DIY: B5 S4 4:1 center differential modification like StaSIS/JHM

**slowww328** · 07-08-2011 03:01 PM

The 4:1 modification for TORSEN center differentials changes a car's torque bias ratio, allowing the car to shift more torque to the rear wheels when necessary. This improves corner exit speed and decreases understeer. Both StaSIS and JHM offer this modification, but they charge $550 and $350 respectively. The same exact effect can be achieved by replacing a pair of low-friction washers in the differential stack with steel ones. This walkthrough will show the reader how to disassemble the differential and install a set of aftermarket washers in a B5 S4.

This modification only applies to S4s with T2 differentials (model year 2000+.) '99 S4s and older European models can be either T1 or T2. If you have one of these cars, you'll have to remove the differential from the housing to find out what type it is. If it's T1, this walkthrough isn't applicable.

The part numbers for T2 differentials:
01E 409 751 E
01E 409 751 F

First, you must remove the differential from the transmission and diff. housing. This procedure is outside the scope of this DIY. For help, see this guide for the B5 A4 or this guide by StaSIS.

Materials:
Aftermarket washers
6-32 tap
3x 6-32 threaded bolts (get a few extras, these tend to get broken)
Hammer
Vice grips or similar

Once the differential is out of the car, the spirol pins holding everything together will have to be removed. These are meant to be single use, but when removed with care they can safely be reused. Use a 6-32 tap to thread all three pins.

Next, thread your 6-32 bolts into the spirol pins and grab them just inside the heads with your vice grips. Using the hammer, hit the vice grips to tap the pins out of the differential like so:

Once these pins have been removed, the rods holding the gears on the unit are free to slip out. Be careful not to let this happen! Tape can be applied over the holes to keep them in for the time being.

Removed pin:

Once all three pins have been removed, the differential and gears must be marked for later reassembly. Each gear will have to be put back in its original location with the correct orientation. One easy way to do this is to stick a piece of tape on the differential housing next to each gear. Next, find the imprinted letter E on each gear in between the teeth and mark its location on the tape. Then, mark each window or opening on the differential housing with a number 1-3. For example:

Next, remove the rods on which the gears turn.

Wrap each gear in tape and label its location (top/bottom and window number):

With the gears removed and labeled, only the guts of the differential remain. This is the washer stack, the part we will be modifying. Pull it out, and keep the washers in order like so:

In top to bottom order, the stock washer materials and rough thickness measurements should be as follows:

* bronze - 1.96mm (can also be teflon coated instead of bronze for T2 version # 01E 409 751 E, but the sizing is identical and it doesn't change the mod)
steel 4.14mm
GEAR
steel 1.56mm
* teflon coated 1.96mm
steel 1.58mm
GEAR
steel 4.74mm
steel (darker and slotted) 1.98mm
steel 4.78mm

The hardened steel aftermarket washers replace the bronze and teflon coated washers, marked with *s in the above list. Simply swap them out and reassemble! Once the gears are back on but before the spirol pins are in, make sure the gears turn relatively smoothly (i.e. don't lock up, there will be some friction since at this point the diff. isn't lubed.)

**RusS4** · 07-08-2011 06:57 PM

Great write up man! Definitely bookmarked.

**Bimmerchop** · 07-08-2011 07:37 PM

Imaginary rep coming your way

**Gumby** · 07-08-2011 08:53 PM

slowww328, i copied this into the FAQ/DIY section. if you have a problem with this PM me and i'll take it down.

**Vinchenzo51** · 07-08-2011 08:57 PM

Wow, well written and informative.

Bookmarked for sure.

**slowww328** · 07-09-2011 04:28 AM

Thanks guys, let me know if you see anything that needs improvement.

**DaveS4** · 07-09-2011 06:31 AM

Great stuff :)

**BITRBO** · 07-09-2011 05:41 PM

Kinda sounds like someone is getting back at one of the "tuners" w/ this write-up...

