Differential

The M5 has a very special computer controlled limited slip differential.

The purpose of the rear differential is to transfer the longitudinal rotary torque of the prop shaft 90 degrees to the wheels, while allowing the wheels to spin at different rates for when going around curves (the outer wheel needs to travel further than the inner). Here's how a differential works in general terms.

The input shaft spins the cage which tumbles the freely rotating blue "spider" gears. As they tumble, the green axles are spun. When the spider gears spin on their own axis, it allows one of the green shafts to turn at a different rate than the other.

In situations where the tires have different traction, a regular "open" differential as described above will direct all the torque to the tire with the least traction. This is a bad thing when going around curves as weight will transfer to the outer wheel and the inner wheel will lose traction. So just when you want more torque going to the outer wheel that has traction, it won't go there, and the torque will just spin the inner wheel uselessly. Likewise for when starting on snow and ice and one tire has more traction than the other.

For this reason, various "limited slip differentials" have been invented over the years. They use clutch plates to lock one of the axles to the cage thus preventing the blue spider gears form rotating about their own axis. Until recently, these have all been of mechanical design. The innovation in the M5 is to have these clutch plates operated by an electrical motor under computer control.

A cutaway of the unit is shown below.

The input shaft enters from the far side. The rear wheel axles are connected to the sides. The protruding device on the right is the computer-controlled motor which tightens and loosens the clutches. Here is another view.

We can see the electric motor which tightens up the clutches.

In this image, we see the inner workings at the front. The large bearings and worm gear that turn the cage are seen clearly.

The control unit (1) is under the trunk of the car near the battery.

It communicates with the central gateway module using the new FlexRay car computer bus standard, indirectly getting information from the stability computer (wheel speed, target transverse torque distribution, stabilization status, braking value), the motor electronics (accelerator pedal angle, wheel drive torque, "engine running" signal), and the integrated chassis management computer (wheel circumferences, lateral acceleration, yaw speed, vehicle speed, roadway inclination, steering angle).

Every 1000 km a quick calibration is run to correlate locking torque with motor current, and also assess clutch wear. The diff contains its own fluid, cooled by a heat exchanger, which are the veins underneath the unit in the under-vehicle airflow.

The operation of the limited slip rear differential is very noticeable when accelerating hard out of corners, especially in situations where the traction is limited. In my previous car, the E60 545i, the traction control would cut in in these situations, applying brakes and cutting engine power. Without traction control, you would need to accelerate out of the curve slowly or risk losing the rear end. In the F10 M5 by contrast, the car feels amazingly composed at speeds which would have easily spun out the other car.

First Impressions

My new M5 finally arrived on the weekend! I took some photos.

Here are my impressions after a week of driving it around.

1. The handling is absolutely amazing. It's on a different level entirely from other cars I have driven. That "M" logo on the steering wheel certainly stands for something.
2. The power in the car is awesome to experience.
3. Brakes are no-nonsense. They do a much better job than they appear to from the "lack of drama".
4. The car is very comfortable and almost all the gadgets work as advertised (including the real-time traffic on the nav). The Blackberry integration for email messages is a bit hit and miss though.
5. It feels like a big car when parking it.
6. When driving, it feels big on the inside, but feels lithe and nimble on the outside.
7. The car has a very different character in "Comfort" settings (the default when it starts up), as opposed to the sportier settings available when you press a pre-programmed "M" button. Turns into an absolute beast in M mode!
8. I am very happy with my colour and trim selections.
9. I look forward every single day to driving her!

Dual Clutch Transmission

The drivetrain is shown below. The gearbox is mounted longitudinally, in line with the engine crankshaft and connected via a driveshaft to the rear differential which directs torque to the wheels, tires, and road.

The gearbox on the M5 is the Getrag GS7D36BG M Double-Clutch Transmission (DCT) with Drivelogic. The "Drivelogic" is  marketing speak for the computer that controls the gearbox: BorgWarner's DualTronic clutch module (also called the "mechatronics" as a hybrid of mechanical and electronic in one package).

