The TH350 and TH400 are two automatic transmissions used in GM vehicles. TH350 and Th400 have 3-speed automatic transmissions and were built in 1969 and 1964. The two transmissions have undergone a series of enhancements from companies such as TCI Automotive, Performance Automatic, and Hughes Performance and are at high power levels. The TH400 is larger and heavier than the TH350. TH400 is built for heavy vehicles and 4WDs while the TH350 is for lighter vehicles. Though there are many similarities, some differences should be considered when choosing the correct transmission.
What are the TH350 and TH400 Transmissions?
The TH350 and TH400 3-speed automatic Transmissions are used in General Motor vehicles.
The TH350 transmission was introduced in 1969 to replace the GM’s Powerglide automatic 2-speed transmission. This transmission became famous due to its power, which could handle 315HP to 400HP or more with few adjustments.
The new transmission was compact, versatile, and able to handle more horsepower. The TH350 came with a cast aluminum alloy body, which made it a lightweight unit and was fitted in a wide range of small vehicles.
An automatic transmission with four speeds was introduced in 1964 to replace the ST-300. In the 1960s and 1970s, this transmission was used predominantly on GM vehicles.
Later on, this material was used in cars such as Ferrari, Jeep, Roll Royce, Jaguar, and others.. The TH400 can handle 400-450 HP; with some adjustments, it can go up to 1000HP.
TH350 Vs TH400
Weight and Size Differences Between the Two
There is a size difference between the Th400 and Th350. The Th400 is more extensive and longer than the 350 Th400, 24 3/8 inches long, and the TH350, 21 3/4 inches long. The TH350 weighs 125 pounds without fluid and the TH400 135 pounds.
Fuel Efficiency Compare Between TH350 and TH400
The TH350 transmission is used for lightweight 2WD vehicles, and the TH400 is for heavy-duty 4WD vehicles. A fuel efficiency comparison will not reveal the correct fuel efficiency. However, the fuel efficiency should be better in TH350 due to its low weight.
Gear Ratio Differ Between TH350 and TH400
A vehicle with a combustion engine generates power by burning fuel inside the combustion chamber. The ignition gases push the piston, which will start rotating the crankshaft.
Rotation of the crankshaft is transmitted to the gearbox where a series of gears will absorb the process of the crankshaft.
The gear ratio is calculated as the number of times the crankshaft has to turn to turn a particular gear wheel. The gear ratio of 3:1 means there should be three rotations from the engine to turn a specific gear wheel once.
When the gear rotation is low there would be less work for the engine and more excellent fuel economy but will give the lower towing capability. When it is high, the vehicle will perform with more work for the engine, less fuel economy, faster acceleration rate, and greater pulling power.
| Gear | TH350 | TH400 | Remarks |
| 1st | 2.52:1 | 2.48:1 | The performances are all similar |
| 2nd | 1.52:1 | 2.08:1 | TH350 has low pressure on the engine. TH400 has more pulling power |
| 3rd | 1.00:1 | 1.00:1 | The performances are all similar |
| Reverse | 2.07:1 | 2.08:1 | The performances are all similar. |
Which is More Durable: TH350 or TH400?
TH400 transmission has been built for heavy-duty and 4WD vehicles. It is more significant, heavier, and more robust as it has to deal with more engine power than the TH350.
The TH400 can work with a torque rating of 450 ft. lb and 400-450 HP; with modifications, it can reach up to 1000 HP.
Since the TH400 transmission lasted so long and was still functional, GM had to reduce the quality of subsequent versions to ensure that they would last as long as possible
Though the TH400 is very durable, it does not say that the TH350 is not stable. Though the TH350 is not as durable as the TH400, it is for lightweight 2WD vehicles and performs well in that category.
How Do TH350 and TH400 Perform in Racing Scenarios?
TH350 and TH400 transmissions perform well in drag, road, and circle track racing. Both are durable and can handle a lot of power and torque. Choosing between the TH350 and TH400 should be done to suit the type of racing.
TH350 is a lightweight transmission with a high gear ratio in first gear and a low percentage in second gear, which is much suited for Drag racing.
The TH400 is more durable and can handle more power and torque, making it more suitable for road and circle track racing.
What Are the Common Modifications Made to TH350 and TH400?
Although many common modifications can be made to TH 350 and TH400, the following should be considered.
- The Type of Driving – Daily Driving, Drag Racing, Road or Circle Track racing. The modifications will be different according to the kind of driving.
- The Engine’s Power Output – This will be an essential factor as more power from the engine will give more stress to the transmission system. The modifications should be done to balance the excess engine power.
- The Budget – Modification can be expensive.
The modifications can be done to the shift kit, valve body, converter, clutch packs, bearings, and gears of the transmission system.
Which is More Cost-effective: TH350 or TH400?
The TH400 is made for heavy-duty applications with high torque and the TH350 is for light-duty applications with lesser torque. Naturally, the TH400 is more expensive than the TH350.
A stock TH400 will cost around $1600, and a stock TH350 will cost about $1200. A used or rebuilt transmission will be available for a lesser price.
How to Decide: TH350 or TH400 for Your Vehicle Needs?
Choosing a Transmission for the vehicle depends on many factors, and a decision should be made considering all the elements.
- Power and Torque – The TH350 can handle up to 350 lb-ft of torque, and the TH400 can handle up to 400 lb-ft of torque.
- Application – For vehicles used for towing or off-road driving, the TH400 is a better choice.
- Rebuildability – The TH400 is more complex than the TH350, so rebuilding may be more difficult.
- Size – The TH400 is larger, so you should consider the space available.
- Budget – The TH400 is more expensive than the TH350.
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Video Credits – Jakes Performance
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