poor mans dccd, the 5mt center diff locker thread

[legsportwag]

Well I've been getting questions about my recent transmission I built so I making this dedicated post on how-to build this franken-box.

In essence this build completely eliminates the viscous type center diff and replaces it with a manual locking center diff (open or 50/50)

Parts needed:
Ej 5mt of your liking (I used a 03 forester 5mt)
Complete Subaru rx or xt6 dual range all time awd 5mt

From the factory Subaru used a very unique transmission in the Rx/xt6. It was a dual range, all time awd 5 speed with center diff locker. Basically it was the pre-curser 5mt to the standard 5mt used in everything 1990+ and because of this a lot of things are interchangeable between the Rx/xt6 5mt and the ej based 5mt..

So here is were it gets a little tricky.

Basically you need to have both trans apart and next to each other. With both cases split you need to transfer the front ring and pinion from the Rx transmission into the ej transmission. Both are easy to get at and to save work swap just the ring gear itself onto the ej front diff (same for doing rear vlsd swap). Basically you want to keep bearings matched up so you don't have to mess with setting pre load and having to jack with backlash and adjusting bearings and gears. Once ring gear is swapped you need to swap the pinion into the driven gear set of the ej trans. This again is super easy. One big nut on the end of the pinion and the whole thing slips out. BE CAREFUL TO KEEP BEARINGS WITH THE PINION! the rx /xt pinion is about 6" longer and is required to make the center diff locker work.. Once your Rx pinion is torqued to spec and the nut has been peened you can set the driven gear set back into the ej case and re assemble.. You will now have this (notice phase 2 case with Rx pinion sticking out the back)

And a regular ej trans for pinion comparrison


From here you simply slide the Rx tail shaft housing (transfer box with center diff) onto the back of the ej transmission. And you end up with this:

See the little lever on the side? That's the diff lock actuator, on the Rx this was hooked to a vacuum can that had two sides so it could push or pull. I managed to salvage the actuator can (tucks under starter) and used some random vacuum solenoids to control it...

A pic of the locker piece itself:


Some other notes:

The diff locker center is 2.5" shorter then a viscous center (requires 2.5" longer drive shaft)
Requires custom shift linkage
Requires custom trans mount

Enjoy!

[86BRATMAN]

One thing to note, the xt6 trans is not a dual range. Only the rx transmission is d/r. Aside from that party on Wayne.

[mike-tracy]

legsportwag: I have seen a couple RX's in the junkyard, and that neat "diff lock" switch. With the center diff "open," (and no more viscous lsd center), where does the power go when a wheel loses partial or all traction? I noticed the RX had a clutchpack LSD rear diff, so I'd guess that car would be a minimum of 3WD, right?

[legsportwag]

In open mode the power will follow the path of least resistance. Essentially if all 4 aren't on the ground it looses all power. I'm personnaly on coilovers so I notice pulling into some parking lots if I hike a tire it will spin free. I also run a ph2 vlsd in my rear (05 Saab 92x axles and diff in legacy turbo wagon 3.9 case)

[MConte05]

This must be a washington only type mod that is possible. I honestly cannot say I've ever seen an RX in a junkyard, let alone on the street! Very cool.

[cj91legss]

What if I wanted to keep 4.11

When my transmission brakes, it most likely will, I may try this out.

I think I'm on my 5th 5 speed now or something like that.

[ride_child]

thats pretty cool, and i happen to have a turbo rx in my driveway....

[Legacy777]

Nice write-up! Any more pictures?

Are you using a custom driveshaft or were you able to find a stock Subaru driveshaft that works?

[legsportwag]

I wish I had more pictures.. I built this trans months ago as a backup for when I blew up my 01 forester 4.11 box.. I built my own drive line actually, I have a spare I need to take to the drive line shop so i t can be extended professionally (mine is straight but not balanced)..

The problem with keeping the 4.11 gears is the pinion. The diff locker center requires the matching pinion (its much longer) and thus requires matching ring gear as well.. Basically your stuck with 3.9 final with this center locker, no way around it...

[cj91legss]

Damn...

[legsportwag]

thats pretty cool, and i happen to have a turbo rx in my driveway....

I was looking at picking up an Rx. The beauty of this "franken-box" is if I would have put it in an Rx, gl10, or xt it would have bolted in without custom drive line, shifter linkage or trans mount.. Basically this would allow you to bolt an ej motor in with out the normal ej to ea adapter plate

[legsportwag]

Same situation let's people wanting to do ej swaps in ea cars run a normal ej clutch. Ea clutches are too small for the power an ej can produce. Especially turbo ej motors

[James614]

This is pretty cool. The parts for it are super rare though.