It is classified as an "Automatic Manual" because it uses clutch plates and a clutch mechanism to connect and disconnect the engine from the geartrain as for a manual transmission, but is has no clutch pedal and the gears can be made to shift automatically as for an automatic. It does not use a fluid-coupled torque converter as would a traditional automatic transmission, instead using hydraulically controlled wet clutches. 

It is a "Dual-Clutch" because it has two set of clutch plates, one for the even gears and one for the odd ones. This creates two sub-transmissions. As one is driving along in a certain gear using sub-transmission A, the electronics know if you are accelerating or decelerating, and will automatically engage the next gear in advance on sub-transmission B. This sub-transmission B free-wheels until the one clutch is disengaged and the other engaged. This allows extremely rapid gear changes with no interruption of power to the wheels.

This type of transmission was first used on a BMW in the M3. The Getrag BG used in the M5 is a beefed-up version of the SG used in the M3, and shown below in a partial cut-away view.

The principle of operation is as shown below.

The engine (A) inputs torque to two clutch assemblies (1,3) within the transmission (B). Depending upon which clutch is engaged, torque flows to either the top transmission sub-assembly (2) or the bottom one (4). These gears engage the propeller shaft which drives the rear wheels through a differential (C).

The dual-clutch assembly pulled away from the gear trains is shown above. It is a wet clutch, meaning that it is bathed in oil for smoother operation and longer life, and uses multiple clutch plates to compensate for any resulting slip. The two clutches are concentric, and the two transmission sub-assembly shafts nest one inside the other (observe the nested toothed gears towards the middle right, each drives its own sub-assembly).

The illustration below represents the inside of the M3 gearbox, but it is similar in principle to the M5's. The gear train on the bottom is called the countershaft. It is permanently rotating and meshed with the constant gear on the output shaft. There is an additional small gear train sticking out the side for reverse gear. The various gears are always meshed with one another. Some are permanently rotating with their shaft, others are free-wheeling on their shaft until a dog clutch pushed in place by the shift mechanism meshes them to their shaft. The dog clutch uses a synchomesh mechanism to match RPMs before locking the gear to the shaft. The main shaft is actually two entirely separately rotating shafts, one nested inside the other. Some of the dog clutches mesh the gear to the inner shaft, others to the outer shaft, and some of the gears are permanently rotating with either the inner or outer shaft.

Below we see the gear diagram showing sensors. The blue rectangles are the sliding dog clutches. They slide right or left to "mate" a gear to its shaft. Each has its own shift travel sensor.

For example, 2nd gear is always turning with the main outer shaft, and it is meshed with its counterpart on the countershaft. That gear, however, is free-wheeling on the countershaft until the dog clutch mates with it.

The system predicts what gear it will likely go to next depending on if the car is accelerating or decelerating. It will then pre-engage the appropriate dog clutch. The next gear is always on the other shaft which is not yet clutched to the drive shaft, so its shaft can free-wheel. The engaged clutch can then start decreasing and the other clutch can start increasing in pressure to match RPMs. 

The following animated diagram shows all the various power channels through the transmission. 

Note how the clutches alternate as we go up and down the gears.

The diagram below shows the entire system.

The transmission (2) is lubricated and cooled by oil via the air/transmission oil cooler (1). The transmission has its own integral oil pump driven from the center input shaft. Therefore the engine must be running for the oil pressure to build up. It is connected by wires to the gear selector level.

Within the transmission there is a pipe running down the side with nozzles to lubricate the gears.

On the drive end of the casing there is the parking brake that prevents the shaft from moving, with the mechanism shown below.

Park is engaged when the engine is turned off and the gearbox is not in neutral. If the parking brake ever needs to be released manually, this is accomplished inside the front cup holder by removing the cover and sliding the parking lock lever as shown.

The "mechatronics" (electronics + hydraulics) module plugs into the side of the transmission, receiving input from the various position, rotation, temperature, and pressure sensors within, and effecting via hydraulics the shifting and clutching functions.