**Dimcorner** · 07-09-2011 08:30 PM

Nice to see a DIY. I would have to see what the final cost is whenever I decide to do this. The washers are about $75-100 bucks so I wouldn't mind paying in the realms of $200-300 to just git er done.

**Scotty@Advanced** · 07-09-2011 08:58 PM

great tech article however
bronze washers will work great but will wear out quickly.
you need to use hardened steel washers that are precision gounr and absolutley flat.
also we've seen some diff require slightly thicker shims to compensate for wear and prvent clunking issues.

as dimcorner said the washers themselves are pretty expensive when having small qtys made and for slightly more than the cost of the washers you get a drop in diff and a warranty.

**Seerlah** · 07-10-2011 12:03 AM

There is a guy on VW Vortex sellng the washers for $55 shipped. I plan on having my mechanic place them in at the same time he does my clutch (when????), but they can be def sought our for cheap.

**slowww328** · 07-10-2011 06:02 AM

Originally Posted by Scotty@Advanced

great tech article however
bronze washers will work great but will wear out quickly.
you need to use hardened steel washers that are precision gounr and absolutley flat.
also we've seen some diff require slightly thicker shims to compensate for wear and prvent clunking issues.

as dimcorner said the washers themselves are pretty expensive when having small qtys made and for slightly more than the cost of the washers you get a drop in diff and a warranty.

The replacements are hardened steel, not bronze. This is not just for durability, however, it's for surface texture and friction properties as well. One of the two stock washers you replace in the S4 diff. is bronze. Why Audi/Torsen chose to use bronze as opposed to a second teflon coated washer, I don't know. I would imagine it's for fine tuning the TBR. In any case, the steel replacements should actually exhibit better longevity than the softer stock washers (not that it matters, they don't go bad.)

150k mile Torsens out of the older 01As with the needle bearings invariably look brand new. In the newer 01As and the S4s, you can see very minor wear on the teflon coated washer, but nothing serious. These diffs are bulletproof. But the beauty of this mod is that the teflon washer gets replaced anyway. Shouldn't be any problems with slop in the washer stack unless you're burning the teflon off your washers like the community used to do to avoid paying $350-550 for this mod. Torsen definitely makes one of the best products in an Audi car - the tolerances and machining are phenomenal. Solid American company.

For me, $55-70 vs. $350-550 is more than a slight difference, but perhaps it isn't to you.

Originally Posted by Dimcorner

Nice to see a DIY. I would have to see what the final cost is whenever I decide to do this. The washers are about $75-100 bucks so I wouldn't mind paying in the realms of $200-300 to just git er done.

If you can get one for $200 w/ a warranty, that's a killer deal. The cheapest I could find was JHM @ $350 + shipping the diff. there + $50 for transmission fluid + core, and that's assuming you don't pay a mechanic to do the swap for you. They also don't modify the older A4 diffs, so my '98 was SOL unless I wanted to pay $550 to StaSIS. In the end, I did the mod myself for just over $100 including trans fluid for my A4, and about $20 more for my S4. Not bad considering I needed to change the fluid anyway. It's not for everyone though - it's pretty technical. If you don't have the chops but you've got the cash, JHM and StaSIS are expensive, but very good, options.

**bw86** · 07-10-2011 09:16 AM

thanks for the diy

**avusen** · 07-10-2011 10:15 AM

do you notice any difference in everyday driving? Would love to reduce my understear for sure!

**GetBoosted84** · 07-10-2011 10:38 AM

Originally Posted by Scotty@Advanced

great tech article however
bronze washers will work great but will wear out quickly.
you need to use hardened steel washers that are precision gounr and absolutley flat.
also we've seen some diff require slightly thicker shims to compensate for wear and prvent clunking issues.

as dimcorner said the washers themselves are pretty expensive when having small qtys made and for slightly more than the cost of the washers you get a drop in diff and a warranty.

I'm sorry to be the bearer of bad news but I have been running these since 09/2010 and put a ton of hard miles on the car and I have had zero issues.

