JDM viscous diff swap

Alcyone Lunacy · #1 12-15-2009, 02:54 AM

Is the JDM viscous rear diff a "plug-n-play" swap with the US open diff. I know the JDM diff is 3.7 ratio and not 3.545 like the US. Just curious or does this require an entire JDM tranny/front diff as well?

subeman90 · #2 12-15-2009, 04:22 AM

all SVX's have an LSD rear diff.

Alcyone Lunacy · #3 12-15-2009, 05:32 AM

But didn't the JDM SVX only get the viscous LSD? I thought the LSD we got was non-viscous?

Manarius · #4 12-15-2009, 05:42 AM

Quote:

Originally Posted by Alcyone Lunacy

But didn't the JDM SVX only get the viscous LSD? I thought the LSD we got was non-viscous?

If our LSD is not viscous, what type do you think it would be?

I've not heard of too many other LSD's that aren't viscous in some way.

(I don't mean to be condescending with this post, I'm interested in this idea).

Now the type of coupling in the transmission on the other hand - that's a different story.

NeedForSpeed · #5 12-15-2009, 09:17 AM

The US and JDM VLSD diffs are identical, except for gear ratios. If your front diff is 3.545, use the US rear diff. If you install a fwd 3.70 front diff, use the JDM rear diff,,,

NeedForSpeed · #6 12-15-2009, 09:22 AM

Your XTs have a mechanical LSD rear diff.
I believe the automatics were 3.70, but the diff is not compatible with SVX axles.

BoxerFanatic · #7 12-15-2009, 09:23 AM

Quote:

Originally Posted by Manarius

If our LSD is not viscous, what type do you think it would be?

I've not heard of too many other LSD's that aren't viscous in some way.

There are viscous LSDs, friction clutch LSDs, and helical-gear torque biasing LSDs. Torsen is an example of the helical gear type. 2007-2009 Legacy Spec.B uses a torsen with 3.9:1 final drive, where the 2005-2009 Legacy GT have viscous LSD rear differentials, and a 4.11 final drive. My 99 miata has one, as well.

I believe the Spec Bs with the 6MT gearbox use a 1.1:1 step-down, with a 3.9 rear diff, and a 3.54 front diff (same as SVX front and rear, with no step-down), and the Spec B rear differential can be pieced together with the SVX diff carrier system, and can be used in an SVX, with the corresponding 6-speed manual conversion. But don't treat that as an iron-clad guarantee... I am not 1000% positive it works exactly that way.

The Spec.B driveline is basically the same as the later model WRX STI drivetrain, except without the driver-override center differential DCCD system, it is basically set to permanent "auto" on the center differential control, and the Spec B didn't get WRX STI's Brembos nor 5x114.3 wheel hubs, and retained the Legacy GT's power level and turbo configuration, rather than the STI's hotter turbocharger, and requisite intake and intercooler setup.

Personally, my ideal SVX would have a tuned-up and freshened EG33 engine, somewhere around 300 naturally aspirated horsepower, or a bit better, backed by the Legacy Spec.B drivetrain... both the 6MT manual transaxle, and torsen rear differential, and the rearward standard torque distribution, with Variable Torque Distribution center differential. My ideal new Subaru would merely transplant the EG33 with a new EZ-series, tuned and equipped similarly.

Otherwise, in general, there are newer systems that couple differentials with electronically controlled clutch packs that actively manage the output of differentials, which is basically the same as the VTD center differential Subaru system I just mentioned, also like the rear diff in Acura RL's SH-AWD, and such. I believe that other torque-vectoring electronically managed differentials, like Ferrari, and now Audi, and others are starting to use, are similar systems, with varying degrees of sophistication.

NeedForSpeed · #8 12-15-2009, 09:34 AM

Quote:

Originally Posted by BoxerFanatic

There are viscous LSDs, friction clutch LSDs, and helical-gear torque biasing LSDs (Torsen is an example. 2007-2009 Legacy Spec.B uses a torsen, where the 2005-2009 Legacy GT have viscous LSD rear differentials. My 99 miata has one, as well.)

My LS has a Torsen as well

ridered777 · #9 12-15-2009, 01:51 PM

Quote:

Originally Posted by BoxerFanatic

2007-2009 Legacy Spec.B uses a torsen with 3.9:1 final drive,
I believe the Spec Bs with the 6MT gearbox use a 1.1:1 step-down, with a 3.9 rear diff, and a 3.54 front diff (same as SVX front and rear, with no step-down), and the Spec B rear differential can be pieced together with the SVX diff carrier system, and can be used in an SVX, with the corresponding 6-speed manual conversion.

If its a 3.9 Torsen, one could assume it would be useful in any 3.9 equipped car - say a legacy/imprezza 5spd, not just the 6speed from the leggy.

Correct?

This could be a option worth looking at, perhaps for the aftermarket options of a 3.9... assuming the axles fit the leggy rear end.

That is to say, rather than an R180, those of us with a slightly older imprezza tranny could have a pretty beefy rear end, without having to do the STi/WRX tranny for alot more!

Alcyone Lunacy · #10 12-16-2009, 12:04 AM

Quote:

Originally Posted by Manarius

If our LSD is not viscous, what type do you think it would be?

I've not heard of too many other LSD's that aren't viscous in some way.

(I don't mean to be condescending with this post, I'm interested in this idea).

Now the type of coupling in the transmission on the other hand - that's a different story.

No offense taken, I am learning more about the mechanical aspect of our cars, thus why I asked.

I was the VTD tranny that the JDM market got(duh this is what I was thinking about in the original post)

However,

Need for Speed- you say the US fwd svxi got the 3.7 front diff? So If I sourced one of these and a JDM rear diff the swap is do-able?

Just trying to figure this out. Thanks for all the info guys! Wow that is some serious homework!

NeedForSpeed · #11 12-16-2009, 11:07 AM

Quote:

Originally Posted by Alcyone Lunacy

No offense taken, I am learning more about the mechanical aspect of our cars, thus why I asked.

I was the VTD tranny that the JDM market got(duh this is what I was thinking about in the original post)

However,

Need for Speed- you say the US fwd svxi got the 3.7 front diff? So If I sourced one of these and a JDM rear diff the swap is do-able?

Just trying to figure this out. Thanks for all the info guys! Wow that is some serious homework!

Swapping front diffs is very do-able, probably a thread here about how to set it up properly. We swapped a 4.11 diff onto an eSVX trans, and many have found it necessary to swap another 3.545 diff for a broken diff. Do a search, should be helpful. Why not replace a broken 3.545 front diff with a fwd 3.70 diff, since you have the right rear diff?