Upgrading your BMW’s differential from the factory options provides unique benefits for different driving styles. We will walk you through pros, cons, and differences between each type of differential found in BMWs and explain gear ratios to help you choose the best differential for your BMW and specific driving needs.
Differentials receive torque from the rotation of the engine through the driveshaft and transfer it to the wheels. A differential allows the wheels to spin at different speeds depending on differences in available traction or cornering radius. A Limited Slip Differential (LSD) allows for the the differential to both lock or unlock depending on the driving conditions, torque being sent to each wheel, and the amount of traction each wheel has.
What is a Differential?
A differential is a set of gears that takes the rotation of the engine and transmits that motion to the wheels. The name is derived from the ability to differentiate wheel speed between the left and right wheels in order to allow them to turn at different rates for cornering. This is necessary as when a car turns a corner, the outer wheel will need to turn more frequently than the inner wheel. Differentials make this possible, which results in a more comfortable driving experience.
Types of Differentials
Inside a differential, you will find a Ring Gear and a Pinion Gear. The ratio of the two gear sizes relative to each other is where we get the numbers associated with the differentials gear ratio. A 3.73 differential means the Pinion Gear, or input, needs to turn 3.73 times before the Ring Gear, or output, spins once. You can also think of this as how many times the Pinion Gear turns for the wheels to rotate one time. The bigger the difference between sizes, the more reduction your differential will have. Increasing the ratio increases acceleration, but decreases overall top speed. A numerically higher ratio such as 4.10 has a mechanical advantage for punching off the line, but a numerically lower ratio such as 2.93 will cruise more comfortably at higher speeds.
For performance use, understanding what your acceleration and top speed needs are will help you determine what gear ratio to choose. If you are going to be driving Auto-X or doing drift events, a high ratio has a better ability to keep your power band in the sweet spot and provide more torque to accelerate out of corners. If top speed over a greater distance is your goal, having a lower ratio will be more suitable. Some prefer taller (numerically lower) gear ratios in turbo cars, as the increased engine loading helps to create more usable boost for better acceleration and power delivery in first and second gear. Turner Motorsport offers BMW differential rebuild kits and components in a full range of gear ratio options to suit your driving style and performance needs.
Types of Differentials
Differentials receive torque from the rotation of the engine through the driveshaft and transfer it to the wheels. An open differential allows the wheels to spin at different speeds depending on differences in available traction or cornering radius. A Limited Slip Differential (LSD) allows for the the differential to lock or unlock depending on the driving conditions, torque being sent to each wheel, and the amount of traction each wheel has.
Open Differentials
Most BMWs, with the exception of M-cars and special orders, have open differentials. Open diffs are great for the average driver as they are much comfortable to drive and considered easier by the general market. The downside of an open diff is the torque is distributed based on how much traction the tires have. Open Differentials send the power to the tire with the least resistance. Nearly 100% of available torque can be lost to one wheel in an open differential during performance driving (Ex: One Wheel Peel). This weakness fosters the common misconception that cars with open diffs are “one wheel drive” or only send torque to one wheel. More accurately, an open diff will always supply equal torque to both wheels, but can only supply as much torque as can be transferred to the ground through the wheel with the least amount of traction. So, if a wheel starts to slip with only 2 ft-lbs of torque applied (such as on ice), an open diff will supply an equal 2 ft-lbs of torque to the wheel on dry pavement.
Different Types of Limited Slip Differentials (LSD)
Helical Differentials
A Helical (such as Torsen, Quaife, or Wavetrac) torque biasing LSD uses helical gears to allow for both locked or independent wheel speeds. Torque biasing means that a helical style differential can supply more torque to the wheel with the most traction. The torque bias ratio is the level of torque (or traction) imbalance that the differential can support and still remain locked. The higher the ratio, the more torque it can send to the wheel with more traction. This creates partial locking of the wheels during acceleration but allows for the wheels to spin at different speeds during cornering. Thanks to these additional gears, your BMW will put more power to both wheels. The wheel with the most traction will receive the most torque rather than the one without it, which means more torque to the ground than an open differential. The torque bias ratio depends entirely on the helical differential type.
