How to Convert 2WD Transmission into 4WD Transmission?

The 2WD and 4WD versions of automatic and manual transmissions have a lot of design similarities. They have similar housing units, gear ratios and frequently have the identical spline connection on the output shaft. The main differences between these two units lie in the fact that the 2WD output shaft is longer than the 4WD version and 4WDs are connected to a transfer case. Some carmakers manufacture their 2WD transmissions in such a way that they can be transformed into 4WD models.

A 2WD transmission is drive recognized by its extended tail shaft with the rear-end part including an overdrive unit. The 4WD transmission also has such unit, but it is paired with a transfer case that transfers the power to both axles. The 2WD gear shifting units have no transfer case. However, some cars are equipped with a dummy transfer case to significantly simplify the process of conversion to 4WD (only a 4WD output shaft will be needed).

Transfer Case

If you see the transfer case connected to the rear-end of the gearbox, you can say for sure that you are dealing with a 4WD car. However, some 2WD cars also may have a dummy transfer unit providing an opportunity for 4WD conversion in the future. A transfer case is intended for distribution of torque from the gearbox between 2 axles with the help of 2 shafts. It also syncs up the discrepancy between the RPM rates of the fore and back wheels, and may be equipped with one or more sets of low range gears for off-highway application.

Learn more about a transfer case

The gear shifting mechanism in 4WD cars without a transfer case has a special gearbox design. The 4WD versions of passenger cars (that are not intended for off-road driving) are designed on the basis of a standard passenger car. Some models are designed only for 4WD application and have no 2WD analogs (for example, Subaru and Jaguar X models).

The FWD models with a longitudinal powertrain positioning (majority of Audi cars) are the easiest to be redesigned to 4WDs. In such a case, torque is transferred to the back axle from the transmission via the shaft/cardan drive. The rear axle is complemented by the main gear, while the central/interaxle differential is embedded in the transmission housing. If you want to convert a RWD car with the front engine positioning into 4WD, you will certainly need a transfer case and a central/interaxle differential, attached to the transmission. Here torque is distributed asymmetrically, most of which is transferred to the back axle. The main gear with independent suspension of the front wheels is connected to the body. It is much more challenging task to convert 2WD transmissions in FWD cars with a transverse engine into 4WD. In this case, the main gear housing ( картер главной передачи) is positioned behind the engine and the cardan gear is used for connection of the front main gear to the additional rear axle main gear. The 4WD conversion for a 2WD car with central engine positioning is too complex, thus such complex design transformations are rarely implemented.

To perform the 2wd to 4wd conversion operation you will certainly need an adapter kit

Actually, a standard 4WD system includes the following components:

• Manual or automatic gearbox;
• Transfer case;
• Center differential;
• Cardan drive;
• Rear and front differentials.

2WD to 4WD conversion: Does the end justify the means?

The value of 2WD to 4WD conversion became the subject of sharp debates on automobile Internet forums.

Learn more about 4WD systems

Nobody disputes that the 4WD transmission gives you a lot of important advantages. It is obvious that in addition to significant improvements in off-road driving (if we are talking about SUVs), a 4WD vehicle performs much more efficiently when driving on slippery roads and provides more intensive acceleration. Four driving wheels improve dynamics and stability of a car on the road, and sharp starts without slippages are not a problem for a 4WD car. Some people claim that the conversion to 4WD is not that hard and can be performed even without a complete transmission disassembly.

However, according to numerous posts on auto internet forums, where this question is actively discussed, 2WD-to-4WD conversion can become a real pain in the neck, because this procedure may include a complete transmission reassembly and replacement of numerous parts. The 4WD conversion implies additional weight and power losses, which have a negative impact on the vehicle’s overall performance capacity (lower top speed) and fuel efficiency. Breaks and tires will wear out much faster.

Some people say that by converting the vehicle 2WD to 4WD the additional weight will put an extra load on chassis and numerous elements of the car. So you also have to modify/replace the other parts of vehicle to deal with additional loads. The engine repositioning may also be needed. Some users claim that you may have problems with balancing. In some cases, entire suspension may have to be changed including lower arm. If we are talking about cars equipped with advanced and electrically operated automatic transmissions, it is hard to predict how the conversion to 4WD will affect the operation of the transmission software and its numerous sensors. Therefore, it is assumed by many that it is better to save your time and money, and save up money for a 4WD car.

Conversion of 6L80 2WD transmission to 4WD

We will use examine the conversion process using the example of the GM automatic transmission 6L80. This 6-speed automatic transmission is an extremely popular OEM unit and an amazing conversion transmission thanks to its adaptability. The process of 2WD-to-4WD conversion for this transmission is not very complex and includes the following steps:

• Remove the tail shaft housing;
• Remove the O-ring as well as the sleeve;

• You’ll see three open areas: 2 holes (used in the 2WD unit for lubrication of the output shaft bushing) and place for a 4WD seal;

• Install a 4WD output shaft seal (make sure that this part is completely flushed);

• As for the two holes, it is required to install two solid plugs. These plugs block ATF from reaching the back-end of the gearbox and send it back into the pan. In the 4WD version, it is not necessary to lubricate anything, because you have a transfer case attached to it. When installing the plugs it is desirable to use a little bit of thread-locker compound (do not use Teflon tape);

• Cut the 2WD output shaft as specified in the manual that comes with the adapter;

• Install the 4WD adapter (4WD tail housing) and test fit the transfer case.

6L80 conversion to 4WD

All in all, it can be concluded that the complexity and reasonableness of the 2WD-to-4WD conversion largely depends on the transmission type/model, its design peculiarities, as well as on the structural design of the car in which this transmission unit is installed. In fact, some transmissions such as 6L80 are terrific conversion transmissions thanks to their adjustability, while other transmission units may require much more efforts to make them 4WD.

In many cases, automakers standardize their car models to be equipped with different types of engines and transmissions with different drive configurations. Thus, the conversion process in such car models is not something supernatural and does not require significant changes in the structural design, but at the same time to perform it properly you need technical knowledge and some special tools. If you are not sure that you can perform this operation properly and you badly want to convert you 2WD transmission into 4WD still, you can buy all necessary parts on the internet and find a professional workshop providing swap services.