[legsportwag]

Not in my neck of the woods.. I can put my hands on at least 3 other Rx/xt6 full time trans within 50 miles.. The one I used to build this was given to me by a good friend , although I'm sure he wanted to see me build this trans as bad as I wanted to build it. They are around!

[James614]

They probably don't use salt in Montana

[legsportwag]

Nope, and we have plenty of land for big junk yards! Lol

[czei]

I had always thought the manual was locked at 50/50, but that was a misreading of the Wikipedia entry:

The manual transmission uses a true gear type viscous limited slip center differential. The secondary shaft in the transmission is hollow with the pinion shaft extending through it to the center differential. It provides a 50:50 torque balance under acceleration and deceleration up to the point of wheel slip, then the VLSD begins to send more torque to the wheels with more traction.

Do I understand how this works correctly under hard acceleration in rally conditions? In rapid acceleration the front lightens and the front wheels get less grip while the rear wheels get more weight and thus more grip:

1. A locked 50/50 transmission will send 50% of the power to both front and rear wheels. Since the front is not limited slip its power could be pissed away on the slipping inside wheel in a corner, but the rear LSD makes sure it's 50% is well used. And the front wheels will still have some grip, and every little bit helps.

2. The standard new 5MT under acceleration will start at 50/50 front/rear and then as the front wheels slip will try to send up to 80% of the power to the rear wheels since the front wheels have less weight and thus less grip. This minimizes the need for limited slip in the front since the LSD in the rear is getting most of the power anyway. This scenario gives the most efficient use of the engine's power.

3. A well used open front/rear 5MT will start at 50/50 and as the front wheels lose grip in acceleration power won't be transferred to the rear, but instead to the front where there's less grip. This implies that if the diff is open my rally car is getting useless wheel spin rather than putting all of the power to the wheels. Crap, that's not good. It makes sense in hindsight since my car puts in much better times if I avoid the best lines and stick to the sections of the track with more grip, regardless of how slow of a line it is. It also would explain why switching from an open to a limited slip rear diff didn't increase speed, which makes sense if power is being sapped away in a spinning front wheel.

Is there any test for an open center diff like there is for the rear LSD? The Wikipedia entry on viscous LSD technology said they were toast after 60K miles but gave no reference so it's likely to have been pulled out of someone's butt, but never-the-less could have some truth to it.

[James614]

Where you talk about a locked 50/50 trans, I assume you mean a fixed 50/50 torque bias and not a locked differential.

One thing ill say id that under motorsport use, the stock center diff really isnt up to the task. On a new trans it might be, but even then it would wear over time and fail to provide as much torque transfer to the wheels with traction as we would like. Ive seen 08 WRXs where the center diff is already acting almost open.

Doing the mod in this thread to lock the center diff will completely lock the front and rear axles together, great reducing the amount of power youll loosse to a single spinning wheel (id imagine that as long as you dont have 1 wheel in the air while rock climbing, you shouldnt see single wheelspin at all). And this mod allows you to unlock it for daily driving, though then you would be completely open and have very little or no traction when one wheel starts to spin . But assuming you just drive like a normal person, thats probabpy fine. This mod is probably ideal for someone like you.

The other options are far more expensive: swapping in a V6 STI true DCCD trans (rrequires a controller, but then you get to set progressive limited slip ratio up to full lock at your desire), an STI 12kg or 20kg viscous center LSD which are similar to the stock diff but can transfer much more torque under wheelspin (they are 3x and 5x stronger than stock, respectively), or adding a front LSD to eliminate single wheel spin upfront. The best solution is to use a combination of these. But obviously, these are EXPENSIVE, an RX trans if you can find one will be very cheap.

[legsportwag]

Your on the money there James!

[Legacy777]

...The other options are far more expensive: swapping in a V6 STI true DCCD trans (rrequires a controller, but then you get to set progressive limited slip ratio up to full lock at your desire)....or adding a front LSD to eliminate single wheel spin upfront. The best solution is to use a combination of these. But obviously, these are EXPENSIVE, an RX trans if you can find one will be very cheap.

This is essentually what I have now with the six speed. I've tested locking the center diff and doing a tight turn. You can definitely feel the resistance/binding in the transmission. Additionally, the front LSD should help with traction events. Like James said it does help, but it's not cheap.