The mechatronics communicate with DME-1 via the PT-CAN computer bus to get pertinent information and to "blip" the throttle on shifts. Blipping means bringing the engine revs up to match the wheel speed at the next lower gear down during down-shifts. The gearbox blips in all shifting modes.

Sequential manual gear shift control can be effected by the '+' and '-' paddle shifters on the wheel.

The main control is by means of the gear shift selector.

Moving the selector to the right toggles between "Drive" (automatic shift) and "Sequential" (manual shift). Pushing the lever forward and back will shift. Pushing the lever to the left will engage neutral. Left and up: reverse.

Each of D and S modes have three "Drivelogic" settings accessed from (5). In D these are Efficient, Comfortable, and Sporty, which will move the shift map higher in the RPM range and shift faster. In S mode, the three are Comfortable, Sporty, and Maximum which refer to the speed of the gear changes. The shifting speed is also affected by the accelerator position and how quickly it is changed. In my experience, D1 mode is dreadful in that it robs the car of all its torque by shifting too early and keeping the revs very low. This is the most fuel efficient setting, however.

There is a mode called "Launch Control" for maximal acceleration off the line allowing the optimal amount of wheel slip (17%) and using the fastest shift speeds at the highest RPMs. This is engaged by deactivating DSC, selecting the third "S" mode, pressing the brake pedal gently, holding the gear selector forward, waiting for the flag symbol to appear, flooring the accelerator, releasing the brake, and then releasing the gear selector switch. It's not meant to be easy!

In order to simulate the bahviour of an automatic gearbox, a light tap on the accelerator when stopped will cause the car to move forward (or backwards if in R) very slowly without holding it. On hills, the brakes hold the car steady for 2 seconds after releasing them until the accelerator is depressed. After that, it starts rolling.

The gear ratios are as follows. 1st=4.8x, 2nd=2.6x, 3rd=1.7x, 4th=1.3x, 5th=1.0x, 6th=0.84x, 7th=0.67x. The final drive ratio at the rear differential is 3.15x. Driving at less than 145 km/h it's never necessary to get out of 3rd gear, though the RPMs are at 6000. 4th gear will get you to well above 200 km/h, and 5th will get you to its maximum speed of somewhere around 300 km/h (though the electronic speed limiter keeps it to under 240 km/h or so). 6th and 7th gears are for fuel economy, for cruising quickly on the highway at relatively low RPMs to conserve fuel.

The DCT gearbox on the F10 M5 is an absolute delight to drive. First it adds to the performance of the car by keeping power applied through the shifts. Second, the shifts are lighting quick, like gunshots, which is just a whole lot of fun to drive, and finally the blips on the downshifts are just right, and make it feasible to get yourself easily into the right gear when tackling a corner.

BMW V8 Engine History

The engine inside the F10 M5 is officially known inside BMW as the S63B44T0. Other names for it are S63 top (for "top performance class") and S63Tü (for "technische überholung", or "technical overhaul"). In this post I'll answer the question as to where this engine came from.

The first ever product that the newly named BMW company produced was the BMW IIIa aircraft engine in 1917. It made a series of successful aircraft engines both before and after WWI, but eventually was forced out of the aircraft engine business altogether at the end of WWII.

Meanwhile BMW also moved into small engines, motorcycles  and cars. BMW's first automobile engine designed exclusively by themselves came out in 1932. for the BMW 3/20 "Dixi". It was an inline 4-cylinder engine.

A series of 4 and 6-cylinder engined followed both before and then after WW2. WW2 created a hiatus, and after the war they re-emerged in 1952 with a very high-end luxury car, the BMW 501,

and very low end compact cars including the Italian-designed Isetta

and the BMW 600

both powered by modified forms of their 2-cylinder motorcycle engines.

While the Isetta was successful, the 600 was a flop, and the the 500-line were too expensive to produce and sold only in small numbers. At this time BMW was almost acquired by rival Mercedes-Benz, averted only by the dramatic last minute intervention of the Quandt family in 1959, who remain major BMW shareholders until today.

BMW's next car, the BMW 700 was also an inexpensive compact car powered by a motorcycle engine.

