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BMW M3 and BMW M4 Inside (part 4): Sound and exhaust system.
28. April 14
M engineer Rainer Drees.

Mr Drees, what area are you responsible for?

Rainer Drees: I am responsible for the acoustic characteristics of M automobiles. This relates to everything that the driver hears as well as everything that can be perceived in the surroundings.

One of my jobs is to ensure that our M cars sound authentic and full of character while at the same time meeting the various worldwide regulations regarding external noise levels. This is above all a matter that concerns the design of exhaust systems.
Let's talk about the sound and the exhaust system of the new BMW M3 and BMW M4.
















THE SOUND.

So what should an M automobile sound like? What does this mean in terms of the sound of the new BMW M3 and BMW M4?

What is important for an M automobile is that it is clearly and unequivocally recognisable. It is this characteristic sound that makes it possible to distinguish it from a BMW AG model or a competitor's vehicle with a similar engine design. But more than that, the sound of a BMW M3 or BMW M4 is different to that of a BMW M5 or BMW M6. The objective here was to create a purist, subtle engine sound, reminiscent of that of racing vehicles.

BMW M4 Coupé: Exterior sound examples.

The driver of a BMW M3 or BMW M4 expects to hear a particularly sporty sound both in the interior of the car and outside the vehicle. The aim is always to determine the best compromise between characteristics that stimulate the emotions and those that promote the vehicle's everyday suitability. A sound that is fantastic for the first 20 km might easily get extremely tiresome after 200 km.

What characterises the sound of a naturally aspirated engine as employed until now in the recently superseded BMW M3?

A naturally aspirated engine has relatively unobstructed lines both from the air intake and to the exhaust systems. A small proportion of the sound that the driver perceives as an acoustic indication of the engine load level in the interior of the vehicle comes from the direct transmission of structure-borne engine noise, while the majority of the noise is transmitted through the air directly from the intake tract; however the noise level is damped to an extent by a combination of the long intake distance and the air filters. In the case of a naturally aspirated engine, this damping can be decreased further by fitting a larger valve to the air box, thus acoustically dethrottling the air intake – as is the case in the BMW M3 CSL for instance. The resulting acoustic effect is very positive.

Airbox BMW M3 CSL.

The situation is similar on the exhaust side of the engine. Only the length of the exhaust tract and the silencers reduce the combustion noise emanating from the engine.

A naturally aspirated engine offers a very broad-banded frequency spectrum, which is useful when it comes to developing acoustic characteristics. A large engine speed range combined with many cylinders and a large engine size have a further positive impact on the available fundamental sound spectrum.

What is it that distinguishes the sound of a turbo engine from that of a naturally aspirated design? And why is this so?

The acoustic properties of a turbo engine represents a great challenge to sound designers.

The turbo charger's paddle wheels that are located in the air intake and exhaust tracts are effectively a barrier that blocks off intake and combustion noise.

Turbo charger's paddle.

As a result, the airborne noise transmitted from the intake is virtually eliminated. And this is immediately noticeable in the interior of the vehicle. Only the relatively weak structure-borne sound transmission remains intact.

The situation is similar on the exhaust side: all combustion gases that flow through the exhaust turbine are subjected to extremely high acoustic damping. Only those exhaust gas flows that are not needed for the turbo charger and which pass through the waste gate past the charger produce a combustion sound that can be utilised. However, these are precisely the acoustically relevant exhaust gas streams that are reduced still further to facilitate the outstanding response characteristics of the M TwinPower Turbo inline six-cylinder engine of the new BMW M3 and BMW M4.

This all sounds somewhat complicated...

Maybe the following example will make it all a bit clearer:

If you imagine a hall (the space beneath the bonnet) in which an orchestra of many different instruments (the engine) is playing. Some of the instruments are able to produce both low and high sounds, i.e. they have a uniformly good bandwidth or a broad sound spectrum.

However, alongside this hall is another, separate hall (the passenger space), which is joined to the first hall through different sized doors (the body, the air intake tract and the exhaust system). In the case of a naturally aspirated engine, most or at least many of these doors are open, which means that the driver in the room next door can still hear a large frequency band of the sounds produced.

However, in the case of a turbo engine, it is as if the majority of these doors were closed. You can hear something – maybe the basic character of the music as it plays (the combustion) – but the audible portion is restricted to the low-frequency range. A large section of the acoustic frequency spectrum is lost through the turbo system.

However, manufacturers have begun taking systematic measures to compensate for this problem.

The new BMW M3 and M4 employ highly modern technologies.

And what are they?

Ever since turbo engines were invented, sound designers have devised various methods of restoring some of the lost engine sounds to the interior of the vehicle. In the case of sports engines, this is not only important from the point of view of emotional stimulation, but, as we will see in a moment, it is also an important element of acoustic feedback from the car to the driver.

The first stage of development comprised mechanical systems. The vehicle interior was coupled to the high-pressure side of the engine beyond the turbo charger through a membrane. These systems were already relatively effective. However, by and large, the acoustic curve followed the torque progression of the engine.

