The Single Best M140i Upgrade?
Straight from the factory, the M140i is an incredible package. With an engine producing around 335hp and 500nm which is matched to a fast-shifting ZF 8-Speed Auto the performance credentials are strong, but one of the MAJOR drawbacks of the M140i, which is passed onto the M140i is the open differential.
Why did BMW fit an open differential?
Initially, BMW did bestow performance plate style locking differentials to their performance models, including non ///M cars like the E30 320is and 325i but as technology progressed they decided to remove this option in favour of a less costly open differential which also had benefits with lower Noise Vibration and Harshness (NVH). This was achieved without any tangible detriment to the driving experience to the novice driver though the addition of electronic systems such as ESP and electric differentials which improve stability and slow down wheel speed by using the ABS or a torque reduction. However, to the performance driver, the difference is instantly noticeable, especially once power is increased.
This reduction in cost, NVH and wanting to achieve greater separation between full ///M models and “regular” BMW models lead to even the high-performance variants of the BMW range having open differentials. We wonder, how would an M140i with a performance differential stack up to an N55 M2?
What is the problem with an open differential?
In its simplest terms the problem with an open differential is that when one wheel begins to lose traction, or “slip”, all torque (people also say drive, power etc.) is sent to that wheel. You then have a situation whereby all of the torque is being sent to the wheel with the least traction; this is a problem. With an open differential, the only way to begin sending torque to the other wheel is to slow down the wheel which has the traction loss; this is often achieved by coming off the power or by the electronic systems actuating the ABS to slow the wheel down. Both options are a slow and unenjoyable way to drive a car.
Whatsmore is that with tuned cars, on an M140i over 400hp is easily achievable this problem gets much worse and can result in excellent vehicle instability and perhaps even accidents if the driver is a novice or gets caught out in changeable conditions.
There are a few different solutions on the market from plate style locking differentials to automatic torque biasing limited-slip differentials. They all have their benefits and negatives.
The optimal solution for you will always depend on your use case scenario. For example, if you are using your car for 50% motorway driving, 40% fast-road and 10% track day we would not recommend a plate style differential as the NVH and service costs to maintain performance would not suit you.
Plate Style Locking Differentials
Plate style locking differentials are the pure motorsport choice, although, there is a BIG difference in performance, driveability, longevity and quality between the low-end £500-£1600 price bracket plate style differentials and the £2500 + price point of a Drexlar or Holinger differential.
The benefits of the plate style locking differential are that the axles can achieve a range of torque transfer or “locking”. This can generally be between 25-100%. With 25% meaning that 25% of the torque can be sent to the slower wheel (wheel with most grip) and 100% meaning that 100% can be sent to the slower wheel, so they move at the same speed.
The drawbacks of the plate style locking differential are many, due to the use of clutches and the axle locking the NVH is noticeably higher, performance reduces with time due to the clutch wear, they require maintenance to maintain performance and also mostly never come with warranties. This is why plate style locking differentials are suited to pure motorsport use where the vehicle is continuously checked over for maintained as required to ensure reliability and performance.
Automatic Torque Biasing Limited Slip Differentials (ATB LSD)
There are two main types of ATB LSDs. One is the typical Quaife design, which most ATB LSDs are similarly designed after in that they use pinions and gears to transfer torque across the axles with a spring washer pack in the centre to provide preload. The other is the Wavetrac design which is a more “modern” ATB LSD with patented features that solve some ATB LSD problems.
The benefits of an ATB LSD is that they are smooth in operation and low in NVH and don’t require any servicing. The Wavetrac takes this one step further than the Quaife and similarly designed differentials by including some carbon fibre bias plates which the pinons rotate on, these provide a consistent friction surface throughout the lifetime of the differential and are a patented feature, so are unfortunately not found in other ATB LSDs.
Other benefits include that they do not lock an axle; their behaviour is very predictable and smooth. Perfect for a novice driver through to motorsport. You can get in and enjoy the experience.
The drawbacks of ATB LSDs, or the Quaife and similarly designed ATB LSDs are that when they experience a “loss of axle load” which can happen when a wheel slips, suspension unloads (cornering), or wheel lift the ATB LSD performs like an open differential, sending power to the wheel with the least grip. This reintroduces the problem you are trying to solve.
The Wavetrac ATB LSD design overcomes this with their patented “wave hub”. This solves this problem by sensing the loss of axle load and creates internal load, which then allows the differential to continue to work. For tech junkies, you can read more information on this here: http://www.wavetrac.eu/
How to upgrade your M140i with an LSD
Upgrading your M140i with an LSD used to be complicated. It used to involve lots of downtime due to the stock differential requiring machining, however, at Regal Autosport we have made the investment to have multiple pre-built Wavetrac LSD equipped axles in-stock at all times. This means that a Wavetrac LSD equipped axle can be installed into your M140i in as little as 2 hours or shipped to you anywhere worldwide.
For a cost of £1999 plus installation, oil or shipping you can have a Wavetrac ATB LSD equipped axle installed into your M140i on exchange with your stock open differential axle.