This car was a great success, and was enough to stave off bankruptcy and fund the development of the "New Class", launched in October of 1962 which set them on the course they follow today. The first New Class was the BMW 1500, really the first "5-Series", though not called that at the time.

It used a new engine design, called the M10, which we will discuss below.

With that context, let us now turn our attention to the sequence of V8 engines that BMW produced culminating in the one in my car, the S63B44T0 engine in the F10 M5, first dialling the clock back to the 1950's.

1. BMW OHV V8: 1954 – 1965

BMW took until 1954 to build a V8 engine. This was BMW's only pushrod-driven overhead valve V8. This engine used a single camshaft going down the middle of the block and pushrods that drive the overhead cams. Modern engines use overhead camshafts and avoid the need for fussy pushrods. This engine's block was cast from aluminum alloy, and it used a dual-barrel Solex carburetor (similar to that show below) for mixing gas and air going into the cylinders.

It was used in the BMW 502, a sporty update to BMW's first car after WW2, the 501 in response to competition from Mercedes-Benz. It displaced 2.6L and made 100HP.

It was later enlarged and tuned to culminate in a 3.2L engine making 160HP that was fitted to the exclusive BMW 507 roadster which achieved a 0-60mph time of under 10s.

This car was so exclusive and expensive to make, and sold so little (only 252 were ever built - it sold for $10,000 - Elvis had one) that it almost bankrupted BMW.

There followed a quarter century hiatus starting in 1965 during which BMW did not make any V8 engines, sticking to the continued development of the smaller inline 4s and extremely smooth-running and relatively powerful inline 6 engines, and leaving the 8-cylinders beasts to the American muscle cars such as the 1970 Plymouth GTX shown here.

While great in straight line acceleration, these cars were heavy in front due to the massive engines, had poor fuel economy, and poor cornering, which were all areas that were much more important for, and in fact defined, European cars of the time.

BMW meanwhile was working on its New Class platform. It developed what became known as the M10 engine line specifically for it.

The M10 engine was designed by Baron Alex von Falkenhaven, an engineer and race car driver, and was a good part of the reason the New Class was so successful. The basic engine design was an inline 4-cylinder with a single overhead camshaft (SOHC), with sufficient capacity to expand up to 2.0 L of displacement. It lasted until 1987 alongside other inline SOHC 6-cylinder engines whose designs were based upon it (notably the M30 "big six" and the M20 "small six").

A new wave of engine designs were released starting in 1987 including the M40 4-cylinder, the M50 6-cylinder, and the M70 12-cylinder.

BMW M50 Engine

Feeling the need to compete at the higher end, BMW was faced with the choice of developing a V8 or a V12. They decided to start with a V12 in order to leapfrog Mercedes and go head-to-head with them at the high end with the E32 7-Series.

As well, a V12 is essentially two inline 6 engines (BMW's speciality) driving a common crankshaft. As each I6 had no first or second order imbalances  the V12 was also extremely smooth running and highly performant.

In fact, the M70 V12 shown here in an exploded view is based on two I6 SOHC M20's set at a 60 degree angle to one another.

2. BMW M60: 1992 – 1996 

The V8 moratorium was finally broken by the M60 V8 engine in 1992, though development began in 1984.

The engine was made in two variants, a 3.0L and a 4.0L which appeared in the E34 530i and 540i respectively.

(Recall the 5-Series lineup: 1972 E12, 1981 E28, 1988 E34, 1995 E39, 2003 E60, 2010 F10).

This engine was made from an aluminium block with aluminium cylinder heads. The cylinders were lined with Nikasil, an alloy of aluminium, nickel, and silicon, which is corroded by the high sulfur content of low quality gasoline.  BMW extended the warranties and changed the design to use a different alloy, Alusil (still in use today) when the problem was first uncovered.

It introduced the use of 4 valves per cylinder, 4 overhead camshafts, two dual chain timing belts, connecting rods made using a sintering process, a plastic intake manifold, fuel injection, and a new ignition system using individual coils over the plugs to replace the distributor and controlled by the Bosch Motronic 3.3 system.

