due to inconsistency in acceleration and downshifting. The Th350 version produced after 1984 used an electronic torque converter which eventually helped boost fuel economy at high speeds.

The th350 trans came in different groups and specs, and which is fitted into a car depends on the car’s engine. Generally, the stronger the engine, the stronger the 350 transmissions fitted. Both ends of the 350 transmissions are the same because it doesn’t come with central support. 

While its portability, power, and versatility made it very famous and wanted by many, these features also made many vintage car owners use it even to date. You should find them fitted on many classic vehicles sold in the market, though with few modifications in certain parts.

The hydramatic 350 was fitted to almost all GM’s rear-wheel trucks and cars until 1982 the 700R4 was launched. People fell in love with the 700R4, invariably slowing down production. The th350 transmission was finally phased out in 1984 and replaced with the 700R4 since the 700R4 had become people’s choice

If you are to compare the th350 vs. 700r4, the primary thing that differentiates them is the number of gears. Unlike the th350 with three gears, the 700R4 is a four-speed automatic transmission.

GM’s Th350 was particularly combined with small block v6 and v8 engines. Some vehicles that featured the 350 include GMC trucks, Pontiac birds, Chevrolet Camaro, Caprice, etc.

Read Also: 4L80e Transmission—All You Need to Know

TH350 Transmission
Image Credit:

Th400 vs. Th350 Comparison Table

This comparison table describes the Th400 and Th350 transmission at a glance.

Th400 Th350
For heavy-duty applications with high torque For light-duty applications with lesser torque
Longer: Th400 is about 24-3/8 inches long Short: the th350 is about 21-3/4 inches long
Heavy: weighs 135 lbs Lightweight: weighs 120 lbs
Bigger in size Smaller
More durable: 250k miles plus Durable, but not as the th400
Performance: More potent, so it can work at higher RPM and engine power Function at lower RPM and engine power
Fitted to both GM’s heavy-duty and other brands 4wd and 2wd trucks Mainly fitted to GM and other brands’ 2WD vehicles
450 ft lbs torque rating 410 ft-lb torque rating
Features 32 spline driveshaft It has 27 spline driveshaft
A stock th400 can handle between 400- 450 horsepower A stock th350 can handle between 315- 400 horsepower



Gear ratios: 2.48, 2.08, 1.0 Gear ratios: 2.52, 1.52, 1.0
A stock th400 costs about $1600, though it can be less if you buy a used Th400 or rebuild. A stock th350 costs about $1200, though it can be less if you are buying a used Th400 or rebuilding

Read Also: 6L90 Transmission—All You Need to know

How do you tell the difference between the Th400 and the Th350?

Here is how to differentiate the Th400 from the Th350

  • Size and weight: Look at both transmissions and, if possible, raise it. The turbo 400 transmission is bigger, longer, and heavier than the hydramatic 350 transmission.
  • Kick-down mechanism: Another easy way to distinguish the th400 and th300 is to look through their kick-down device. The turbo 400 uses an electrical slide button regulated by throttle linkage. In contrast, the 350 uses a mechanical wire attached to the throttle linkage.
  • Spline driveshaft: While the turbo 400 has 32 spline driveshaft, the 350 has only 27.
  • Vacuum modulator: While the Th400 vacuum modulator is placed at the rear right-hand side of the transmission, the th350 vacuum modulator sits at the front right-hand side.
  • Face size: The Th400  face is wider than the Th350.
  • Center support: The th400 features a rigid mounted support with a sun shaft, while the Th350 doesn’t have.
  • Performance: The Th400 is more potent, so it can handle vehicles with more power, hence an excellent choice for heavy-duty or high-performance vehicles.
  • Durability: The Th400 has a stronger construction, so it lasts longer than the th350.
  • Cost and budget: The Th400 is more expensive than the th350. So if you want something cheaper but with less power, opt for the Th350.

Read More: 200R4 Transmission—All You Need to Know

Th400 vs. Th350 similarities

While both transmissions are entirely different, they share certain features.

  • Production year and producer: Both transmissions were built in the ’60s by General Motors.
  • Cars fitted with the transmission: Both transmissions were used in almost the same cars. However, the th400s were used in those with bigger blocks that have much torque since they function well with higher RPMs
  • Number of gears: Both transmissions are three-speed automatic transmissions.
  • Production material: Both transmission casings were made with cast aluminum alloy
  • Modification: Both transmissions can be modified to handle more torque and horsepower
  • Versions: Both transmissions have the 2wd and 4wd options
  • Torque converter: Both transmissions used the same torque converter

Is there a size difference between the Th400 vs. Th350?

There is a difference in size between the Th400 and Th350. Generally, the Th400 is larger and longer than the 350. While the Th400 is about 24-3/8 inches long, the th350 is about 21-3/4 inches long. Even comparing the th350 vs. th400 weight, the turbo 400 is heavier.

Read More: 545RFE Transmission—All You Need to Know

Final Words

This article has explained and compared the th400 vs. th350 extensively. In terms of size, the 400 is bigger and longer. Performance-wise, the 400 is more potent, hence can withstand more power—a reason it’s fitted to big blocks with high torque. Comparing the weight of the th350 and th400, the turbo 400 also comes heavier.

So technically, if you want something for high-performance or heavy vehicles without considering cost, opt for the Th400. If, however, you’re looking for a transmission that is cost-effective, lightweight, and can fit into smaller vehicles, the th350 is your best shot. While reading these helps you know the capacity of both transmissions, it’s also a guide to making your choice if you were ever confused about which to buy.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

Recent Posts