EDIT - Additional information added June 2009 to reflect current pricing for bolted and welded crown wheels
Right, so it's now been almost three weeks since I had the LSD installed on my car, and I suppose it's about time I posted my thoughts. It is the first differential to be installed on a customer car (the very first and prototype exists on the Birds Hartge demonstrator) so I thought it best to wait a while for everything to settle in before writing a review.
Bottom line, it's out-of-this-world extraordinarily amazing!
Here's the original BMW open diff
And another more detailed pic
Here's the new Quaife LSD, fully enclosed obviously
The two side by side
The OEM diff on the left, minus the crown wheel, and the Quaife LSD on the right with the crown wheel installed. See how the crown wheel on the right is secured by bolts? This is the original style of mounting on the BMW diff - for cars produced in 2007 onwards, the crown wheel is welded
to the OEM diff and cannot be dismantled by unbolting. Therefore the new style OEM diff has to be machined to release the crown wheel, before an LSD can be installed. More work, greater cost, but same benefits and results
So, other than being amazing, what is it and how different does it make the car feel?
Well, as most of you will know, the fundamental point of a limited slip differential is to facilitate a seamless and transparent transfer of torque across the driven axle to the wheel that at the time has the most physical grip. The Quaife LSD uses a system of helical gears as opposed to locking clutch plates to transfer the torque from side to side. It is installed in the BMW diff housing, filled with oil, and does not need refilling or servicing. It doesn't need running in either - install it, and drive it like you stole it.
In a FWD car, an LSD helps prevent torque-steer and vastly reduces understeer, as when cornering the torque transfers to the outside wheel where there is most grip and will generate much higher cornering forces.
For us, the gains from using an LSD are mainly in vastly enhanced traction and consequently less wheelspin. All of us have experienced the DSC system cutting in, flashing the yellow light, when pulling out of a junction either turning left or right, and needing some speed and acceleration at the same time. It's frustrating when the engine cuts the power to regain traction, when what you actually want is for the car to rocket away so you can safely merge into that gap in traffic.
When I installed the LSD, the first thing I noticed was how much massively hugely better traction was when turning out of a junction. In a normal car the DSC system senses that one of the two wheels is spinning due to lack of traction, and cuts power and/or applies the brake to regain traction. With the LSD, if one wheel starts to spin, the torque is transferred mechanically and seamlessly to the wheel with the most grip, and hey presto you start moving very quickly without the DSC system intervening.
The feeling is quite surreal, because now my exit speeds from corners are so much higher, all down to the increased mechanical grip. No longer do I have to wait for the car to straighten up before I bury my right foot - I can accelerate through a corner and maintain momentum all the way through. Also, hard acceleration in a straight line is improved, as there is a higher threshold before the onset of the DSC system intervening, and therefore a greater rate of acceleration.
However, there is a BUT. Not a big one, but enough to generate caution. You have to learn how to drive your car again if you have a LSD. The reason is that although the threshold before spinning is much higher, when you do finally break traction on BOTH rear wheels, you'll end up drifting and sliding like the pros. The DTC won't intervene early enough to prevent the rear sliding a bit, but the DSC system still works and will brake the rear wheels enough to stop you doing a 360. However, this does mean that if you're not judicious with applying the power with your right foot, you'll end up power-sliding out of corners and around roundabouts, which is great fun, but expensive on rear tyres!
Although I advocate caution above, the reality is that the DSC system flatters our driving skills and all we have to do is to relearn how to regulate our right foot! A huge upside now is that the feel of the car is completely alive in my hands, and I can tell to the absolute last degree what the back end of the car is doing. I can tell when the onset of rear grip is being reached, and when the car is about to slide a bit, and it is sooooo easy to adjust and correct as you're hooning along. Hammering along sweeping bends is a complete revelation, as you can feel the torque transferring from inside to outside, the rear end biting harder into the tarmac and because of the greater feel you have much more confidence in the way the car is handling and can carry greater speed safely through corners. The difference is startling.
Right after installing the LSD I drove down to see James and to compare how my car felt next to his Proceeded 335i. Although we don't have empirical data to show, both of us felt that with the LSD, my standard ECU car was as fast if not faster on initial acceleration and cornering especially than his remapped car. Certainly on the slightly greasy roads on that day, James' DSC light was doing its nut and would have been better just left on rather than flashing a million times a second. He just could not get any traction going up the road. Doing the same thing in mine, we came out of the junction turning 90 degrees right, hammered the throttle mid turn and literally smashed it up the road without even the merest hint of DSC. For sure, once moving up and beyond 60mph, his car is faster due to the extra power, but for cornering and the daily cut and thrust of driving, my standard car is infinitely more driveable and just as quick, if not quicker.
It surprises both of us that in the US, so many people are jumping up and down for big dyno numbers, and wanting 400+hp etc etc, but so few have gone out and addressed the fundamental key of how to go faster - by increasing mechanical grip. On a race track like Brands Hatch, if I were to race James, he wouldn't see which way I went after just a few corners. The difference really is night and day.
What are the downsides? None to my mind - I think it's actually good value at around £1200 fitted for the bolted crown wheel version. Some cars have welded crown wheels, and this adds extra cost due to the additional machine work required - fitted cost of this is around £1700. The only reliable way to tell which form of crown wheel a car has is to visually inspect the OEM differential. However, Birds can give an 80% accurate indication of the likelihood of the installed component.
It is completely invisible to BMW and has no bearing on engine or gearbox reliability. It takes three to four hours to fit, and works with both manuals and autos, petrols and diesels, and although it has been said before that the petrol 335 will benefit the most, I have driven a 535d with an LSD and the difference there is ASTONISHING, because of the amount of torque that diesel mill can generate. With a remap, I think the advantages of the LSD will be even bigger, as you'll have so much more torque and traction available for more of the time.
It's available for 325/330 petrols and diesels as well. I cannot stress how much of a revelation it has made - it makes me think that BMW actually designed the 335i with extra power and an LSD, then decided it was just too damn good so down-specced it with the M3 in mind.
As an aside, I think the ultimate everyday car for the road would be an E92 335d, remapped and with an LSD installed. If only I wasn't such a rev merchant and petrolhead....