Mitsubishi DSM Automatic Transmission Modifications

The focus of what we're talking about here are the Mitsubishi built turbo (and V6) transmissions, the W4A33 (AWD) and the F4A33 (FWD). Internally, these two transmissions are very similar so I'll lump them together for the purpose of simplicity.

These relatively weak transmissions can be made quite suitable for performance use. As with anything else, there are varying degrees of what can be done with them.

At the entry level of making these transmissions able to cope with more horsepower is the installation of a shift kit or modified valve body and the addition of an upgraded end clutch kit.

The valve body mods are really the foundation of any performance trans rebuild. An automatic transmission utilizes a network of valves, servos, solenoids, orifices and passages that control and route hydraulic pressure to the appropriate places to apply the clutches and bands.

Without going into elaborate detail which would fall outside the scope of this thread, suffice it to say that modifying the valve body effectively changes the rate of clutch and band apply and also increases the clamping force with which these components have to work with. This is why shift kits and modified valve bodies produce that firm performance type shift that increases the transmission's capacity for power handling.

Next up is the installation of a performance end clutch kit. The function of the end clutches are to help produce an overdriven gear ratio (4th gear). Due to original factory design and vehicle space considerations, this clutch pack is designed to 'fit in the car' rather than to be durable. The steel reaction plates, which are sandwiched between the factory clutches, are of an inferior material. This problem makes the plates very subject to warpage and subsequent failure when exposed to excessive heat.

The performance end clutch kit helps to rectify this situation by using better steel plates and friction plates with clutch lining that has a greatly improved coefficient of friction compared to the factory components. The beauty of the first two modifications is that they can both be accomplished with the vehicle still in the car.

The next thing to be considered, from a durability and a performance standpoint, is the addition of a performance torque converter.

As well as being made with many strength improvements over the factory piece, a performance converter enables a vehicle to better take advantage of the RPM range at which maximum power is made. In simple terms, a properly designed converter will allow a vehicle to launch under more power by increasing the rpm at which the launch takes place. In essence, a torque converter is an infinitely variable transmission in and of itself, and altering its torque multiplying characteristics becomes absolutely necessary when other variables in the vehicle are changed (bigger turbos and bigger cams, etc.).

Converter designs range from the strengthening and restalling of the factory core- all the way to reduced diameter customized pieces made with a cover that is machined from a piece of steel billet.

The ultimate step in improving the automatic transmission is a full performance rebuild.

First off, the transmission needs to be disassembled, thoroughly cleaned and inspected. During this process all questionable parts are discarded. Many of the parts that are reusable may undergo various processes such as heat treating, Meta-Lax stress relieving, and cryogenic treatment (freezing them in liquid nitrogen at -300 degrees Fahrenheit to 'reset' the part's structure on the molecular level).

Next, all critical support bushings are replaced to insure that the centerline of the transmission runs in perfect alignment with the centerline of the crankshaft. Ignoring this step puts a lot of undue stress on the internal transmission components.

At that point, all of the required seals and gaskets are changed to materials that are better able to cope with the heat and stress that is common in high power applications. Performance clutches and steel reaction plates are then installed and their clearances are then set to much more exacting specifications than those that are used by the factory.

We have also recently developed a means of increasing the holding capacity of the front clutch by installing an additional clutch in the pack. This results in an automatic increase of 25% in the torque handling ability of what has historically been the weakest link in this gearbox.

The aforementioned valve body modifications are performed and the remainder of the transmission undergoes a precision reassembly and blueprinting process where more critical clearances are checked and adjusted and, of course, all fasteners are torqued to spec. This is somewhat similar to what is done during engine assembly.

The W4A33 and F4A33 are an excellent starting point for the foundation a performance transmission. They have relatively 'beefy' hard parts and gearsets and feature quite a lot of clutch and band surface area, which is critical when it is time to handle increased power levels.

The 900+ horsepower level is achievable with these units with the right components and modifications, but certainly not in factory form.

More info can be found here: Mitsubishi / DSM Custom High Performance Transmissions


About The Author:
John Lombardo, a.k.a. Transdude, is co-owner of IPT Performance Transmissions and has been in the transmission and high performance industry for over 20 years. Visit his website- IPT Performance Transmissions for more drivetrain information articles and performance products.

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