Th350 vs Th400: Which transmission should you choose?

The fact that there is still quite high demand for th350 vs th400 automatic transmissions in the LS engine swap enthusiasts that we have to create a detailed comparison between them even though they are long out of manufacturing that is since 1990, shows the premium offerings these both automatic transmissions provide over the existing stock engine transmissions.

Th350 vs Th400

For those who are complete beginners in an automobile and looking for the selection between th350 vs th400 let’s first understand what even is the use of transmission in a vehicle?

Need of Transmission in a Vehicle

In layman’s terms, transmission is primarily required to provide a turbo to your vehicle means it can be considered a vehicle’s gearbox.

Now a turbo, for those who don’t know, is simply an air pump that pushes more oxygen to the engine whenever required.

Hence it boosts the engine capacity to up to 50 % more horsepower depending on the model of the engine.

A turbo consists mainly of two parts, a turbine, and a compressor that is responsible to enhance the horsepower of an engine where the automatic transmission is going to be installed.

The turbine spins up to 15000 revolutions which can be 30 times more than the engine speed when installed directly in the exhaust stream.

A compressor on the other hand does the opposite of a turbine by using its fans to suck the air from the engine to pressurize it.

Hence both a turbine and a compressor work hand in hand within a turbo of automatic transmission to enhance the speed of the engine by controlling the air pressure.

Th350 vs th400

Before comparing the detailed head-to-head functionalities of th350 vs th400 it is important to note that both these automatic transmission are quite different from each other when it comes to their use case.

Hence one can get the impression that one might be lacking from the other based on the comparison below.

But it is not the case as both transmissions fulfill a whole different set of use cases on their own. 

Th350 or turbo 350Th400 or turbo 400
The major noticeable difference between every automatic transmission is calculated with its horsepower and torque. Both terms are related to acceleration gained by a vehicle but people don’t have clarity between them. Horsepower is total acceleration that is dependent on torque defined by the rpm of transmission. In the case of th350, the rpm is around 1800-2000 which generates 500 horsepower. It is used in lighter vehicles such as cars etc.Th400 is a high torque transmission as compared to th350 because in this transmission the generated rpm is around 5100 to 5300 which generates a total horsepower of 650 Hp which is way higher than th350. The best use case of this much horsepower is generally in heavier engines like loading trucks, heavy manufacturing machinery, etc because Hp is directly proportional to load-bearing capacity at a particular speed.   
Gearset usually people ignore when it comes to transmission as the most demanding thing from installing an automatic transmission is the performance boost it provides to the engine. But gearset also impacts the overall functioning of the engine and in th350 a reverse cut planetary gear set is used which although looks complicated but is less efficient than th400 because it is designed primarily for general purpose vehicles. In the thy400 transmission the forward cut spiral gear set is used which produces fewer vibrations and noise as compared to the reverse cut planetary gear set that is used in the th350. The two main reasons for the better performance of reverse cut planetary gearset is its simpler design that comes with helical teeth that are smooth to shift kit in a synchronized spiral system. For this reason, th400 is generally a choice of high-performance based cars as it has efficient energy and more power transition. 
From the design perspective, the th350 has a vacuum modulator placed at the right front of the transmission that is relatively small in comparison to the th400 hence driving less vacuum. Also, the th350 lacks the rigid mounted center support that stabilizes the transmission and also removes certain noise and vibrations. The sun shell is also missing in th350.The th400 transmission has a vacuum modulator which is much broader in size and is placed in the rear front of the transmission. The main aim of this position shift kit of the vacuum modulator is to provide a larger area to convert more engine vacuum into the transmission’s valve body input. The th400 also comes with both rigid mounted center support and a sun shell or drive shell that prevents vibrations, noise, hub breakage, spine stripping, etc.
In terms of performance, th350 is decent when it comes to general-purpose vehicle engines. The main motive of th350 is to provide an enhanced acceleration to general cars that required moderate performance. As a lightweight low torque transmission of its targeting category is quite efficient in its functioning. In the category of high-performance requirements th400 no doubt has an upper edge over th350 because it is mainly designed for high torque applications. This is the reason it is the choice of heavy load-bearing vehicles and also can be used in drag racing cars or more power-consuming vehicles. 
The cost-effectiveness is superior in th350 although it depends on the needs. The durability in the average pricing is quite efficient in th350 as it is designed for general purpose cars for achieving daily traveling goals of a family. It is an expensive engine transmission and is only required for the specified motive of intense performances like in drag racing cars or heavy load vehicles. The durability is also enhanced along with the prices because a heavy engine demands an intense durable transmission to avoid breakdowns.

Similarities between th350 vs th400

As it gets clear by now that th350 vs th400 is quite irrelevant in terms of comparison because they both are targeting a whole different set of vehicles.

