MotorTrend

Project BMW 335i Sleeper - WaveTrac Limited-Slip Differential Install

Finding Traction: LSD upgrade to unlock tons of performance
Writer

So you bought a 335i, because let's face it, they are a performance and luxury bargain, and you want to modify it, make it faster and improve its handling. But BMW never offered a factory limited-slip differential (LSD) option for the 335i and 135i, and the cars come with an awful open differential. All that “Ultimate Driving Machine” stuff goes right out the window when you try to leave a stoplight with force and end up instead leaving a long, single, shameful skid mark.

We feel your pain. Our 2007 BMW 335i, with its six-speed manual transmission and 75,000 peachy miles, was in dire need of a traction overhaul. How could we ever give E9x M3's a run for their money with an open diff? We got in touch with Wavetrac, Autotech Driveline's differential company, and got ourselves a bookshelf-worthy piece of equipment, the Wavetrac Torque Biasing Limited Slip Differential. And since we didn't want to be responsible for making a terrible mistake, we enlisted the expertise of our good friends over at European Auto Source, in Anaheim, Calif., to fit this bad boy in our 335i's rear-end.

First, let's clear things up a bit by defining some key terms – the big ones are Differential, Open-type diff, Locking-type diff, and Limited Slip diff.

Differential (Diff)

Essentially, the differential is a set of gears powered by the engine and transmission (and in RWD and AWD vehicles, a driveshaft) that work together to turn the axles going out to the drive wheels of a vehicle. It allows the wheels to spin at different rates of speed, which is necessary if you want your car to turn without tire slip. For example, if you were to drive in a circle, the inside wheel needs to travel slower than the outside wheel to allow smooth movement. There are three basic types of differentials: Open, Locked, and Limited Slip.

Open Diff

This is the most common differential. Open differentials always supply the wheels with the same amount of torque. The maximum amount of torque supplied is limited to the greatest amount that will not make the wheels slip. In a situation where, let's say, one tire is on ice and the other is on dry concrete, you're in bad shape; due to the tiny amount of torque the ice-bound tire can handle without spinning, the tire on the road is receiving that same, which is much less than necessary to get you moving.

Locking Diff

If you follow drifting, you may have heard of a locking differential before. There's a reason drifters on a budget will often remove their factory open differential, open it up, and weld the gears together. With the gears welded, it is locked. A locking diff will send 100 percent of the torque to the wheels, regardless of the traction situation. If all you need to do is go straight, or drift, this is a great option. In our icy situation above, the tire on the road would be getting all of the torque, since the tire on the ice couldn't put any of it down, and you'd be back on the road driving in no time. However, a locking diff is awful for turning, again where the wheels need to be spinning at different rates. If the inside and outside wheels were directly connected and forced to spin at the same speed, tension could build up, the wheels could skip, or worse, a driveshaft could snap.

Limited Slip Diff

In certain situations, you want locking differential behavior, such as accelerating in a straight line, being in a poor traction situation, or exiting a corner and getting on the throttle. In other situations, like simple turns, navigating parking lots, and so on, you want the differential to behave like an open diff, so the tires can spin at different speeds. You can see why, in a car like the 335i, and especially after we've modified the motor to create upwards of 400 lb.-ft. of torque, an LSD is a necessity if you desire a fun car that can put the power down.

With all the options out there for LSDs, you might be wondering why we chose Wavetrac. The Wavetrac LSD is different from every other gear differential on the market because it solves a very real problem: loss of drive during zero or near-zero axle-load conditions.

Zero axle-load is a condition that occurs during normal driving, but it rears its ugly head most when driving in extreme conditions. Zero or near-zero axle-load is when no load is applied through the drivetrain, generally where a drive wheel is nearly or completely lifted, something that can happen in aggressive cornering. Zero or near-zero loads also occur during the transition from acceleration to deceleration and back, even with the drive wheels on the ground. And for you guys who've got low, rigid vehicles and stiff sway bars, you may be familiar with lifting a back wheel when entering a driveway – that lifted wheel is experiencing zero load, too.

Nearly every diff on the market except the Wavetrac will not power the gripping wheel, and during the transition from accel to decel all other diffs, except Wavetrac, do nothing. Why is this? Other torque biasing diffs divide the torque between the two axles up to the available grip of each tire. The amount of drive torque one wheel can get over the other is called the bias ratio, a measure of torque split across the axle. Open differentials have a bias ratio of 1:1.

Torque biasing diffs, on the other hand, offer increased bias ratios. For example, if a diff has a bias ratio of 2.5:1, it can apply drive torque to the wheel with the most traction (like our icy-dry surface situation from above) at 2.5 times the traction limit of the wheel with the least traction. This is a huge improvement over an open differential, but what if you've got a zero-load instance? 2.5 times zero equals what? Zero. That means during the zero-load period the gripping wheel gets zero drive.

How did Wavetrac solve this problem? There is an innovative, patented device at the center of the Wavetrac diff that responds during these specific conditions. In zero or near-zero axle-load situations, each side gear in the differential starts to turn at a different rate. This speed difference causes the Wavetrac device to step into action. Wave profiles are precisely engineered and placed on one side gear and it's mating preload hub. As the two side gears rotate relative to each other, each wave surface climbs the other, causing them to move apart. This creates enough internal load with the Wavetrac to stop the zero axle-load condition. The drive torque is then applied to the wheel on the ground – the one that has grip and traction – and the differential will keep the power down.

The Wavetrac unit uses 9310 steel gears housed in case-hardened billet or forged steel bodies and ARP fasteners are used throughout. Wavetrac claims you NEVER have to maintain its diff. It will perform a lifetime of service without maintenance or rebuilds. If the standard setup isn't quite right for you, you can alter the bias ratio and the diffs behavior to suit your needs with optional components. And to top it off, it's got a limited lifetime warranty.

The guys over at EAS know BMWs better than most, and if you're reading this you've likely seen the fruits of their labor, from shop cars like their E92 M3, Z4 M, and now M4 Coupe, to our own projects, like our crazy Turbocharged E46 M3. They had our factory pumpkin (differential case) out of the car in just about one hour. We threw it in the EAS work truck and brought it straight to the Autotech headquarters in Aliso Viejo, where the expert technician started disassembly to install the Wavetrac LSD.

Very early six-speed manual transmission equipped 335is had a traditional “bolted” setup, where the ring gear was fastened to the gear carrier on the differential. Then the engineers in Munich decided to weld, not bolt, the ring gear to the gear carrier. This complicated things a bit. We did our research and the car's build date assured us that our 335i had a standard bolted diff; to our painful surprise, it was in fact welded. In order to install the new Wavetrac LSD, the ring gear had to be precisely machined to separate it from the ring carrier in order to bolt on the new assembly. This comes at an added cost, so do your research and be prepared, as the machining process has a two to three week lead-time.

With our newly machined differential and the Wavetrac happily nestled in the pumpkin, EAS installed the rear end back into our 335i.

All we can say is WOW. The car finally felt like it should, from second gear power-on oversteer to first gear donuts, and hard throttle with plenty of traction leaving corners on the track or in the canyons. Our 335i was finally ready for the next level. We know it's our job to put into words how cars feel, but this is truly a difference you can only feel if you are driving. If you commute to and from work and only accelerate hard when you're getting on the freeway, perhaps this isn't a crucial modification for you. But if you're like us and want confidence-inspiring traction and behavior of a true, rear-drive sports car, then this should be at the very top of your list for your 335i or 135i.

We've got lots of plans for our E90, so stay tuned for the next phase of the build!

LSD - $1295

Machining - $700

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