REVIEW: Quaife Limited Slip Differential on 335i E90 LCI / Alpina_B3_Lux
Quaife limited slip differential
Once you increase the engine power and, in particular, the torque output of your car, you are faced with another problem: traction! I experienced that, after having flashed my car with the Evotech flash (400hp / 560 Nm at the crank), it got increasingly difficult to transfer the power on the road, the tires spun too easily and much of the desired forward momentum went up in black smoke, decreasing the life span of the tires. This was even more true in corners or in the wet - unless the road is dry, there's wheelspin sometimes even in fourth gear, and even when dry, powering out of bends is near impossible in second or third gear.
Why is that, you may say, doesn't the 335i come with a differential? Well, BMW has labeled a function of its DSC "electronic differential", but that is quite a misnomer: All it does is brake the rear tire that has lost traction, thus decreasing the life span of your rear brake pads but not helping with forward momentum. The 335i unfortunately is not equipped with a limited slip differential as all M models are, for marketing reasons in order to differentiate it from the M3. As a consequence, it has an open differential which means that the power of the engine is always directed to the wheel with the least resistance. In corners, that means obviously the inner wheel as the weight of the car is transferred to the outside - and if you apply full throttle now, the inner wheel starts to lose traction and spin, the DSC cutting in to prevent this, and you get sort of a hickup-driving-style. That's not fun! :mad0259: You can't determine the curve radius with your throttle, you can't drift, and most of all you can't properly apply the power of the mighty N54 to the road. I experienced that on numerous occasions during my trip to the famous Nürburgring in August 2009 and by driving through mountain passes in the Italian Alps, and this led me to the conclusion that a limited slip differential (LSD) was definitely needed here.
What does an LSD do?
An LSD (at least the so-called torsen or torque sensing variety) senses that a wheel loses traction and transfers the torque to the wheel with more traction and/or locks both wheels together - as opposed to the OEM open differential where the torque always goes the way of least resistance, making the tire with the least grip spin. In the example above, if you corner with an LSD, it will automatically transfer the engine power to the outer wheel which is under load and therefore has more traction, enabling the car to apply more power to the road. Such differentials are also called "torsen" (torque sensing) or "automatic torque biasing" (ATB) differentials. They can lock up the normally open differential until 80% (a complete lock would be too dangerous) to enable the torque transfer from one wheel to the other. The effect is typically progressive, making it easy to drive.
There are several limited slip differentials available for the 335i - e.g. from Drexler Motorsport, OS Giken, Quaife Engineering or Wavetrac. BMW themselves as well as Alpina for their B3 S Biturbo (as an option) and the B3 GT3 model use a Drexler differential. Drexler is a "plate type" LSD that uses clutch plates to do the torque vectoring, whereas Quaife relies on gears for their operation. The advantage of the latter technology is that, as there are no clutches that are used, such LSD are almost maintenance free, "fit-and-forget" solutions. For me that was the decisive factor, as I did not really see fundamental advantages of a clutch type LSD (except in motorsport which was irrelevant for me).
So then, Quaife or Wavetrac? There's a lot of discussion going on on the forums about the advantages of one over the other. Wavetrac is supposed to work even if one wheel is completely off the ground - but that seemed a rather irrealistic scenario for my driving style, so that argument was of no concern for me. On the other hand, Quaife had a long-standing track record (decades, really) and first rate reputation, in particular for BMWs, and I knew several forum members personally who had it installed in their cars. Furthermore, Wavetrac didn't have an option for a welded differential like mine at the time (I later learned that there may be a workaround, but I wasn't interested in a solution that wasn't perfectly established), so the choice was easy in the end - Quaife.
Now, where to obtain it? If I was living in the US, I would have gone through HP Autowerks as I had already dealed with Harold in the past and was quite satisfied with his services. Being in Europe, however, I was recommended Birds in the UK as they are the European distributor of all Quaife differentials and have unmatched experience in this sector. As I wanted to have other modifications done at the same occasion and wanted to benefit from their experience with BMWs, I combined the installation of the differential with a nice trip to London where I hadn't been for a long time - killing two birds with one stone! You can look up their prices here on their website - due to my welded differential I needed a final drive unit which cost me (installation included) around 1800 EUR. I also thought about obtaining an LSD with a shorter gear ratio which is available as an option (for quicker acceleration), but in the end I thought that it was unnecessary as I'm not into 1/4-mile-racing and wanted to retain a high top speed (unlimited Autobahn! ).
So, in February 2010 I took the ferry over the Channel and drove on to Birds whose shop is located in London, not far from the Heathrow airport. They're really nice guys, and I got shown around their shop where several cars were in surgery, the most impressive being a conversion of a Z8 from left- to right-hand drive (wow! :bulge. I was also shown what the LSD looked like and where under the car it would be installed (as I don't really have on-hand mechanical knowledge). - The installation itself was routine for them, and the new LSD went in without any problem.
A few photos:
Original BMW differential
Comparison BMW and Quaife differential
The LSD was the most cost-intensive modification I had done so far on my car, and I was therefore quite curious to see whether it was worth the expense. First of all, it's completely noiseless, I could never hear any sound whatsoever coming from the differential, under any circumstance and load condition. I attribute this to the excellent install done by Birds.
As to driving - while driving normally, you don't feel anything different. But as soon as you push the throttle harder, there's no blinking christmas tree in the dashboard, no power cut off through the electronics, just acceleration pushing you into the seats. Under dry conditions, with good tires and off the race track, it's much more difficult than before to get the tires to spin - in a straight line, that's basically only possible in first gear or in second but at very high revs. The biggest difference can be felt in corners: Whereas before traction control cut power and the car hobbled around, now it just zooms along and you can feel how the torque is transferred to the outer wheel. It practically grips the road and pulls the car along, and long bends or motorway ramps are much, much more fun now. That's as it should have been as a factory car, in my opinion. I also believe it's safer to drive, as the situations where suddenly and unpredictably the DSC cuts your power and you're left with a car that doesn't accelerate as it should will be much less frequent.
Of course, the LSD can't defeat physics - when it's wet and the road slippery, wheelspin can still be induced easily, but that's to be expected with a 400+hp rear wheel drive car. However, wheelspin and traction control intrusions happen to a considerably lesser extent. I was able to experience that on the Nürburgring as it was mostly wet there last time - without an LSD, it would have been close to undriveable, with it I could still go quite fast around the corners, as long as I avoided second gear and full throttle in third.
• Problems / disadvantages?
For the LSD, the price tag is probably the first deterrent for most people, at least if they lease their car. If it's your own car, however, it's definitely worth it and I would not consider the price performance ratio to be bad. For installation I would recommend a shop with experience, and that can sometimes be slightly difficult to find; in Germany, for instance, Evotech distributes and also installs Quaife differentials.
An additional comment after having driven about 70.000km with the differential: There are no negative side effects that I can discern. I have since added an aftermarket differential cover from VAC to my car in order to optimise the cooling of the differential oil - if you track your car, this might be a good option to consider.
Current: Audi R8 V10 2013 S-Tronic, daytona grey, carbon side blades, MTM tune, Michelin PSS tires, Capristo x-pipe
Gone: Audi R8 V10 2010 manual, ice silver, grey side blades, MTM tune, MTM air filters, Michelin PSS tires
Gone: BMW 335i Individual (Öhlins, PFC brakes, RB turbos etc.)
Gone: Alpina B3 E46 3,3