As you can see from the animation, supplied by Jose Morano, the worm gears are allowed to turn in either direction, which gives the wheels the ability to spin at different rotations per minute around a corner. When the gears are resisting each other, they lock up and simulate a solid axle for better traction and acceleration.
More instantaneous response.
No special fluid or maintenance needs.
Does not wear out or reduce lockup with wear.
Works great as an upgrade to modern traction control and stability control systems, reducing the intervention required by the electronic driver aid system.
Clutch Disc Differential
The other common LSD is a Clutch type. This LSD uses friction plates and clutch packs rather than additional gears to lock the wheels together. This provides a similar effect seen in the Helical LSD style. The more clutches you add to a limited slip differential the more friction you will have, which provides you with a stronger lock. A characteristic of the clutch type differential is that it will always lock the wheels together, regardless of how much traction there is to either wheel. Inversely, a Helical differential relies on the available traction to each wheel. The clutch packs in clutch differentials do eventually wear and will eventually need servicing, though. As a result, older M-Cars with the original differentials are most likely due for a refresh or upgrade. If you notice signs of differential wear such as reduced traction during cornering, or more inside wheel spin than usual, it is a good time to refresh your differential with an OEM rebuild, upgrade from your factory BMW differential to a LSD with more clutch plates, or switch to a Helical type LSD.
Gearing & Anatomy of the BMW Differential
Inside a differential, you will find a Ring Gear and a Pinion Gear. The ratio of the two gear sizes relative to each other is where we get the numbers associated with the differentials gear ratio. A 3.73 differential means the Pinion Gear, or input, needs to turn 3.73 times before the Ring Gear, or output, spins once. You can also think of this as how many times the Pinion Gear turns for the wheels to rotate one time. The bigger the difference between sizes, the more reduction your differential will have. Increasing the ratio increases acceleration, but decreases overall top speed. A numerically higher ratio such as 4.10 has a mechanical advantage for punching off the line, but a numerically lower ratio such as 2.93 will cruise more comfortably at higher speeds.
For performance use, having an idea of what your acceleration and top speed needs are will determine what gear ratio to choose. If you are going to be driving Auto-X or doing drift events, a high ratio has a better ability to keep your power band in the sweet spot and provide more torque to accelerate out of corners. If top speed over a greater distance is your goal, having a lower ratio will be more suitable. Some prefer taller (numerically lower) gear ratios in turbo cars, as the increased engine loading helps to create more usable boost for better acceleration and power delivery in first and second gear. Turner Motorsport offers BMW differential rebuild kits and components in a full range of gear ratio options to suit your driving style and performance needs.
Ramp Angles
Ramp Angles are the angles of the friction plates where the pinion wheel will push against the inner faces under acceleration or loaded deceleration. The steeper the angle, the more aggressive the lock you will experience. The ramp angles of the friction plates determine whether it is a 1 way, 1.5 way, or 2 way differential. The 1 way differential will have angles that allow the pinion wheel to push the friction plates apart to cause locking under acceleration, but has flat surfaces on the bottom side which allows for open characteristics under deceleration. A 1.5 way differential will have two different ramp angles, which lets the differential fully lock under acceleration, but only partially lock under loaded deceleration. This makes them particularly exceptional at retaining traction in corners and then maximizing torque to the ground upon acceleration out of the corner. A 2 way differential has equal angles and causes equal lock under loaded acceleration and loaded deceleration.
OEM M Variable LSDs
One of the big advantages of a traditional clutch-type or helical LSD over the OEM M Variable diff found on the E46 M3 is there is no wasted motion in the LSD. When slip is detected the mechanical diff will instantly transfer load to the other wheel. In the OEM M Variable Viscous Diff there is a delay as wheel spin happens first and then the diff begins to lock up.
Dynamic Traction Control
Newer BMWs can benefit from an LSD since the modern open differentials are regulated with brake inputs, also called 'DTC' (Dynamic Traction Control). The computer tells the rear brakes to help simulate the effect of locked wheels, but this can rob performance. We tested our F30 335i on a skid pad and experienced faults with the DTC after a few passes. The mechanical advantage of a true LSD is seen in both the acceleration and cornering abilities.
