The main differences that make the turbo legacy 4eat better is that is has one more clutch plate on the forward & high clutch assembly.
Is it really worth the money, and or hassle to try and find a turbo legacy 4EAT? I don't know...probably not, unless you're making gobs of power, even then, the stock turbo leg tranny wouldn't hold up too long. Only big catch as mentioned is the different drive ratios. However the 92-94 normally aspirated 4EAT's had a 3.90 final drive ratio, so if you found one of those, it'd be a direct swap.
Here's a post from Larry Witherspoon from the yahoo bc-bf legacyworks group about the tranny differences
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Sent this to a "generic" Subaru forum last nite as an addendum to an auto trans discussion
For Turbo Legacy and SVX owners...ESPECIALLY
Turbo Legacy Auto Trans Part Number TZ102ZJ2AA
SVX Auto Trans Part Number TZ102ZM1AA
The MY 91 - 94 Turbo Legacy auto trans is unlike most other Sube auto trannys but is also found in the MY 92 - 97 SVX
Some would look to a WRX for a replacement trans and that's fine, but if you can find one in good shape from a Turbo Legacy or SVX you will have the added advantage of a trans built for a heavier car
Be careful trying to use an SVX trans in a turbo Legacy. The SVX had two different gear ratios...3.9 and 3.545. The torque converter for the 3.545 won’t fit the Legacy Turbo 3.90 flexplate.
As far as the torque converter differences, my info has it that the SVX and Turbo Legacy converters are not interchangeable. They have different overall diameters and different mounting bolt circles.
The Subaru Factory '91 Model Year Update Course gives the following tranny info comparing the Legacy Turbo auto tranny to the auto trans for it's normally aspirated sister;
Of great importance in my opinion is the last sentence, which provides information about a condition I believe unique to the Turbo Legacy, and why it is so important to try and stay with a Turbo Legacy trans and electronic control units if you have one of these rare cars;
"The TCU and ECU do NOT interface on the Turbo Legacy" (unlike all other Subarus)
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In order to handle the increased torque produced by the turbocharged engine, the 4EAT Transmission uses a larger Torque Converter. For additional strength, the input shaft has also been shotpeened. In both the Forward Clutch assembly and the High Clutch assembly, the number of drive and driven plates has been increased by one (1), while the retaining plate thickness has been reduced. The Low/Reverse Brake assembly has increased the number of plates by two (2), but the thickness has been reduced. The outer diameter of the Servo piston (band application) assembly has been increased in order to provide more band clamping force. Of course, the servo piston seals have also been enlarged to match the servo piston diameter. Due to the new piston shape the number of "O" ring seals has been reduced by one (1) Lubrication passages have been added to the Planetary Carrier assembly for increased gear durability. Finally the logic in the Transmission Control Unit (TCU) has been changed to match the mechanical changes to the 4EAT. And a new Select Monitor Cartridge (P/N 498348400) has been introduced which supersedes P/N 498347500 and P/N 498347501. This cartridge will also operate on the 90MY Legacy. The TCU and ECU do NOT interface on the Turbo Legacy
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For rebuilt Sube transes my first choice for a stock rebuild would be Colorado Component Rebuilders http://www.ccrengines.com/ though I have found local auto trans specialty shops that could do the job. You just have to ask questions you already know the answers to so that you weed out the wannabees.
I now have a "bulletproof" trans from Level 10 Performance Transmissions http://www.levelten.com/ which I went to because the SVX forums speak well of them, and since the SVX only came with an auto trans, those guys know where to go
JMO of America http://www.jmousa.com/ did my torque converter, and are one of the few torque converter performance specialty shops around. An insight to their torque converter performance upgrade can be seen here http://www.hondatuningmagazine.com/tech/0211ht_converter
The downside to the extra performance and durability in my case is an uncomfortably hard upshift at low speeds from first to second and second to third...I hear the tires chirp on dry pavement all the time
A place called TransGo http://www.txchange.com/transgo.htm has a kit for the Legacy 4EAT auto trans which is less expensive than the Level 10 reworked valve body, but I understand there's machining to be done on the valve body plus the replacement parts so once you locate a good tranny man maybe it would be a wash as to which is less expensive. However a still less expensive alternative is their "Shift Improver" kit (part # SKRE4R01A). This $50 kit has 4 plastic rings for the transmission that are designed to handle greater temperatures than stock Subaru parts
ABSITIVELY POSOLUTELY install an external cooler, it's so important for increased longevity the JMO torque converter upgrade includes one in the price!
Now if I can just get my engine done maybe I'll be able to enjoy all the money dropped into the trans!
Larry autotransking Witherspoon