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TH350 vs TH400: How Do They Differ & What About Mods?

difference between th350 and th400
Last Updated on Aug 16, 2023 By Paul Lucas

TH400 is stronger in its stock form while you can make TH350 just as strong. That’ll be enough for most people if you rebuild with expert help.

Ultimately, the choice depends on what type of car you have and what type of performance you want from your car.

We recommend that you stick with the tranny that your car came with. Both can be modified but the question is, do you actually need it?

You should also choose a lighter and smaller tranny if it meets your requirements before considering heavier and larger automatic transmissions.

This is because a smaller tranny will have a lighter rotating mass that’ll be easier on the crank and increase the lifespan of your main bearing.

Differences Between TH350 and TH400?

Comparing factors TH350 TH400 
Materials Aluminum alloy and steel Aluminum and iron
Size Smaller Bigger
Weight Less More
Horsepower Less More
Torque controlling ability Low High
Designed for Small vehicles Large (heavy-duty) vehicles
Position of vacuum modulator  Front right side Rear right side
Gear set Reverse-cut planetary gear set Forward-cut spiral gear set
Gear ratio First gear: 2.52 – 1.00, Second gear: 1.52 – 1.00, Third gear (Direct drive): 1.00- 1,

Reverse: 2.07- 1

First gear: 2.48 – 1,

Second gear: 1.48 – 1.00,

Third gear: an even 1 to 1

Reverse gear: 2.08 – 1

Performance Okay Better
Durability Less More
Price Less expensive More expensive

TH350 vs TH400: Head-To-Head Comparison

Build Materials

TH-350 is made of aluminum alloy and steel while TH-400 has aluminum and iron. The TH400 is thicker and more durable than TH-350.


TH-350 is 21? inches long while TH-400 is 25 inches. There are 6/9/12- inch tail shafts available for TH-350 and 4/9/13- inches for TH-400.

Both have long tail shafts: TH-350 – 33 ? inches, TH-400 – 38 inches.


Weighing 135 lbs, TH400 is 15 lbs heavier than TH350.


TH400’s pan looks like a “hump” face with a curved side inside. On the other hand, TH350 comes with a corner cut and a square format.


TH-350 features a reverse-cut planetary gearset while TH-400 has a forward-cut spiral gearset.

The planetary gearset looks a bit complex but the forward-cut spiral gearset is more efficient. So, the TH-400 gear set is capable of ensuring efficient power transfer.

Vacuum modulator position

In TH-350 the vacuum modulator is placed at the right front of the transmission while TH-400 has its vacuum modulator at the right rear of the transmission.


As stated before, TH-350 has been built for smaller, low-performing cars while TH-400 suits heavy-duty, high-performing vehicles.

TH-350 has a lower 500 hp capacity than TH-400’s 650 hp. Moreover, TH-400 has a better torque control ability of 450 lb-ft than the 400 lb-ft of TH-350.


TH-400 is costlier simply because it’s an upgraded version of TH-350. TH-350 costs you $1200-$1500 while TH-400 can set you back around $2000.

What about modification?

Both of these automatic transmissions can be modified and you will find many trans shops around ready to do that for you.

It is even possible to get almost the same horsepower and durability as the stock th400 from a modified th350.

But the question is whether it would be worth the effort because the total cost of the modification (thousands of dollars) along with the TH350 price will be almost the same as buying a new stock TH400.

The same type of modification is also possible for the TH400, and you can achieve 800-900 hp.


1. What does TH stand for in TH-350 or TH-400?

Ans: TH stands for Turbo Hydra-Matic – a registered trade name given by General Motors for a group of automatic transmissions developed by them.

2. Is a TH350 good for racing?

Ans: With 450 hp, you can pull it up for a street race or race on a track.

3. Is a TH400 a 3-speed or 4-speed?

Ans: TH400 is a 3-speed, automatic shift, and longitudinally positioned transmission.

4. How do I know if I have TH350 or TH400?

Ans: If your turbo has a fitting (vacuum modulator ) attached to the front right side of the transmission then it’s TH350, while the same thing located at the rear right side means TH 400.

5. Does TH400 have overdrive?

Ans: No, TH400 doesn’t have overdrive gear and you can’t try highway cruising with this tranny.

About the author

Paul Lucas

I grew up in a classic car-oriented family, and was taught how to differentiate between a Ford and a Chevy by my dad when I was just five years old! With special feelings for anything Italian, I enjoy driving and restoring old cars.

Over the years, I became tired of those cookie-cutter auto blogs that copy content from each other and decided to take matters into my own hands! So, I am here to bring you the latest auto industry news, tech innovations, the hottest cars and their specs, reviews, test drives, tuning, and more.

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