REM Polishing
In addition to offering both BMW gearing and LSD upgrades, Turner also offers REM polishing service on bearings and gears. The friction from each tooth on each gear in the differential needs to be taken into account in order to maximize the power output to the wheels. The available REM polishing will finely polish your gears and bearings to provide a noticeable reduction in friction, ultimately increasing the power transferred to the ground over a non-polished differential. This is a great way to get the most power from your engine to the ground through your differential.
Takeaways
Now that we have addressed the types of differentials, what they do, and how they are beneficial for your performance BMW, you should feel confident in selecting a gear ratio and differential type that is right for your car. The open differential in non-M BMWs from the factory provides a comfortable daily driving experience, but for a real increase in power going to the ground, a Limited Slip Differential is your best choice. Check out our custom differential options for your BMW.
E30 Custom Differentials
Upgrading or rebuilding the differential is one of the keys to the performance of the E30 325 or E30 M3. If you're stuck with a tall gear ratio you're going to be left behind on most acceleration runs. And if you have installed an M50/S50/S52 engine the gearing should be changed as well to make better use of the new torque curve...Click for more.
E36 Custom Differentials
Upgrading or rebuilding the differential is one of the keys to performance on your E36 325i/328i or E36 M3. Adding a shorter gear set will make the car accelerate quicker with almost no downside. And adding or changing the limited slip unit will improve traction and handling. The diff is crucial to getting the most out of popular engine upgrades such as an M50 manifold swap and cams where...Click for more.
E39 M5 Custom Differentials
A custom differential is one of the best, and smartest, upgrades to have done to your E39 M5. Not only can you get more performance but you can also address the weak points in the M Variable limited slip diff. It's easy to forget about the rear axle when you're considering performance upgrades but changing the gearing is in the top 3 things...Click for more.
E46, Z4, E39 Custom Differentials
One of the best upgrades to be made is custom building a differential. There are a lot of options to make improvements to the diff including gear sizing changes, added limited slip units, and limited slip lock-up options. It's easy to overlook the differential when planning your car's upgrades but it's one of the best areas to get return from your modification spending. Using a shorter (numerically higher) gear ratio will give you better acceleration...Click for more.
E46 M3 Custom Differentials
A custom differential is one of the best upgrades to have done to your E46 M3. Not only can you get more performance but you can also address the weak points in the M Variable limited slip diff. It's easy to forget about the rear axle when you're considering performance upgrades but changing the gearing is in the top 3 things to do to make the car faster. A custom diff is also a rebuilt unit with new seals, bearings, and limited slip unit...Click for more.
Z4 M Custom Differentials
A custom differential is one of the best upgrades to have done to your Z4 M. Not only can you get more performance but you can also address the weak points in the M Variable limited slip diff. It's easy to forget about the rear axle when you're considering performance upgrades but changing the gearing is in the top 3 things to do to make the car faster. A custom diff is also a rebuilt unit with new seals, bearings, and limited slip unit...Click for more.
E90/E92 Custom Differentials
One of the best upgrades to be made is custom building a differential. There are a lot of options to make improvements to the diff including gear sizing changes, added limited slip units, and limited slip lock-up options. It's easy to overlook the differential when planning your car's upgrades but it's one of the best areas to get return from your modification spending....Click for more.
E9X M3 Custom Differentials
A custom differential is one of the best upgrades to have done to your E90 or E92 M3. Not only can you get more performance but you can also address the weak points in the M Variable limited slip diff. It's easy to forget about the rear axle when you're considering performance upgrades but changing the gearing is in the top 3 things to do to make the car faster. A custom diff is also a rebuilt unit with new seals, bearings, and limited slip unit...Click for more.
F30/F32 335i/435i Custom Differentials
One of the best upgrades to be made is custom building a differential. There are a lot of options to make improvements to the diff including gear sizing changes, added limited slip units, and limited slip lock-up options. It's easy to overlook the differential when planning your car's upgrades but it's one of the best areas to get return from your modification spending. Using a shorter (numerically higher) gear ratio will give you better acceleration and the limited slip effect will help get that power to the pavement...Click for more.
