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Subaru Performance Automatic Transmission Upgrades

Dec 6, 2007
There is probably a transmission shop in almost every town in the country that can build a bulletproof transmission for American made vehicles. Whether it is a late model Mustang or a 60's Chevelle, there are lots of aftermarket components and many years of technology available for the transmission rebuilder to utilize.

The same cannot be said for the 4EAT automatic transmissions found in Subaru vehicles. In factory form, these transmissions demonstrate several problem areas that can cause failure in as little as 20,000 miles with a stock engine and even sooner in modified applications.

Poor shift quality, anemic factory components, and above all, improperly configured torque converters plague the automatic transmission equipped vehicle. This is in conjunction with the automatic's propensity to burn out the high clutch pack (third gear) and the 2-4 clutches (second and fourth gear) with even moderate increases in engine output.

Realizing the need to take import and sport compact automatics to their fullest potential, IPT Performance Transmissions- a New Jersey based company, committed a large portion of their research time and dollars toward improving the Subaru automatic transmission.

First up on the hit parade of internal modifications is the need to address the issue of valve body calibration. The valve body mods are really the foundation of any performance trans rebuild. An automatic transmission utilizes a network of valves, servos, solenoids, orifices, accumulators 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 article, suffice it to say that modifying the valve body effectively changes the rate of clutch 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 overall capacity for power handling.

Next, all critical support bushings need to be 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.

Many of the other parts also such as the pinion shaft, the front differential ring gear and the planetary gearset undergo various metallurgical 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).

Lastly, 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 powered applications. Performance clutches and steel reaction plates are installed and their clearances set to much more exacting specifications than those that are used by the factory. Most importantly, IPT utilizes a proprietary six disk high clutch pack and a five disk 2-4 clutch pack which increases the second, third, and fourth gear torque capacity by over 25% versus the factory set up- this is the key to getting a high powered vehicle make a 1-2, 2-3, and 3-4 shift instead of just bouncing off the rev limiter.

As for the modifications outside of the gearbox, the next thing of importance is of course the torque converter. In an automatic equipped vehicle, one of the most important components in the entire car in regards to optimizing performance is a correctly configured converter.

In addition to 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).

Even in otherwise stock turbocharged Subaru vehicles, ET reductions of over one full second are not at all uncommon with the addition of a properly configured converter.

Transmission, torque converter and valve body upgrades are not only for American made vehicles anymore. Increased performance, durability and more efficient shifting is now available for the vehicles of almost every manufacturer- thanks in part to the few companies that are willing to do the research and development work required for this rapidly growing segment of the automotive aftermarket.
About the Author
John Lombardo is owner of IPT Performance Transmissions and has been in the high performance industry for over 20 years
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