Also, had Stasis mod my diff for me before getting these washers and I pulled apart the diff they modded. I can say without a doubt that the washers were exactly the same.

**Scotty@Advanced** · 07-10-2011 12:46 PM

Originally Posted by GetBoosted84

I'm sorry to be the bearer of bad news but I have been running these since 09/2010 and put a ton of hard miles on the car and I have had zero issues.

Also, had Stasis mod my diff for me before getting these washers and I pulled apart the diff they modded. I can say without a doubt that the washers were exactly the same.

Unless you put over 125,000 miles on it in the 8 months you've had this your experience isn't as accurate as the high mileage modded diffs we've torn apart. Thanks for the comment though.

**Scotty@Advanced** · 07-10-2011 01:00 PM

Originally Posted by slowww328

The replacements are hardened steel, not bronze. This is not just for durability, however, it's for surface texture and friction properties as well. One of the two stock washers you replace in the S4 diff. is bronze. Why Audi/Torsen chose to use bronze as opposed to a second teflon coated washer, I don't know. I would imagine it's for fine tuning the TBR. In any case, the steel replacements should actually exhibit better longevity than the softer stock washers (not that it matters, they don't go bad.)

150k mile Torsens out of the older 01As with the needle bearings invariably look brand new. In the newer 01As and the S4s, you can see very minor wear on the teflon coated washer, but nothing serious. These diffs are bulletproof. But the beauty of this mod is that the teflon washer gets replaced anyway. Shouldn't be any problems with slop in the washer stack unless you're burning the teflon off your washers like the community used to do to avoid paying $350-550 for this mod. Torsen definitely makes one of the best products in an Audi car - the tolerances and machining are phenomenal. Solid American company.

For me, $55-70 vs. $350-550 is more than a slight difference, but perhaps it isn't to you.

If you can get one for $200 w/ a warranty, that's a killer deal. The cheapest I could find was JHM @ $350 + shipping the diff. there + $50 for transmission fluid + core, and that's assuming you don't pay a mechanic to do the swap for you. They also don't modify the older A4 diffs, so my '98 was SOL unless I wanted to pay $550 to StaSIS. In the end, I did the mod myself for just over $100 including trans fluid for my A4, and about $20 more for my S4. Not bad considering I needed to change the fluid anyway. It's not for everyone though - it's pretty technical. If you don't have the chops but you've got the cash, JHM and StaSIS are expensive, but very good, options.

Not too sure where you got your pricing information... 01E center diffs $275.00 shipped

These diffs are pretty stout but they can develop internal issues, the worm gears can eat away at the pinion gears eventually leading to them failing, and the shim packs do wear over time, taking apart one diff and then using it as an example for all diffs is a pretty bad representative sample. I've seen a few exploded units as well. There are also a couple of older posts of a couple modded Stasis units giving up the ghost.

Originally Posted by GetBoosted84

I'm sorry to be the bearer of bad news but I have been running these since 09/2010 and put a ton of hard miles on the car and I have had zero issues.

Also, had Stasis mod my diff for me before getting these washers and I pulled apart the diff they modded. I can say without a doubt that the washers were exactly the same.

Your experience is very limited get back to us after 125,000 miles of use on a bronze shimmed diff.

Great tech write up!

**slowww328** · 07-10-2011 01:10 PM

Originally Posted by avusen

do you notice any difference in everyday driving? Would love to reduce my understear for sure!

Night and day. It's the way the car should have been from the factory. It feels much more confident coming out of turns - more planted. At the limit, you have more control with your right foot. On the downside, it will bring out the weakness of the open diff in the rear if you track it hard, but that's better than the ever famous Quattro understeer. If you want the real deal, do a 4:1 mod and then find a way to get a limited slip diff in the rear

.

Originally Posted by Scotty@Advanced

Unless you put over 125,000 miles on it in the 8 months you've had this your experience isn't as accurate as the high mileage modded diffs we've torn apart. Thanks for the comment though.

