A great upgrade for drivers looking for added acceleration and grip for the E30 325i and E30 M3!
A differential upgrade/rebuild for your E30 delivers performance far above the original factory spec. Diffsonline has 20 years of expertise in BMW differential upgrades and rebuilding. Conventional limited slip differentials like the E30's contain clutches that wear out over time (just like the clutch in your transmission). Repeated hard launches, burnouts, drifting, and track/autocross will shorten the life even further. What good is a worn-out limited slip diff in a sporty car? Diffsonline specializes in BMW differentials and makes it easy to replace and upgrade your diff.
When planning a diff upgrade in your E30 there are a few factors to consider: limited slip type, gearing choice, upgrade plans, and intended use.
E30 Limited Slip Options
There are a number of limited slip options for the E30 325i but they fall into two groups - clutch and helical. There was also a viscous LSD used in the 325iX but we're not covering that here. The traditional BMW limited slip uses a clutch layout. Clutches are simple, reliable, and the less expensive. The stock E30 LSD uses a traditional 2-clutch layout which is fine for daily driving and moderate street use. We also offer a heavy-duty 3-clutch and a 3-clutch with more advanced ramp angles to change lock-up behavior on acceleration and deceleration. The OS Giken is a multiplate design that is recommended only for track use and no warranty is provided.
Several non-clutch type LSDs are also available. Wavetrac and Quaife both make a helical-style LSD that are more expensive but offer advantages in wear resistance and performance. Helical diffs use multiple sets of gears instead of clutch discs and plates. They are always engaged and have quicker and smoother response and are less affected by heat than a conventional clutch. There are a few downsides: if a wheel becomes airborne (hopping a curb, lifting an inside wheel) the lockup goes away. There is also additional upfront cost and some slightly higher gear whine than a traditional clutch LSD.
E30 Gear Ratio
Multiple gear options are available to meet your acceleration needs. Shorter gears have a higher number and will deliver faster acceleration at the expense of top speed. The stock E30 325i manual ratio is 3.73 which is pretty short but keeps the stock M20 in the powerband. However, if popular upgrades like an M50-S52 swap are planned you will most likely select a taller gear ratio (numerically lower), such as 3.25 or 3.38. This will give you a wider powerband and not be too short for highway use (for reference a stock E36 M3 had a 3.23 rear end). Refer to the charts below for comparisons of MPH in each gear as well as new RPM levels at cruising speeds. And if you have an LS swap we offer super low 2.79 and 2.93 gears.
Charts for E30 with M20, stock trans, stock tires:
Charts for E30 with M50/S52, ZF trans, common 17" tires:
SpecE30 Diff
The SpecE30 rulebook calls for a stock-type 2-clutch limited-slip with original-spec clutches and ramp angles. One wheel must spin (break-away) when 65ft-lbs of torque is applied. The only gear ratio allowed is the 3.73 set. Don't go out with the wrong diff or a blown out original diff! Worn internals mean you cannot put all of the power down early enough or force you to correct throttle until traction returns. To keep up with the pack and maintain momentum you will need all of the traction coming out of turns and that means a proper-functioning limited slip.
REM Polishing
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.
Core Charge
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 separate core charge, in addition to the advertised price, 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
limited-slip 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 receive only a partial refund.
The core charge on a E30 diff is $900, which is not included in the price and will be charged separately at the time of processing. 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.
E30 non-limited slip (open) differentials returned for core will receive a partial refund up to $200*. E30 limited-slip cores will have the full core charge refunded*.
* - subject to core inspection and evaluation.
E30 Diff Rebuild Process
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.
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.
A great upgrade for drivers looking for added acceleration and grip for their E30 - especially S52 and LS swaps!
BMW Fitments:
3 Series
E30 (84-91) > 325i
E30 (84-91) > 325is
E30 (84-91) > M3
