Expert dialogue with Maximilian Ahme via M-Power.
Maximilian Ahme will answer the questions of MPW members. He is responsible for the technology of the entire vehicle with the exception of the electronics.
When did work begin on the development of the BMW M5 and how many kilometres were covered during testing?
Two to three years must be allowed for testing the development of a BMW M. In some cases predecessor vehicles are used in the first phase, enabling initial engine tests to be carried out, for example. However, this is just a short phase. The fine tuning then involves the latest models. The development departments for engine, electronics, chassis, etc. each have their own test vehicles to enable them to put the latest technology to the test. Approximately 2 million kilometres were covered throughout the world by the new BMW M5 in this way.
How does the engine of the BMW M5 differ from that of the BMW X5 M/X6 M?
The engine of the BMW M5 has been almost completely redesigned. In essence, the only feature it has in common with the engine of the X5/6 M is the bank angle (90 degrees) and the cylinder spacing. The specification sheet also indicated a higher maximum rotational speed of 7,200 rpm.
A lot of new features have been added in order to cater for the demands for low fuel consumption and good response: starting from the continuous regulation of the variable valve timing (VALVETRONIC) by means of the optimised cross-bank manifold to the dethrottling on the intake and output side, where we use larger charge-air coolers and turbochargers with more air throughput according to the M-TwinScroll concept. This required a new cylinder head with new valve drive mechanism and high precision injection. VALVETRONIC is combined with direct injection with an eight-cylinder drive – BMW M has done some pioneering work here. This measure also saves fuel. The oil circulation system has been optimised: a wet sump with an additional oil pump, which has already proven itself in the previous engine. Engine and transmission control are also new. The overall package guarantees a responsiveness familiar from earlier M models with no sign of the so-called “turbo lag”.
Why wasn’t the automatic torque converter transmission adapted from the BMW X5 M/X6 M?
The new V8 engine in the M5 provides maximum performance between 5,750 and 7,000 rpm. In conjunction with the specially tailored M DKG dual-clutch transmission, we use the maximum engine speed of 7,200 rpm as we move up the gears in sporty driving style to ensure that we start off in each new gear with maximum output. This is precisely what we wanted to achieve by increasing the engine speed range. An automatic torque converter is not designed for the required speeds and could not be used for this reason. In addition, only the DKG dual-clutch transmission allows you to change gears without interrupting traction. This is very important for good acceleration times… There are no disadvantages in terms of fuel consumption as this is more dependent on overall transmission ratio rather than the number of gears.
How do you get a turbocharged engine in a car to reach speeds of up to 7.200 rpm? Until recently this seemed impossible.
This isn’t so much a question of the speed of the engine or turbo-charger, after all we have already had speeds of up to approx. 14,000 rpm with the BMW Formula 1 engine. The characteristic zone of the charger in terms of pressure and volume has to be adjusted to the gas throughput of the engine. This in turn depends on the displacement, speed and required output characteristics. The principle is as follows: small chargers are good for fast responsiveness and high torque at low engine speeds, while large chargers are good for high maximum output. I think we have achieved the best possible compromise here with the new M5 in terms of responsiveness and power characteristics.
How does the cooling system work with 10 coolers?
With an output of 412kW / 560 HP, the new BMW M5 has power to spare and offers excellent driving performance. However, this enormous output creates the same level of combustion heat in all high-performance engines. In the case of the Formula 1 engine, the coolers are housed in an exposed position in the side boxes – a different solution needs to be found in the case of a sports limousine. For the new BMW M5 we have developed a tailor-made cooling package that covers all our requirements to perfection. A total of 10 coolers ensure that the temperature is kept just right, while also supporting the spontaneous response of the V8 engine.
The principle of direct or indirect charged air cooling applied to turbocharged engines. Air-to-air heat exchangers are used in the case of direct turbocharged cooling, which would have required much longer paths to the front of the car for the charged air with a lot of dead volume in our case. That’s why we use an air-water heat exchanger placed between the compressor and the intake manifold for every turbo charger. The heat generated while compressing the air is fed to three water-air coolers in the front of the car by means of the medium of water. The charge air thus takes the shortest path with a large diameter from the compressor via the charge air cooler directly to the intake channel. This improves the engine response. We have an air-water cooler and a total of three water-air heat exchangers for each cylinder bank: one radiator in the wheel arch to the left and right and one in front of the main heat exchanger. This means that a total of five coolers with their own reservoir and 2 electrically powered water pumps are used to cool the charge air.
In addition, there is the radiator for the main cooling circuit (high temperature circuit), familiar from every car. In the classic configuration, additional coolers are provided for the engine and gearbox oil and the steering hydraulics. Finally there is the condenser for the air-conditioning system. That makes ten coolers in all.
The rear axle differential has an aluminium oil tray with cooling fins so that a separate radiator is not required there. A smaller offset in the differential reduces friction and therefore the build-up of heat. Less friction means less fuel consumption here too.
What role is played by the Efficient Dynamics measures in reducing ECE normal consumption by about 30%?
Such a reduction in consumption to 9.9 l/100 km in the ECE cycle is achieved through BMW EfficientDynamics measures and an improvement in efficiency. We have already explained how this works with the rear axle differential. In the case of the engine, the variable valve control and dual variable valve timing and the high torque at low engine speed make a significant contribution. On top of this comes the automatic engine start/stop, varioserve power steering pump, volume-regulated engine oil pump, roll-resistance-optimised tyres and brake energy recovery in order to charge the battery when coasting. The entire package helps in achieving consumption targets without compromising on driving pleasure. Another benefit: when consumption is taken together with the larger tank, the vehicle’s range is significantly increased.