Considering he used precision ground hardened steel washers (as I did) in exactly the same way StaSIS does, I would say the method has far more than 125,000 total miles of testing (which you also benefit from, by the way.) But regardless, if you're using steel washers... and he used steel washers... doesn't your experience support his point, not go against it? Anyway, if something is going to wear out, it's incredibly unlikely to be your differential. Like I said earlier, these things are bulletproof.

Originally Posted by Scotty@Advanced

Your experience is very limited get back to us after 125,000 miles of use on a bronze shimmed diff.

Don't spread misinformation. NOBODY uses bronze washers. These are precision ground hardened steel, the same as yours.

Originally Posted by Scotty@Advanced

Not too sure where you got your pricing information... 01E center diffs $275.00 shipped

Fair enough. $55-70 vs. $275 from Advanced Automotion. I think I'll still do it myself, and I hope this DIY helps others who feel similarly. Please don't pollute my thread in which I try to help other AudiZine members save money and enjoy working on their cars. After all, isn't that what this forum is about?

**GetBoosted84** · 07-10-2011 01:56 PM

Originally Posted by Scotty@Advanced

These diffs are pretty stout but they can develop internal issues, the worm gears can eat away at the pinion gears eventually leading to them failing, and the shim packs do wear over time, taking apart one diff and then using it as an example for all diffs is a pretty bad representative sample. I've seen a few exploded units as well. There are also a couple of older posts of a couple modded Stasis units giving up the ghost.

Taking apart a stasis diff to see the washers they installed then comparing them to the ones that I got from the guy on Vortex to ensure they are the same (EXACT) size, material, and finish isn't a good way to check and see if the washers are the same? Same weight too :-). I'm thorough. So that's a bad sample? Really... how exactly would you have gone about it any different?

Originally Posted by Scotty@Advanced

Your experience is very limited get back to us after 125,000 miles of use on a bronze shimmed diff.

I think we already established that it's not a bronze washer.

**Scotty@Advanced** · 07-10-2011 04:48 PM

Originally Posted by slowww328

Considering he used precision ground hardened steel washers (as I did) in exactly the same way StaSIS does, I would say the method has far more than 125,000 total miles of testing (which you also benefit from, by the way.) But regardless, if you're using steel washers... and he used steel washers... doesn't your experience support his point, not go against it? Anyway, if something is going to wear out, it's incredibly unlikely to be your differential. Like I said earlier, these things are bulletproof.

A misunderstanding on my part. But my comment on these diffs being bullet proof are still valid, they are stout but bulletproof.. They are not.

Originally Posted by slowww328

Don't spread misinformation. NOBODY uses bronze washers. These are precision ground hardened steel, the same as yours.

I'm not spreading mis information. Your wrong also as Torsen Issued a DIY modification guide on how to mod the 01A diff and specified a hard bronze washer. This is what I was basing my comments off. Visit http://www.motorgeek.com/viewtopic.php?t=7037 for an excerpt from torsen.com.

Originally Posted by slowww328

Please don't pollute my thread in which I try to help other AudiZine members save money and enjoy working on their cars. After all, isn't that what this forum is about?

I wasn't polluting your thread, I can appreciate it and support the do-it-yourselfer. Just dispelling some inaacuracies in this thread. Although I did misunderstand the use of bronze shims, that's my bad.

Originally Posted by GetBoosted84

Taking apart a stasis diff to see the washers they installed then comparing them to the ones that I got from the guy on Vortex to ensure they are the same (EXACT) size, material, and finish isn't a good way to check and see if the washers are the same? Same weight too :-). I'm thorough. So that's a bad sample? Really... how exactly would you have gone about it any different?

My comment wasn't in response to yours. I mis understood about the bronze washers. I thought the OP used bronze as a replacement washer.

**slowww328** · 07-10-2011 05:09 PM

Originally Posted by Scotty@Advanced

I'm not spreading mis information. Your wrong also as Torsen Issued a DIY modification guide on how to mod the 01A diff and specified a hard bronze washer.

