VAC Motorsports Performance Built Rear Differentials, BMW 135/335

Q: I'm confused: I thought all I needed was a differential? or LSD... What I am actually buying? What do I need?

There are three parts of this upgrade that customers should understand:

1. The selection of the carrier; It is the actual mechanical device that replaces the factory spider gears with a unit that puts power to both wheels at once, rather than just the wheel with the most traction. This is often referred to as the LSD (Limited Slip Differential), but sometimes the entire housing once built is also referred to as the "LSD", so this can be confusing. This listing is for a complete housing, ring and pinion, and LSD carrier.
2. The housing: The actual differential itself with the ring and pinion gear, with the carrier unit installed inside of the housing. That's the part you see under the car with the gears and carrier inside.
3. The Ring gear and Pinion gear: These parts take the power coming from the drive shaft and split it horizontally so it can go to both axles.

Q: Why does your 135i or 335i benefit so greatly from a performance rear differential?:

Short Answer: Improved acceleration, traction, and stability potential; that's why all M-Series are so equipped, AND why BMW left them out of all the 135/335/535i, so they would not steal sales away from the M-cars!
Long answer in simple terms:
Standard open-type differentials, equipped in most non-M Series, only send power to one wheel at a time in all conditions. The more powerful a car becomes, the more traction is needed to effectively make use of this power. "Traction control" is inefficient for performance driving, and becomes more and more cumbersome as power levels rise with tuning.

A 'Limited Slip Differential' or 'Torque Biasing Differential' is able to effectively split power between both wheels at crucial moments while keeping the car stable and easy to drive (Note: This is by different means in different brands. LSD Vs TBD... OEM, Kaaz, Quaife, Carbonetic, Wavetrac ect... see below).
When you launch your car from a stop, having the power continuously sent to both wheels increases traction exponentially! You leave the line with more power getting to the ground, and less power spinning your wheels; this makes a car dramatically faster!

When your vehicle is turning, the outside wheel is rotating faster than the inside wheel; the inside tire is doing less work. The vehicle wants to 'lean' on the outside tire more, further increasing its load. While the outside wheel is forced to do more work, the inside has less traction. Therefore, in an open differential, the power wants to go to the wheel with the least amount of resistance and it becomes wasted, spinning the inside wheel when cornering hard.

Through various means, a LSD or TBD effectively splits power to both wheels and can aid in powering the car forward when coming out of turns as both wheels are utilized most effectively. When entering a corner, this balance of traction helps to increase the stability of the car when coming in at high speed, improving confidence and safety. This leads to faster corner entrance potential.

All of this extra traction can also have improvements in low grip conditions like snow or rain.

Q: What is in my car?

Application Information

Model/ Transmission	Build Date	Ring Gear	Housing Type	Gear Ratio
335i [Manual]	Aug, 06 - Feb, 07	Bolted	Large	3.08
	Mar, 07	(Transition period) may be Bolted or Welded	Large	3.08
	Apr, 07- On	Welded	Large	3.08
335i [Automatic]	Aug, 06 - Aug, 07	Bolted	Large	3.46
	Sep, 07- On	Bolted	Small (NO OS GIKEN)	3.46
335is [DCT]	All Production	Welded	Large	2.56
335d  [Auto]	All Production	Welded	Large	2.81
135i / Manual	All Production	Welded	Large	3.08
135i / Auto	All Production	Bolted	Small (NO OS GIKEN)	3.46
135i / DCT	All Production	Welded	Large	2.56

Q: What does a torque biasing differential like Quaife or Wavetrac do?

We have run a Quaife HARD in our daily driven and often tracked cars with absolute success. Additionally, we have installed dozens of Quaife units in platforms other than E8x/E9x and they always perform flawlessly. We have also installed and sold many Wavetrac units, and can remark that they also perform well for slightly less cost.

An ATB's (automatic torque biasing differential) job is to improve acceleration, traction, and stability potential of the chassis. Rather than using clutch plates as a method of operation, the Quaife ATB uses sets of helical cut gear-pinions that run in pockets, and mesh during normal driving. Should one of the driven wheels begin to spin, these diffs seamlessly and immediately start to generate a torque-bias thanks to the axial and radial thrust of the helical gear pinions in their pockets. Wavetrac is similar but uses its "Wave" that they assert allows for additional traction in '100% bias' (or zero traction) situations such as when a wheel is lifted from the ground in a corner.

The result is a progressive, smooth transfer of torque away from the spinning side of the axle to the driven wheel, which is now capable of transmitting a greater proportion of torque. Unlike a clutch style LSD, the gear type units deliver performance without the need for strip-downs or rebuilds. There are no plates to wear or break and these units retain standard vehicle gear oil without special requirements (Wavtrac recommends gear oil without limited slip additive).

All of this extra traction can also have improvements in low grip conditions like snow or rain. Quaife units are silent in operation, a Wavetrac unit may also be, however some have a slight noise when entering first gear. They do not need any electronics to be re-worked or traction control to be affected in any way.

Very cool and great for both street and track use!

Q: What does a Clutch-type Limited Slip differential like an OS Giken do?

Through clutch plates with internal mechanisms, a clutch type LSD applies load to both rear wheels and aids in powering the car forward when the wheels are turning at different rates, or when acceleration off the line. As with powerful cars, taking advantage off all the traction available from a standing start and in the corners is important, however - clutch type LSDs for BMWs are setup with some locking affect on deceleration as well: when entering a corner, this load and balance of traction helps to increase the stability of the car when coming in at high speed improving confidence and safety. This leads to faster cornering potential. This is what Japanese companies refer to as a 1.5 way LSD, and is excellent for everything from daily drivers to all out competition vehicles. A 2-way LSD has a strong locking effect on deceleration- this is typically utilized for those who like drifting as it can enhance stability off throttle over-steer. These units are supplied by special order.