Why is an all-wheel drive still not an option with the BMW M5?
In view of the performance and torque of the BMW M5, this might be considered an option. However, we believe any advantage is outweighed by the disadvantages of the all-wheel drive. More weight due to four driven wheels means higher fuel consumption and a shift in the axial load in the direction of the front axle. However, we are particularly concerned with a balanced distribution of weight for the driving performance typical of the M. We also had to position the engine higher, which would have led to a higher centre of gravity.
Why wasn’t the fuel tank volume increased to even more, e.g. to 90 litres?
We made the best possible use of the space under the rear seats. That’s why a steel tank is used instead of a plastic one. Steel allowed us to achieve the same rigidity with thinner walls and enabled us to increase the tank volume from 70 to 80 litres. We genuinely fought for every millimetre. After all, safety clearances also have to be met.
The press folder reads “innovative active M differential for optimising the transfer of power to the back wheels”. What is the innovation in the BMW M5?
The variable differential lock in the previous model is controlled by the speed differential between the drive wheels. In other words, this is a passive system in which the lock is activated by different wheel speeds. This is now achieved proactively by means of an electric motor with downstream transmission and ball ramp. We use a similar package of discs which is pre-stressed by means of the electric motor. The driver’s preferences are also taken into account, i.e. when the driver accelerates, the system starts to lock, even in response to fast-changing frictional conditions, e.g. on a “patchwork” of snow and asphalt. The system also operates when the car is coasting – the lock is activated when the wheel sensors detect the relevant values. This makes for greater driving stability. A passive system does not react in this case. While a passive system always operates with pre-stress, the active lock can be completely unlocked. One advantage of the reduced friction is the reduction in fuel consumption. When the M differential lock is active, we can still start off on mountain slopes even with a 25 percent incline and extremely variable frictional values (ice/asphalt). When you are on the slope, you find it hard to believe that you will be able to start, but is really does work. This is a better alternative than walking on the ice…
What is the role played by the tyres in determining the complex chassis regulation systems?
A large part of the car’s driving performance is determined by the tyres. The force transferred when the car is in a bend, or accelerating or braking passes through the tyres. The same is true of the response from the road. Chassis adjustment and steering are at least equally important. It takes tyres that are precisely tailored to the chassis to achieve the superior driving performance associated with the BMW M5. We develop our cars in close cooperation with the tyre manufacturers and for a particular tyre type. Tyres need to meet very stringent criteria, which is why it is not easy for tyre manufacturers to receive approval from us. I would therefore advise you only to use the tyres we approve.
Why was it decided to have two M Drive buttons?
Just one button only allows two adjustment options: the basic setup when starting and the personalised M Drive button configuration. This comprises the setting options for the engine and accelerator pedal characteristic curve, DSC, EDC, M DKG, Servotronic and Head-Up Display. The M5 is now always started in Efficient and Comfort mode, in other words the best setting for fuel consumption and comfort. One M button is now permanently set to Cruise mode, i.e. an automatic setting still applies, although the engine speeds increases and combustion has not yet reached its highest point in the chassis, so that I am still travelling at a reasonable speed. The second alternative is the sporty variant, where I shift gears myself (via the steering wheel paddles or using the gear stick), activating higher damper forces, giving me greater steering forces and making the engine respond even more aggressively. Many customers will probably be happy enough with these three setups. Everybody else will find it easy to adjust the assignment of the two M keys to their needs on the basis of the permanent basic setup.
How much more output is available in Efficient mode? How do the three modes differ?
There is no difference in terms of output, however the ratio of accelerator pedal position to power development varies. In “Efficient” the characteristic curve is flatter for a more gentle use of output, while is it much steeper in “Sport plus” for a very spontaneous and direct development of performance. Thus, unlike its predecessor, the BMW M5 always has full power available as and when you need it.
How has the chassis been altered in comparison with the BMW Series 5?
The front axle has a more rigid connection to the bodywork. In addition, the track has been widened, when meant that we had to move the wings further apart. We have broadened the support base of the front axle bracket, also reinforcing it with a shear panel – a metal plate screwed onto the front axle bracket at calculated points. This means that the forces are applied to the bodywork over a large area. This increases torsional rigidity, so that the body of the car is subject to even less distortion. In terms of kinematics, the rear axle follows the excellent principle applies in the predecessor model. However, the rear axle support is rigidly connected to the body, i.e. there are no rubber bearings. This results in greater driving precision and good responsiveness. A traction field is used following the same principle as on the front axle. On the rear axle we apply forces that affect the chassis, side skirts and transmission tunnel via a package of struts. In conjunction with the hydraulic rack-and-pinion steering with variable ratio, the drive receives very precise feedback regarding the road and the car’s reaction.
How high is the maximum lateral acceleration?
Depending on the road surface and ambient conditions, it can reach levels of up to 1.2 g with our standard tyres. This is not a constant value. Irregularities and unevenness on the road mean that short peaks of up to 1.3 g can be achieved, or perhaps even a little more on the Nürburgring circuit, for example, when the tracking force is briefly increased through compression. Thus, the 1.2 g simply relates to a so-called “standard curve”.
Which detailed solutions posed the greatest challenges?
The whole is more than sum of the parts. In view of the enormous performance, the coordination of the regulating systems is a particular challenge. In the final analysis, our customers and their families should be able to discover the versatility of the BMW M5 for themselves, whatever the weather. On the other hand, the BMW M5 should also shine on the race track. Reading the press reports, I’d say we have obviously succeeded.