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I was wanting to know if a 90-94 Legacy twin turbo transmission is a direct replacement for a 91 Legacy SS Turbo transmission, and is it any different?

It is physically a direct replacement, but you would need to make sure the ratio in the rear differential matches the ratio in the twin turbo transmission.

The Legacy SS has a 3.9:1 ratio. IIRC the Twin Turbo has a 4:11:1. 4:11 ratio rear diff are available here in the states on other models. However, your '91 most likely has a viscous limited slip differential, which most of us would not give up without a fight.

PhyrraM wrote: The Legacy SS has a 3.9:1 ratio. IIRC the Twin Turbo has a 4:11:1. 4:11 ratio rear diff are available here in the states on other models. However, your '91 most likely has a viscous limited slip differential, which most of us would not give up without a fight.

Thanks!

So the rear diff on the US 90-94 USDM turbo Legacy cars are quaife?

No. Viscous coupling.

And although it was "technically" an option for all years, reality has shown that only '91s got them. And almost all '91s at that.

PhyrraM wrote:No. Viscous coupling.

And although it was "technically" an option for all years, reality has shown that only '91s got them. And almost all '91s at that.

Now that is cool.. I didn't know my car year was a special year of a special car.

Would anyone know if I could possibly swap the front ring/pinion and the transfer drive gears in my turbo legacy transmission into a twin turbo transmission case and leave the center and rear diff alone?

Matt Monson would, he's our local transmission guy. It's more common to swap a compatable rear ring and pinion into the LSD unit. Of course your desired usage detemines the "best" way.

PhyrraM wrote:No. Viscous coupling.

And although it was "technically" an option for all years, reality has shown that only '91s got them. And almost all '91s at that.

It was not an option for all years. The rear VLSD was only available on the 91 MY, and it was standard, not an option.

For starters, the first generation twin turbos started in a 1994. Any 1990-1993 JDM Legacy or export Legacy/Liberty would have been a single turbo car with Ej20G engine.

If you provide the 10 digit alphanumeric transmission code off the bell housing I can tell you exactly what it is. If it's a TT gearbox it's mostly likely the same as early WRX:
3.454
2.062
1.448
1.088
0.825
4.111

However, if it's from a 1989-1993 Legacy GT or RS it could be any number of ratios, some of which are the same as an early WRX-RA but with 4.11 FD. Regardless, if it's a foreign Legacy gearbox it's most likely a 4.11

I have requested the bell housing info and I'll post it as soon as I get it.

I'm still wanting to know if I can swap the front diff stuff if necessary so that I can keep the rear LSD the way that it is.

Sure you can but it's not an easy job. It requires splitting the case and going deep inside the transmission.

It's easier to find a 4.11 rear LSD and install it. And you should be able to sell your 3.9 for about the same money it costs you to acquire a 4.11. Or you can install the LSD from your 3.9 into a 4.11 diff. housing.

Furthermore, unless your stock transmission is screwed there's just no point in making this change. You'll basically have a different transmission that performs almost exactly the same as your stock transmission. Part of why people make this change is to gain the 4.11 and make the gearing a little shorter, thereby making the car quicker.

Matt Monson wrote: ... Furthermore, unless your stock transmission is screwed there's just no point in making this change. ...

Thanks,

Hehe!

The transmission is screwed!!

One day I was just commuting to work and accelerating after turning left at an intersection shifted into 2nd then (kaboom rattle thwap-thwap-thwap). At first I thought it was my clutch but then I realized that the only gear that didn't grab right was 2nd. The clutch flywheel and pressure plate are fine.

There you have it...