JHM 4:1 Center Diff Upgrade (Manual Trans Only)
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The factory torsen center differential in the S4 and A4 provides a 2:1 torque distribution between front and rear. What does this mean? This enables the end of the car that still has traction (most of the time the rear) to deliver double the torque to the tires with traction. Our experience has proven that this closer to even distribution is best for drag racing. So this modification is not for you if you are into drag racing primarily. However for Auto Crossers, Road racers and others who prefer the twisties JHM recommends a bias ratio of 4:1. This is the same bias ratio that Audi has provided in the stock RS4 driveline.
So if you are tired of going through corners while using a lot of throttle and still getting "understeer" then this product is for you and will help you get a little more "oversteer" and allow you to drive the car a little more like a RWD car but still have the superior control of AWD.
For Manual Transmissions Only (Please use the drop down boxes below to select your transmission, use the picture below to help determine which center diff your car is equipped with. 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. If you have a T1 we can only modify half of the diff due to its older design so you will not get a full 4:1 diff, it will be somewhere in between a 2:1 and 4:1. We won't know 100% for sure until we dissasemble the diff if your diff does not have one of the part numbers listed above. Some of the older diffs have been replaced over the years from other cars or due to wear so there is no other way to tell until we receive your diff and dissasemble it. NOTE: For 5-speed (01A) transmissions: We DO NOT offer a core exchange due to a high failure rate on these diffs. We will only modify yours that you send in. Also there are 2 different diffs in the 5-speed 01A transmission, we only do the late 99.5 and up. Use this PICTURE to confirm you have the right diff to send in. CLICK HERE. These diffs are interchangeable, so if you have the older diff you can buy a newer diff from a wrecking yard to have modifed by us, then you can replace your older 2 piece unit with the modified late JHM unit.
Click to enlarge picture
To help make the proper selection below use this chart.
01E - 6-speed EARLY OEM PN 01E409751E = B5 S4 (all), C5 A6 (all), C5 Allroad (all) - See note above regarding older 01E diffs (pre 2000) that may not be fully modifiable.
01E - 6-speed LATE OEM PN 01E409751F = B5 S4 (all), C5 A6 (all), C5 Allroad (all), B6 A4 6-speed (02-03 3.0l V6 only all other 6-speed A4s have the 02X) - See note above regarding older 01E diffs (pre 2000) that may not be fully modifiable.
0A3 - 6-speed OEM PN 0A3409751B = B6 S4 (all) and some 05.5 B7 S4s (note: we have seen some 05.5s with the T3 diff which is already a 4:1 and does look different than the one in the picture above. If you have that diff there is no need to send it in for modification)
02X - 6-speed OEM PN 02X409751A = B6 A4 (04 and up) and B7 A4 (all).
01A - 5-speed = B5 A4 (99.5 and up) and B6 A4 02-04 with 5speed transmissions . NOTE: We do not modify the older A4 diffs, see picture below for more info. Also we only modify your diff and do not supply cores on these diffs due to the higher failure rate on these units. INSTRUCTIONS:
--Here is a write up on how to perform the install on a B6/B7 S4. It will help with other models since the general idea is the same. Check it out --> HERE
--Here is a new write up on how to perform the install on a B6/B7 A4 with the 6-speed 02x transmssion. It will help with other models since the general idea is the same. Check it out --> HERE
There is a core charge if we send out a JHM center differential to you (see drop box below for amount). When you exchange your center differential for a JHM unit, send your differential back to JHM and we will refund the core charge. As of 8-30-10 we currently have B5 S4, B6 S4, 05.5 B7 S4 and B6/B7 A4 6-speed cores in stock. We do not have cores in stock for the 5-speed 01A and do not plan on it due to a high failure rate, please send yours in ahead of time for us to modify. NOTE: If your core is damaged or worn out your core fee will not be returned or only partially returned. Also if we shipped you a modified 01E T2 style diff (the only ones we ship out in advance if you pay a core charge) and you send back an OLD style 01E T1 diff for your core (see note above) you will not receive any core credit. Since this is just a modification you need to send us a good center diff. However if you have a damaged center diff contact us and we can discuss your options.
You may also send your differential to us for modification and not incur a core charge. We can usually modify and ship out your differential within the same day. If you will be sending yours in, unselect the core option and specify in the comments that you will be shipping it in. Also put the invoice number on your box when shipping it in for modification. NOTE: It is common with this mod to notice a slight chatter when making u-turns at low speeds. This is due to the modification to make it bias differently under power and at high speeds. Normal daily driving it is not noticeable.
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Do I get this right? I get in a tight corner, accelerate out and no understeer??
In fact, a slight, controlled oversteer? My car has the Neuspeed tubular rear sway, which helped reducing understeer, but now: wherever I point the car it goes. The rear tires may break loose like in an RWD car and I have to correct by steering in the direction of the slide, but more often just a slight 4 wheel slide is what I get and the car quickly regains composure.
I thought others’ claims were exaggerated, but indeed, this is how this car should come from the factory.
The chatter during low speed turns just got a slightly more pronounced than it was before the upgrade. I gladly take this tradeoff. FWIW, the low speed chatter in turns happens ONLY if you give it gas - if you just coast through a turn it does not chatter. (Posted on 1/2/2011)
- From the first hard turn with this Mod,i was hooked at how this diff "shot" me through each turn seeming as if my 04 B6 S4 was indeed more like an RWD car! Turn around time was superb. Custom support was fantastic as usual. This is yet another bit which audi should have given us B6 S4 owners out of the box.Well worth the price of admission for this german car enthusiast... Thank you Jay and JH Motorsports for your dedication to excellence in crafting parts which make my daily driver even more fun to drive day in and day out. Driving is even more of a past time with mods from these guys!! (Posted on 3/29/2010)
Warning: This product may contain a chemical known to the State of California to cause cancer, birth defects, or other reproductive harm.
Learn More: www.p65warnings.ca.gov
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