Th350 is solely designed for general use vehicles where the intense performance is not a huge concern over efficiency.

On the other hand, th4000 is required where the need for intense performance is must over efficiency like in power racing car, heavy load-bearing vehicles, etc.

Then the most obvious doubt one can have is, why is it a great fuzz among automobile enthusiasts when it comes to an aftermarket transmission for an engine?

The answer lies in the similarities of the two transmissions and which are:

Similar featuresth350th400
3 speed transmissionTh350 offers 3 forward gear changes, 1  gear(reverse gears), and a neutral gear.Th400 also offers 3 forward gear change, 1 reverse gears, and a neutral gear.
Input Spline countTh350 uses 17 to 30 input spline counts.Th400 also has exact ame 17 to 30 spline count including same converters for transmission. 
Roller clutch designIt is sort of a limitation as the roller clutch is a weak spot of the th350 transmission. It has an intermediate roller clutch design that directly affects the clutch efficiency of an engine. An Intermediate Roller clutch design is also the same in th400 and that provides a low clutch application on the engine where th400 transmission is installed. 

Although these are only a few similar features in both th350 vs th400 transmission they create a strong impact on the vehicle owner.

The reason behind it is that the major concern during selecting a transmission is to rely on the power speed transmissions that can provide a primary efficiency boost to an engine.

Hence one willing to buy a transmission believes that only power speed transmission count matters when it comes to installing an aftermarket transmission.

How to differentiate between a th350 and vs th400 transmission?

In the above section, we have done a head-to-head comparison between the features of th350 vs th400 covering every aspect.

Hence it might confuse you then what is the need to know the differentiation between both transmissions again. But here we are mainly focusing on the physically visible differences.

Both these transmissions are quite popular that is the reason we are discussing them in the first place, but one needs to understand that let aside the whole demand scenario both transmissions are officially not in production from general motors since 1990 as we mentioned in the intro section of this article,

so there are a lot of duplicate parts and even fraud involved by replacing interior parts, sold you the wrong transmission like you asked for th350 but you get th400, etc.

To prevent you from these duplicate parts and fraudulent related issues below are some points that help you select the right transmission both in terms of originality and the transmission suitable for your engine model.

  1. Vacuum Modulator Location

The first thing to notice is that the vacuum Modulator location is different in both th350 and th400 transmissions. In th350 it is located in the right front of the transmission while in th400 it is in the right rear. One more thing is that th350 comes with a kick-down cable, unlike the th400.

  1. Transmission Pans Size

Transmission pans might be tricky to locate because although both th350 and th400 have 13 bolt transmission pans their shape and size are different. In the case of th350, the transmission pan is square-shaped with cut corners from one side. The th400 transmission pan has an elongated shape with two opposite sides having hump and curve while the other two sides are straight, sort of like medicine capsule-shaped. It also has two circular-shaped indentations on the hump.

  1. Transmission Measurement

It is the most important one because the first thing one can easily measure is the whole length of transmission and in the case of th350 the length of the transmission is 33 ⅝ inches and that of th400 is 38 inches precisely measured only if both are using the longest shaft available. The shaft sizes for th350 are 6,9, and 12 inches while the shaft sizes for th400 are 4,9,13 inches. In terms of body length of transmission, th350 is 21 ⅝ inches long and th400 is 25 inches long.

Read: Flowmaster 40 vs 50

Frequently Asked Questions(FAQs)

How many gears are there in th350 vs th400 transmissions?

Both th350 and th400 have the same gears which are three-speed automatic transmission. Although there are differences in them as mentioned above in the article like higher torque in th400 than th350 etc.

Do th350 and th400 have overdrive?

In both th350 and th400 three-speed transmissions lack the overdrive gear ratios hence they are not capable of highway cruising with maintaining top speed to lessen fuel consumption ratio, but one can install an overdrive gear ratios system with both of these transmissions.


From the general motors first impression, whether it is solely based on the physical square pan design of the th350 vs th400 lighter transmission or their primary features and good shape, it may seem like both the transmission looks the same and need to be compared.

This misconception is quite often over the internet when it comes to comparing th350 vs turbo 400 difference change gears and that is the reason why we selected this topic as our point of discussion.

After reading this article, we hope you get the gist that in practicality both th350 and th400 have huge differences that are not even comparable because they contain diverse features that have very different use cases.

Anyhow this general motors big blocks turbo 350 vs turbo 400 article helps you get the every minute details about both the gm transmissions starting from the shift kits , power, overall size, tranny, wear, modification,less hp, corner cut and overall length to help you do the right selection based on in-depth knowledge instead of selecting a transmission based on half information or recommendations.

Leave a Comment

Your email address will not be published. Required fields are marked *