No, I'm not.

Originally Posted by www.torsen.com

For the M021-DHU, the needle bearings between the side gears and housing/cap can be replaced with steel washers of the same dimensions. Another variation would be to replace only the bearing on the left side of the differential (as installed in the car) with a steel washer.

Torsen suggests steel washers for 4:1 modifications. The posters in the link you provided misinterpret Torsen's website. The bronze washers in the cited table are meant to be replacements for stock washers, they are not meant to be used for modifying the differential. You can see a copy of this table on Torsen's website here. Note the table's title (which those users neglected to copy over to their posts) - "Washer Replacement Parts".

**lucky** · 07-14-2011 12:00 PM

This is my interior Torsen:

Uploaded with ImageShack.us

I have no washer bronze,
but two Teflon coated steel

From top
I have a washer in more than steel from 3.16 mm

see photo

**slowww328** · 07-14-2011 04:34 PM

I take it this is an S4? What year?

Originally Posted by lucky

From top
I have a washer in more than steel from 3.16 mm

Meaning you have an extra steel washer on top of the stack that is 3.16mm? Maybe there is one and I just didn't remove it from the diff. Either way, so long as the teflon/bronze washers are both 1.96mm (or close to that) then you shouldn't have any problems replacing them. As long as there's less than 0.20mm of total wiggle room in the stack, you're within Torsen's specifications. Can you reply with their thicknesses?

**Scotty@Advanced** · 07-14-2011 04:52 PM

Originally Posted by slowww328

I take it this is an S4? What year?

Meaning you have an extra steel washer on top of the stack that is 3.16mm? Maybe there is one and I just didn't remove it from the diff. Either way, so long as the teflon/bronze washers are both 1.96mm (or close to that) then you shouldn't have any problems replacing them. As long as there's less than 0.20mm of total wiggle room in the stack, you're within Torsen's specifications. Can you reply with their thicknesses?

There are a few different variations of torsen, one with a single bronze shim in the middle, one with two teflon coated shims, one with a bronze and teflon shim and one with two teflon shims but the shims are much thinner than the previous versions. There maybe more versions but these 4 are the only ones I've handled.

**GetBoosted84** · 07-14-2011 06:36 PM

Originally Posted by Scotty@Advanced

Your experience is very limited get back to us after 125,000 miles of use on a bronze shimmed diff.

Hey Scott, I wanted to ask if you have any details of testing that Advanced has done on their diffs? I'd love to see the information you folks have taken over the course of 125k miles you referenced. Perhaps some photos and some information on what kind of testing you have done would be really helpful for the community.

**Scotty@Advanced** · 07-14-2011 07:08 PM

Originally Posted by GetBoosted84

Hey Scott, I wanted to ask if you have any details of testing that Advanced has done on their diffs? I'd love to see the information you folks have taken over the course of 125k miles you referenced. Perhaps some photos and some information on what kind of testing you have done would be really helpful for the community.

Being that the bias testing machine cost a hair over 300 grand, I just have decreased lap times and positive customer impressions.

The 125,000mile remark was an referring to an instance in which I overhauled a gearbox which had a modified diff with some worn bronze shims in it.

**slowww328** · 07-14-2011 07:34 PM

Originally Posted by Scotty@Advanced

There are a few different variations of torsen, one with a single bronze shim in the middle, one with two teflon coated shims, one with a bronze and teflon shim and one with two teflon shims but the shims are much thinner than the previous versions. There maybe more versions but these 4 are the only ones I've handled.

Yeah, I gather there can be either double teflon or one teflon one bronze, but that the washers are all the same size. This difference is reflected in the P/N on the T2 styled diff. The exception is the pre-2000 S4s, which CAN have a different stack altogether (type T1, although these cars can have T2s, just have to pull the diff. and check) For the T2 diffs, the same mod washers can be used.

