E82 128i, E9X 325i/328i 3.73 Differential Upgrade

This is a rebuilt 3.73 open differential used as an upgrade for 2007-2013 E82 128i and E90/E91/E92/E93 325i/328i. The 3.73 gear ratio is used from the automatic-transmission models but instead of buying a used high-mileage diff with its own issues, we present this professionally rebuilt diff using the genuine 3.73 gear set, a non-limited-slip carrier, and new OEM bearings and seals.

The 3.73 gears provide much more acceleration for a manual transmission E82 or E9X. The stock ger ratio is 3.23 which provides excellent fuel economy on the highway but lacks acceleration. The 3.73 is a popular choice for enthusiasts because it came from the E82/E9X automatic transmission cars and it can be found in wrecking yards or salvage cars. Moving from a 3.23 to a 3.73 is a drastic change in acceleration and is like adding 13.5% more torque to the engine.

However, buying a used diff is a roll of the dice. BMW did not offer regular fluid maintenance service on differentials. They considered them "lifetime fluid" without specifying which lifetime. A high-mileage used diff may have had the original fluid in it for its entire life. Our differentials are fully rebuilt using new OEM bearings and seals. The process is the same as one of our performance limited-slip upgrades:

• The used diff is disassembled and cleaned
• The outside case (carrier) is stripped of grease, grime, and rust
• The 3.73 gear set is assembled
• 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

A $500 core charge is applied to your order to ensure we have an original unit to rebuild for the next customer. Without a core charge the rebuilt diff cost would be much, much higher. Consider that a Genuine BMW 3.73 automatic diff lists for $2,500 plus a core charge.

This E82/E9X 3.73 differential is offered as an open carrier without a limited-slip unit. We also offer limited-slip upgrades for the E82/E9X which you can find using the Related Products menu below. The open diff is the same as what BMW used on the E82/E9X automatic models (only M cars has limited-slip). A limited-slip is strongly recommended for any track, autocross, or serious performance driving. Our limited-slip options start at just $600 more. The open 3.73 diff still delivers the improved acceleration even without the additional traction.

A shorter gear ratio (higher number) will put the engine higher into the rev band for any given speed. For example, using the stock 3.23 gear at 70mph will have the engine at 2,570 RPM. Moving to a 3.73 at the same speed will put the engine at 3,000 RPM. Top speed will also be affected - losing 21 MPH in 5th gear at 7,000 RPM (161 vs 140 MPH). But those are the trade-offs for much more acceleration at lower speeds. If a 3.73 is too extreme for you, we also offer 3.38 and 3.64 gear sets that are a better acceleration/drivability trade-off (see below).

A core charge applies to this purchase. The supply of original parts for rebuilding keeps the final purchase price low. The core must match the part that was supplied in terms of fitment, type, application, accessories, and hardware. Damaged or modified cores will not receive a refund. A core charge can only be refunded once the core has been inspected and approved. Click here for our Core Policy.