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TH700 Transmission Control with the COMPUSHIFT Mini

TH700 Transmission Control with the COMPUSHIFT Mini

History of the TH700

In 1982 General Motors released the Turbo-Hydramatic 700 automatic transmission (also called the TH700 or 700R4). It was a big upgrade from the GM TH350 transmission and the first in a new series of overdrive automatic transmissions.

The TH700 was introduced at a time when oil prices were reaching record highs in the early eighties. Emission standards were also getting stricter and this led car manufacturers down the route of using smaller displacement engines and a lower axle ratio. These changes brought about very poor performance which wasn’t very well received.

GM’s decision to work on improving the transmission paid off well. Developing the TH700 evolved into the 4L60 which ended up being manufactured for over 30 years after the release of the initial TH700. It is considered to be one of the best and most successful transmissions of its time.

Specifications of the TH700

The TH700 is a light duty transmission rated to handle about 350 ft-lb of input torque. If built properly with upgraded internals, it can handle over and above 450 ft-lb of input torque.

From its release in 1982, it gradually got better with internal improvements over the years until it was subsequently replaced by the 4L60 in 1990.

Manufacturer General Motors
Type 4 Speed Longitudinal Automatic
Predecessor TH350
Length 23.4"
Input Shaft 1982-84: 298mm/27 spline
1985-90: 298mm/30 spline
Overdrive Yes
Case Material Die Cast Aluminum
Weight 155 pounds dry
Fluid Capacity 11 quarts
Max Input Torque 350 lb-ft

Gear Ratios of the TH700

1st 2nd 3rd 4th R
3.06:1 1.63:1 1.00:1 0.70:1 2.29:1

Strengths of the TH700

The creation of the TH700 was the foundation of the 4L60 which ended up becoming one of the most successful transmissions in America due to its good drivability, reliability and fuel economy at the time.

The extra overdrive gear, with the 0.70 ratio leads to a 30% drop in engine rpm when it gets engaged at cruising speeds. On top of that, the torque converter clutch (TCC) can lock up which provides a direct drive from the engine to the drivetrain. This reduces engine rpms even further and subsequently keeps the transmission cooler, helping save fuel, reduce noise and engine/transmission wear.

Overall, many users praise the good launch from standstill and its smooth shifts between gears. Compared to other GM transmissions of the time like the TH350, the TH700 is a much more practical transmission for daily driving.

Weaknesses of the TH700

Initially when the TH700 was released, reliability of the transmissions wasn’t very good. However, General Motors did take it onto themselves to constantly upgrade this transmission and solve its issues. We’ll go over some common issues that were seen in early models and their subsequent solutions.

The original 27-spline input shaft that came on the earliest models was a common failure point, especially under high torque applications. GM upgraded the input shaft to a 30-spline shaft within a few years of release, which helped prevent its failure. If your engine has been upgraded or tuned in any way, you can look at upgrading to a stronger aftermarket hardened steel input shaft to handle more torque.

Other than the input shaft, the TH700 underwent several other changes from 1984 to 1987. The oil pump was upgraded from a 7-vane design to 10-vanes, and better pump rings and seals were used for improved reliability.

The TH700 heats up quickly under high torque requirements and this hurts its life expectancy. An external cooler might be necessary depending on climate and use of the transmission.

Tuning the TH700 can be time consuming and difficult to get right especially when pairing various different engines with the transmission. As the TH700 doesn’t have electronic shift controls and relies on a governor and throttle cable to operate the shifts, it takes a bit of work to get right. A lot of shifting issues on custom builds are a result of misadjusted TV cables.

For this reason, the TH700 was soon replaced by the 4L60, whose electronic controls were quickly adopted due to their numerous advantages in controlling gear shifts and the transmission operation.

Popular Custom Vehicles that use the TH700

Despite its age, the TH700 is still a very popular transmission with classic vehicle enthusiasts.

As the transmission is manufactured by General Motors, it is most commonly found in Chevrolet vehicles. Chevrolet muscle cars like the Corvette, Camaro and light pickup trucks such as the Chevy S10 and C/K (3rd Gen) had models that came with the TH700 installed.

A First-Generation Chevrolet S10 Truck

For custom builds such as hot rods or jeeps that use an older Chevrolet engine like a 4.3L V6 or a 305/350 small block V8s, the TH700 pairs well with them thanks to the compatible bellhousing pattern.

Vehicles that previously used the GM TH350 transmission, such as the Chevrolet Impala, Bel Air and Caprice are often upgraded to a TH700 transmission since the TH350 doesn’t have an overdrive gear.

A 6th-Generation Chevrolet Impala which first came with the TH350 and then TH700 in later years

When upgrading from a TH350 to a TH700 transmission, you’ll need to have a mechanism to lock the TCC in overdrive gear. Without a torque converter that locks up, the transmission will overheat and this will lead to component failures and other problems over time.

While there are many methods used to control the TCC lockup such as with a timer or a manual driver-operated switch, one of the best and most efficient methods to control the lockup is based on throttle position and vehicle speed. This is where a transmission controller like the COMPUSHIFT Mini comes in.

TH700 Transmission Control with the COMPUSHIFT Mini

The COMPUSHIFT Mini uses a throttle position sensor, vehicle speed sensor, and transmission pressure sensor to decide when to lock up and unlock the torque converter clutch.

This is a step up from other more basic torque converter clutch lockup solutions. Under certain throttle application and cruising speed conditions, you might need to unlock the TCC, and at other times it would be better not to unlock it. This point would vary depending on your engine’s characteristics, the speed the vehicle’s travelling at, and various other factors.

The COMPUSHIFT Mini lets you analyze the transmission pressure in real-time via the accompanying application which connects via Bluetooth. This allows you to adjust the TV cable slack and travel properly during setup.

Installing and using the COMPUSHIFT Mini is by no means a complicated task. On our  website's product manual page you can find plenty of information on the accessories you’ll require to use the COMPUSHIFT Mini with the TH700 transmission.


Transmission Controllers for the TH700 or 700R4


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