Select Vehicle From Dropdown

JHM 4:1 Center Diff Upgrade (Manual Trans Only)


Select options & add to cart to view final price
From: $350.00

To: $850.00

* Required Fields

For Manual Transmissions Only (Please use the drop down boxes above to select your transmission and correct matching core if you are not sending yours in first, use the picture below to help determine which center diff your car is equipped with.

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
Audi JHM 4:1 Center Diff Upgrade

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.

--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

Core Exchange
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.

For shipping quotes: Please sign in or join then put the items you want in your shopping cart and proceed to checkout.

This product fits the following vehicles:
Make Model Chassis Engine
Audi S4 B5 (1997-2002) 2.7t V6 30v
Audi S4 B6 (2003-2005) 4.2l V8 40v
Audi A4 B6 (2002-2005) all
Audi A4 B5 (1996-2001) all
Audi A6 C5 (1998-2004) all
Audi A4 B7 (2005.5-2009) all
Audi RS4 B5 (1999-2001) 2.7t V6 30v
Audi S4 B7 (2005.5-2009) 4.2l V8 40v
Audi allroad C5 (2001-2005) all
Customer Reviews (2)
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.
Review by Zsolt (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!! Review by brian (Posted on 3/29/2010)
Write Your Own Review
You're reviewing: JHM 4:1 Center Diff Upgrade (Manual Trans Only)
Off-road Use

By Purchasing any Off-Road use ONLY products from JH Motorsports, Inc. I acknowledge and understand the following disclosure. (This notice is located at the bottom of the page description of affected products on our website.)

Vehicle Code § 27156:
(b) No person shall operate or leave standing upon a highway a motor vehicle that is required to be equipped with a motor vehicle pollution control device under Part 5 (commencing with Section 43000) of Division 26 of the Health and Safety Code or any other certified motor vehicle pollution control device required by any other state law or any rule or regulation adopted pursuant to that law, or required to be equipped with a motor vehicle pollution control device pursuant to the National Emission Standards Act (42 U.S.C. Secs. 7521 to 7550, inclusive) and the standards and regulations adopted pursuant to that federal act, unless the motor vehicle is equipped with the required motor vehicle pollution control device that is correctly installed and in operating condition. No person shall disconnect, modify, or alter any such required device.
(c) No person shall install, sell, offer for sale, or advertise any device, apparatus, or mechanism intended for use with, or as a part of, a required motor vehicle pollution control device or system that alters or modifies the original design or performance of the motor vehicle pollution control device or system.

Health and Safety Code § 43000:
43001. The provisions of this part shall not apply to:
(a) Racing vehicles.

As the aforementioned code sections outline, it is against the law to operate or use a vehicle on public streets or highways with modified, altered, or disconnected motor vehicle pollution control devices. This may include but is not limited to, Evaporative Fuel Controls, Catalysts, DPF, PTOX, PCV Systems, Exhaust Gas Recirculation Systems, Oxygen Sensors, Thermostatic Air Cleaners, Air Intake Systems, Fuel Injectors, Regulators, Adjustable Cam Gears, ECU Programming, Mapping, or other Computer Modifications. Air Pumps, Air Injection, Timing, and any other Emission System Modification(s).

Purchaser acknowledges and certifies to JH Motorsports Inc., that Purchasers vehicle will only be used for racing applications or off road vehicle use only. Purchaser requests these modifications for the purpose of enhancing the vehicle for competitive Racing Applications and will not use this vehicle on public streets or highways. Purchaser agrees to indemnify JH Motorsports Inc., all employees, officers, owners, and subsidiaries for any civil or criminal liability arising out of use of Purchasers vehicle on public streets or highways as Purchaser has been duly notified of the laws and regulations regarding disconnecting, alteration, or modification of a vehicle’s pollution control devices. Purchaser understands and has been advised the driving of a vehicle on public streets or highways could result in criminal or civil penalties.

Prop 65

Warning: This product may contain a chemical known to the State of California to cause cancer, birth defects, or other reproductive harm.

Learn More:

Q and A

Product Questions

No questions yet. Be the first to ask the question!
You may also be interested in the following product(s)