On the other hand, drivers with sporting ambitions prefer to have an acoustic progression that reflects the actual power being emitted from the motor, as with naturally aspirated engines. It was for this reason that in the wake of the mechanical systems, the first electric sound systems were developed and employed by many among the competition. They involved screw-fitting a so-called shaker to the body of the car. The shaker is a kind of mechanical actuator that can be used to transmit vibrations into the body (e.g. the A column). These vibrations can then be felt in the passenger space.

However, the new BMW M3 and BMW M4 meanwhile use a more intelligent system. The technology employed here makes it possible to restore the sound that is actually produced, albeit damped by the turbo system, and render it audible in the passenger space. This can only be done systematically in those areas in which excessive damping takes place, but the driver desires and requires such acoustic feedback to support or more-sport oriented driving style.

By adopting this approach, it is possible to give the driver a very precise impression of the engine's load conditions. This enables the driver to use his ear as a guide, in the same familiar way as with naturally aspirated engines; after a certain period of acclimatisation, he is able to change gear at the required engine speeds, based on the acoustic feedback received from the engine speed/load conditions, without having to look at the tachometer. This is also important because while the torque plateau offered by turbo engines is broad, it breaks off abruptly when the engine speed becomes too low or too high.

To return to the orchestra analogy: we systematically open certain doors to bring the music to the audience.


THE EXHAUST SYSTEM OF THE NEW BMW M3 AND BMW M4.

Exhaust system of the new BMW M3 and BMW M4.

What is it that typifies the exhaust system of the new BMW M3 and BWM M4?

Optimum power development requires the lowest possible counterpressure from the exhaust.

Moreover the specification sheet called for a significant weight reduction in comparison to the system used in the previous model.

What this has resulted in is an exhaust system that differs significantly from the conventional engineering and functional concepts that we had implemented previously.

However, we have now for the first time designed the rear silencer in such a way that we have enough space in the dual exhaust system for two electrically adjustable exhaust valves ahead of the silencer.

Flow of gas through the rear silencer when exhaust valves are closed.

When the exhaust valves are closed, the gas branches off into a reflection chamber in the rear silencer. If we were now to feed the exhaust gas flow from one chamber to the next to create the necessary damping, this would lead to a high level of unwanted exhaust gas counterpressure.

Our solution is smarter: from the reflection chamber, the exhaust gas passes once right through the rear silencer to the tail pipes located on the opposite side. Hence the damping is primarily produced by artificially extending the length of the exhaust pipe, accompanied by absorption.

Middle silencer.

Moreover, the exhaust system is equipped with a small middle silencer, which enables crosstalk between the two exhaust lines, accompanied by absorption damping.

Gas flow through the rear silencer when the exhaust valves are open.

When the exhaust valves are open, there is virtually no damping and the exhaust fumes exit directly through the tail pipes.

This type of exhaust system allows us to implement an external sound that meets the motor sporting demands of the new BMW M3 and BMW M4. The flow of gas is at all times through all four tail pipes.

Complete exhaust system (excluding tail pipe covers) of the BMW M3 and BMW M4.

How does the sound vary? What are the criteria on which this depends? For instance by opening and closing the exhaust valves?

From the outside-vehicle perspective, there is a distinct difference in the sound emitted when the exhaust valves are closed and when they are open. For control purposes, we vary the engine characteristics; this allows us to open and close the exhaust valves in accordance with variations in engine speed and load conditions and corresponding with the selected drive mode.

For instance, if you select the COMFORT mode, the exhaust valves are fully closed in the lower revs ranges. If there is a greater load demand at higher engine speeds, they open up. When driving with a constant load in COMFORT mode, the valves generally remain closed to achieve a maximum level of everyday comfort over long distances.

In SPORT and SPORT PLUS modes, other engine characteristics are selected for controlling the exhaust valves. At lower gears, the exhaust valves are always open, while when higher gears are selected under constant driving conditions, they are closed; however, if the load conditions change accordingly, they open up in a flash. There are also ranges in which we have chosen to close the valves, for instance at low engine speeds in high gears. In these ranges, which are frequently selected in the automatic mode of the M DKG Drivelogic for driving at maximum efficiency, the exhaust tract in turbo engines would have a tendency to drone if there were no damping.

This special valve controller allows us to keep the exhaust gas pressure extremely low at all times. It gives us this conspicuous and unmistakeable M sound over the entire engine-speed range and enables precise feedback regarding the load status. The driver selects from a choice of coherent set-ups available with the different modes, which also takes into account the acoustic feedback from the engine.

What is the reason for that very specific sound that follows a cold start?

When starting the car for the first time of the day, the engine and exhaust system are still cold. To fulfil current exhaust gas standards, the exhaust gas treatment system must be enter working temperature extremely quickly. For this purpose, very high exhaust temperatures are generated for a short length of time.

What are the differences between the rear silencer offered by BMW M Performance Parts for the new BMW M3 and BMW M4 and the standard system?

The components in the similarly designed BMW M Performance Parts system are made of titanium; in addition to the lower weight, the material characteristics also offer acoustic advantages.

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Techniques in detail: Part 4.
Sound and the exhaust system of the new BMW M3 and BMW M4. Interview with M engineer Rainer Drees.
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Techniques in detail: Part 3.
The new engine of the new BMW M3 and BMW M4. Interview with Juergen Poggel.
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