A great upgrade for drivers looking for added acceleration and grip for their E28 M5!
Don't let a worn-out differential ruin the M5 driving experience! The original M5 came from the factory with a limited-slip differential that will wear out, especially with aggressive driving, and leave you with "one-wheel drive". New or rebuilt diffs are no longer available from BMW. Restoring the limited-slip diff and possibly choosing a different gear ratio can dramatically change the driving characteristics of the E28 M5. Diffsonline has 20 years of expertise in BMW differential upgrades and rebuilding and we make it easy to replace and upgrade your diff.
When planning a diff upgrade in your M5 there are two factors to consider: limited slip type and gearing choice.
E28 Limited Slip Options
There are a number of limited-slip carrier upgrades for the E28 M5 but they fall into two groups - clutch and helical. For purely street use we would recommend a helical such as Wavetrac or the in-house Diffsonline carrier. Helical diffs offer advantages in wear resistance and performance over traditional clutches. Torque transfer happens instantly and it doesn't need to sense a spinning wheel first. Lock-up is quicker and smoother and it's less affected by heat than a clutch. The only downside is potentially higher gear whine (but not in all cases).
The E28 M5 was equipped from the factory with a 2-disc clutch-type limited-slip unit with 25% lock built in. We're fans of the clutch-type differentials because they are simple, quiet, reliable, original to the car, and easy on the wallet. They are great for street cars that see a few track days a year. Our other clutch options are designed around serious track or race use since they can be easily customized around car setup and driving style. These diffs feature multiple clutch and spring discs - more clutches equal greater capacity for torque and heat management. Custom ramp angles are machined into the carrier to dictate lockup behavior under acceleration and deceleration (coast). Lower values equal a more aggressive lock-up.
We can even update the diff to the later M Variable viscous design that has 0-100% lock, although a clutch-type is our favorite. The BMW M Variable viscous diff uses plates and heavy fluid to transfer torque between axles. When new and fresh the system works quite well for street use. However, it is out-performed by traditional clutch differentials on the track and loses performance and refinement within 60,000 miles.
The OS Giken is a multi-clutch design that is recommended only for track use and no warranty is provided.
Multiple gear options are available to meet your acceleration needs. All E28 M5 came with a 3.91 stock gear ratio. This is actually quite short and keeps the engine in the meaty part of the power band. Shorter gears are also available but we would probably advise against them unless absolute neck-snapping acceleration is what you want for the street. Multiple taller gear options are available if you want to lower revs on the highway. To maintain original performance (such as for restoration or resale purposes), keep the stock 3.91 ratio. The gear ratio you choose is based largely on personal preference and how you plan to use the car.
Refer to the charts below for speed and RPM data. Note that we used original-spec tire sizes (225/50-16) so a different tire size will have other values. For example, a 235/45-17 has a larger rolling diameter and will result in lower RPM values than stock.
REM treatment is a multi-step light surface polish of the gears and/or bearings that removes small imperfections and high spots on the surface. BMW does some final detailing on their ring & pinion sets that removes most of these imperfections but there is still room for improvement. High spots create heat risers which raise the surface temp of the surrounding metal and leads to imperfections and wear. REM polishing reduces internal temps and friction, improving performance and efficiency. It also makes the ring & pinion quieter with smoother engagement. REM polishing is done on a as-ordered basis and may result in additional build time. REM is a registered trademark of REM Chemicals, Inc.
We're able to offer rebuilt and performance differentials at a reasonable cost due to the use of cores. A "core" is your original used differential that is returned to our diff builder to be recycled into a new, fully-rebuilt diff. If we did not have cores available, and had to buy new factory differentials from BMW, our performance diffs would be four times the price they are now. To keep this upgrade feasible for the entire enthusiast community we rely on having your good, reusable core returned to the diff builder (Diffsonline) within fourteen days of receipt of your new diff. In order to cover the cost of sourcing another used diff, a core charge is applied to your order at the time of processing. This core charge can be refunded when you send back your original limited-slip
diff. Only complete and reusable cores will be eligible for a refund (no blown out, abused, or modified cores please). You must return a diff equal in components and accessories to the one sent to you. Non-limited slip (open) differentials returned for cores will not receive a full refund.
The core charge on a E28 M5 diff is $2,500
, which is not included in the price and will be charged separately
at the time of processing. Click here
for the core policy and instructions. You can also avoid the core charge altogether by sending in your original diff to be rebuilt. Note that this will mean downtime for your vehicle, so we recommended using the core service.
While we take in used diffs as cores, what you get is a fully rebuilt and properly set up unit that is often better than what BMW can provide. There is a lot of specialty knowledge required in the diff building industry and Diffsonline has over 20 years of experience in BMW differentials. Nearly all of the major retailers of performance differentials use Diffsonline, even when sold under other brand names. Rebuilding a diff is a very involved process -
- The used diff is disassembled and cleaned
- The outside case (carrier) is stripped of grease, grime, and rust
- The desired limited slip unit is prepped
- The specified gear set is assembled with the LSD
- Any machining of the carrier is done in-house
- New OEM bearings are fitted
- New OEM seals are fitted
- The entire internal assembly is aligned (crucial in avoiding gear whine later on)
- Final assembly and checks
- Carrier is given a fresh coat of black paint
- Shipment to your door in special heavy-duty packaging
The packaging can be reused to send back your core. Diffs are shipped without covers and with no fluid inside. Diffsonline recommends Red Line 75/110 fluid for all of their differentials, except M Variable (Castrol SAF-XJ) and OS Giken (OS 80W250).
BimmerWorld offers FREE GROUND SHIPPING on all diffs.
Upgrading the differential in your BMW is one of the best ways to improve handling, acceleration, and all-around performance. Whether you're looking to alter the final drive gear ratio, or add limited-slip functionality to get all that power to the ground in the turns, we've got the knowledge to guide you in the right direction. Should you have any questions on BMW diff upgrades or the process, please call or email one of our knowledgeable sales reps.
BMW Fitments:5 Series
E28 (1988) > M5