The larger engine made 282HP, and generated 295ft-lb of torque at 4500rpm.

3. BMW M62/M62TU/S62: 1994 – 2005 

The M62 was a straightforward successor of the M60. Slightly larger displacement, and amongst other minor changes, it also replaced the dual timing chains with a single one.

The M62TU (Technical Update) was used in the 1999 model year E39 540i.

This engine displaced 4.4L and made 282HP with 325ft-lb of torque at 3600rpm. It added variable valve timing on the intake camshafts, called VANOS by BMW, and an electronically controlled throttle.

The M Motorsports division put out the S62B50 with enlarged stroke and bore, one electronically actuated throttle body per cylinder for quicker response, and variable valve timing on both the intake and the exhaust for the first time in a BMW (called "Double VANOS"). It was used in the extremely popular and well-loved E39 M5.

This engine displaced 4.9L, made 394HP, and generated 369ft-lb of torque at 3800rpm.

4. BMW N62: 2002 - present

The N62 is the engine used in the E60 5-Series, including the 540i, the 545i, and the 550i (by increasing bore and stroke). It incorporates double VANOS, variable valve lift, and a variable length intake manifold.

The N62B48 used in the E60 550i produced 362 HP and 361ft-lb of torque at 3400 RPM.

This engine was highly regarded and won International Engine of the Year in 2002 for its advanced technology, performance, and reliability.

There was no higher performing "M" variant of this engine. Instead, M built the S85 V10 for the E60 M5, inspired by their involvement in F1 racing engine construction.

5. N63: 2008 - present

The N63B44 engine is BMW's first twin-turbocharged V8 produced in 2008 for the X6 and later for the 750i and F10 550i.

The switch to turbocharging was against BMW's prevailing philosophy of direct naturally aspirated throttle response. They started the switch with the N54 Inline 6 engine in 2006 for use in the E90 335i.

It has two small turbos plus direct fuel injection, however they had to give up variable valve lift. The engine won numerous engine of the year awards, however reliability was plagued by problems with the high pressure fuel pump at the fuel rails.

The V8 N62 was introduced two years later, in 2008. Unique to this new engine is the switch of the usual intake and exhaust valve orientations to allow for twin turbochargers mounted on top in the "V". This lowers the distance from cylinder head to turbo for the exhaust gasses leading to less lag and more low-rpm torque. It uses air-to-water intercoolers to cool the compressed air before it enters the cylinders. The engine displaces 4.4 L and makes 402 HP and 440 ft-lb of torque between 1750 RPM and 4500 RPM, with a 6500 RPM redline. The first application of the engine was in the F01 750i.

Unlike its predecessor, it does not use variable valve lift, as it was not as needed due to the compressed air from the turbochargers minimizing the partial vacuum problem the Valvetronic system was designed to solve. It does, however, use double VANOS to control the timing on both the intake and exhaust camshafts. The turbochargers are of the single scroll design and are isolated one cylinder bank from the other, which because of the uneven cylinder firing inherent in a V8 causes uneven pulses of exhaust gas to flow over the turbos, reducing their efficiency.

The N63 was later upgraded to the N63TU for 2013 following development in Valvetronic for turbocharged engines pioneered in the S63TU engine in the F10 M5.

6. S63 & S63Tu: 2009 - present

The S63B44 is M Motorsport's higher performance variant of the N63 used in the X5M and X6M.

The main differences from the base N63 are the use of twin-scroll turbochargers and a more efficient exhaust manifold that crosses over the exhaust gasses from the cylinder banks for smoother driving of the turbos, and larger intercoolers.

The 2012 model year F10 M5 has the S63B44TU ("Technical Update") engine.

This engine also displaces 4.4 L, but makes 560 HP and 502 ft-lb of torque between 1500 RPM and 5750 RPM, and redlines at 7200 RPM. It adds Valvetronic variable valve lift, doing away with the throttle, which improves throttle pedal responsiveness. It also runs at a higher compression ratio of 10:1, has a higher turbo boost (1.5 bar), larger intercoolers, and larger, more free flowing air intake and exhaust.