[mike-tracy]

How much was having your drive shaft extended? I've seen 3 rx's in over a decade of going to the junkyard. I grabbed one of their rear LSDs, and the cover said it was 3.7 final drive, is that what you meant, legsportwag? I noticed the same generation (as the RX) Subaru's open rear diffs had a FD of 3.9.

Guess what I'm trying to say overall, is the RX is a rare beast. I did end up buying a 5mt DCCD tranny for ~800, and the rear biased power delivery is a hoot. Don't have the ability to lock the center diffy without the $400 Spiider, so your point about this RX tranny hybrid being the poor mans dccd is well taken

[legsportwag]

I did the drive shaft extension myself (I'm a welder by profession and an a hobby machinist) my driveshaft is straight but not balanced (can feel slight vibration under heavy load (20psi). Yes this a true poor mans dccd. Its a driver controlled center diff. But only options are open or locked..

Anyways this last labor day weekend was a local track event. Needless to say I ran the legacy with this trans, multiple locked launches as well as many many 65+mph power slides and not a single issue. I'm super happy

[Alphius]

Very cool that someone has actually done this. I've always suspected it was possible.



To clarify: whether open or locked the torque split of the center diff is and always will be 50/50. Doesn't matter if you have a completely worn out center VLSD, a 4, 12 or 20 kg center, a locked or unlocked RX center diff or any amount of wheelspin. The torque split is always 50/50. This does exclude the DCCD transmissions, as they have a planetary gearset that splits torque unevenly. That split ratio has varied over the years but is generally something like 35F:65R.

I have driven a 5MT with a completely shot VLSD center. If you dropped the clutch from a stop it would roast one front tire all the way down the street. Lift any one tire off the ground and you were stuck.

If you don't understand the physics behind the torque split being forever fixed at 50/50, have a read of this post. It explains it much more succinctly than I have time for. http://www.clubwrx.net/forums/sti-trans ... ost1121543

[czei]

Very cool that someone has actually done this. I've always suspected it was possible.



To clarify: whether open or locked the torque split of the center diff is and always will be 50/50. Doesn't matter if you have a completely worn out center VLSD, a 4, 12 or 20 kg center, a locked or unlocked RX center diff or any amount of wheelspin. The torque split is always 50/50. This does exclude the DCCD transmissions, as they have a planetary gearset that splits torque unevenly. That split ratio has varied over the years but is generally something like 35F:65R.

I have driven a 5MT with a completely shot VLSD center. If you dropped the clutch from a stop it would roast one front tire all the way down the street. Lift any one tire off the ground and you were stuck.

If you don't understand the physics behind the torque split being forever fixed at 50/50, have a read of this post. It explains it much more succinctly than I have time for. http://www.clubwrx.net/forums/sti-trans ... ost1121543

If the split remains 50/50 on an open diff then why would one get stuck of a tire was lifted off the ground? Even if one of the front wheels ws lifted at took all of the torque from the front the 50% of the torque in the back would still find traction,

[Alphius]

Read the link for a better description. Here's a short version:

To understand the torque split, you have to understand Newton's third law of motion: every action has an equal and opposite reaction. To understand this as it applies to traction, think of how the wheels move the car. The wheels push against the ground with a force equal to the torque the engine is producing to move the car forward. Assume our Subaru has an open center diff with a torque split of 50/50. The front and rear wheels will each push against the ground equally with exactly 50% of the torque. On dry pavement, it is likely that the engine cannot produce enough torque to overcome the wheels friction with the ground, and the car moves forward. If the engine produces 300ft-lb of torque, the front wheels and rear wheels each apply 150ft-lb of torque to the ground and the car moves forward.

Now let's assume we have our open center diff Subaru but with one important difference: the front wheels are on ice and the rear wheels are on pavement. If you apply torque, the front wheels will spin and the car will not move. In this situation, the max torque that can be applied to the ground is limited by the least amount of traction available to any wheel. So, if it takes 10ft-lb of torque to spin the front tires on the ice, your open 50/50 diff will still transfer 50% of that torque to the rear wheels. In this situation, 50% of the torque is 5ft-lb. The center diff applies that torque to the rear driveshaft, but it's not enough torque to overcome inertia and move the car forward. In this example, the center diff is still applying 50% of the torque to the front and 50% to the rear, but the maximum torque applied to the car in total is limited by the traction of the front wheels.

Make sense?