Originally Posted by JHM

NOTE: For 6-speed (01E) transmissions: We can only perform the FULL modification on the T2 style differentials originally equipped in 2000 and newer B5 S4s, C5 A6s, C5 Allroads and B6 A4 (3.0L only, 02-early 03). These T2 diffs have one of the following PNs on the outer case; 01E 409 751 E or 01E 409 751 F. Some of the pre 2000 diffs may be T2 diffs and have these part numbers as well. For those the full modification could be performed. If your diff is older and does not have one of these part numbers or has NO part number at all then it is most likely a T1 style diff.

**lucky** · 07-14-2011 10:57 PM

Hi,
Yes,S4 1998 years

**lucky** · 07-14-2011 11:13 PM

Originally Posted by slowww328

I take it this is an S4? What year?

Meaning you have an extra steel washer on top of the stack that is 3.16mm? Maybe there is one and I just didn't remove it from the diff. Either way, so long as the teflon/bronze washers are both 1.96mm (or close to that) then you shouldn't have any problems replacing them. As long as there's less than 0.20mm of total wiggle room in the stack, you're within Torsen's specifications. Can you reply with their thicknesses?

Hi,
Yes,S4 1998 years

Thickness from the top:

3,16 steel
1,96 steel teflon
3,20 steel
gear
1,53 steel
1,96 steel teflon
1,54 steel
gear
4,73 steel
1,96 steel (darker and slotted)
4,73 steel
They are measures approx mm

**lucky** · 07-14-2011 11:20 PM

Originally Posted by slowww328

Yeah, I gather there can be either double teflon or one teflon one bronze, but that the washers are all the same size. This difference is reflected in the P/N on the T2 styled diff. The exception is the pre-2000 S4s, which have a different stack altogether (type T1.) For the T2 diffs, the same mod washers can be used.

I have a Torsen T2 number 01E 409 751E

My S4 is pre-facelift, but I live in Italy
Perhaps this is different for European car

**slowww328** · 07-15-2011 08:32 AM

Originally Posted by lucky

I have a Torsen T2 number 01E 409 751E

My S4 is pre-facelift, but I live in Italy
Perhaps this is different for European car

Well, it still has a Torsen T2 so these washers will work. The washers you need to replace are the same size as in the differential I modded.

**Scotty@Advanced** · 07-15-2011 10:59 AM

If your diff looks like this, the different versions share the same shim dimensions.

If your diff looks like this (found in later generation 01E's) the shims are a different thickness.

**slowww328** · 07-15-2011 01:46 PM

Gotcha. That's a B6 S4 diff. then?

**Scotty@Advanced** · 07-15-2011 01:51 PM

Originally Posted by slowww328

Gotcha. That's a B6 S4 diff. then?

Nope it's an very late model 01E diff (mfg dates after 2002), typically only found in euro boxes or the B6 01E equipped cars.

The B6 S4 cars used an 0A3 box. That stasis website uses and 01E diff in the picture.

**slowww328** · 07-15-2011 04:00 PM

Originally Posted by Scotty@Advanced

Nope it's an very late model 01E diff (mfg dates after 2002), typically only found in euro boxes or the B6 01E equipped cars.

The B6 S4 cars used an 0A3 box. That stasis website uses and 01E diff in the picture.

I see. Not B5 S4 euro boxes though, correct? After all, they stopped making the B5 S4 worldwide in September 2001.

**Scotty@Advanced** · 07-15-2011 04:54 PM

Originally Posted by slowww328

I see. Not B5 S4 euro boxes though, correct? After all, they stopped making the B5 S4 worldwide in September 2001.

No I've seen them in B5 S4 gearboxes as well but I think most if not all of them were factory remans gearboxes. The 01E was used up until the 2005 model year in the Allroad but phased out in the rest of the model line in 2004.

**slowww328** · 07-15-2011 05:08 PM

Originally Posted by Scotty@Advanced

No I've seen them in B5 S4 gearboxes as well but I think most if not all of them were factory remans gearboxes. The 01E was used up until the 2005 model year in the Allroad but phased out in the rest of the model line in 2004.

Oh right, cool.