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      04-29-2014, 01:17 PM   #1
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Lightbulb BMW M3 / M4 Sound and Exhaust System: Deep Dive and Interview

Sound and Exhaust System Deep Dive

The following comes courtesy of

Also see:
BMW M3 and BMW M4 Inside (part 1): The new M Servotronic.
BMW M3 and BMW M4 Inside (part 2): The Brake Systems.
BMW M3 and BMW M4 Inside (part 3): The new engine.
BMW M3 and BMW M4 Inside (part 4): Sound and Exhuast System.
BMW M3 and BMW M4 Inside (part 5): smartly light, rigid, precise.
BMW M3 and BMW M4 Inside (part 6): The new chassis.
BMW M3 and BMW M4 Inside (part 7): The Active M differential.

BMW M3 and BMW M4 Inside (part 4): Sound and Exhuast System.
Interview with M engineer Rainer Drees.

April 28, 2014

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Rainer Drees, M engineer responsible for the acoustic characteristics of M automobile.

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.


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.

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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.

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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.

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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 *******, 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.

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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.


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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.

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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.

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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.

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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.

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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.

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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.


Some various sound/exhaust videos of member uae247's delivered F80 M3:

Video: Accleration and fly-by (exterior)

Car was in Sport+ mode and in S3

Video: Different sounds of the driving modes (Efficient, Sport, Sport Plus)

Video: Tunnel acceleration (pre-break-in)

Video: Cold Start, Idling, Acceleration/Driving

Video: Interior and idle sounds

Exhaust Video:

Engine/Interior Video:

Engine Idling Video:

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      04-30-2014, 07:00 AM   #2
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M3/M4 exhaust in detail
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      04-30-2014, 08:05 AM   #3
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you sure do keep an eye on ; that is a very interesting and informative article!
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      04-30-2014, 08:19 AM   #4
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That is very very cool. It explains the method to their madness. It also might explain why when revving it with the car at a stand still, a lot of people complain that it sounds like crap, or like a weedwacker. That weedwacker sound is mostly probably because the valves are closed when revving it and the sound is damped by the middle and rear "silencers" (mufflers). I really enjoyed learning about that. Thanks!


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      04-30-2014, 08:33 AM   #5
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It seems as though BMW is embarrassed to answer the question.

Maybe I missed something but the BMW PR machine is in full force when it comes to the sound of the ENGINE (not the exhaust):

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

After reading the article there is no actual mention of what this new BMW technology is!
Is it all mechanical simply using membranes and ducts, or a more advanced shaker to get the sound into the passenger area, or as has been previously mentioned is it actually using microphones and speakers to play the engine sounds in the cabin?

I am not saying that using speakers is cheating, I understand that there are important reasons needed to be able to hear the engine (especially while on track and spirited driving), but why doesn't this article actually state what this more intelligent system is? when the author clearly asks "what are they?".
It seems as though BMW is embarrassed to answer the question.

Last edited by ZGM3; 04-30-2014 at 09:38 AM..
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      04-30-2014, 08:52 AM   #6
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A lot like the Ferrari F430 exhaust, with the "flap". I like it.
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      04-30-2014, 09:33 AM   #7
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Very awesome read.

By looking at the exhaust it seems one can do the OEM exhaust mod fairly easily as it was done to the e92/e90 M3. It actually looks easier you just have to cover up 2 proiferated pipes instead of 4 on the previous M3.
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      04-30-2014, 09:51 AM   #8
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Great insight on what goes into each and every component on the M3/M4!
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      04-30-2014, 10:09 AM   #9
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Great explanations of the way the system works. Very interesting read.

I am 100% ok and satisfied with the exhaust clip in the second video, quite contrary to some of the other clips I've heard.
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      04-30-2014, 10:17 AM   #10
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what is that arrow up with number 7 ? the forth video at 1:12.
which feature is it?
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      04-30-2014, 10:18 AM   #11

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People who complain about this exhaust/ sound are probably just poor loser audi drivers
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      04-30-2014, 10:25 AM   #12
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In that first flyby video you can't even hear the exhaust.
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      04-30-2014, 10:45 AM   #13
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hmmm is it me or is the exhaust is missing a true x-pipe/crossover pipe? I see an H-pipe, which is probably why it sounds like 2 inline 3's put together. I'm willing to bet a true center x-pipe will give it a nicer/smoother/warmer overall tone, like a true i-6 engine. For thosse complaining about the sound (me included), it appears to be a simple fix.
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      04-30-2014, 11:12 AM   #14
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The car farts just like a Honda with a coffee can on the back of it.

Some of you drink the BMW Marketing diarrhea straight from the bottle it seems.
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      04-30-2014, 11:17 AM   #15
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Originally Posted by M6-Coupe View Post
what is that arrow up with number 7 ? the forth video at 1:12.
which feature is it?
That's the suggested gear for the current driving mode. Typically shows up in other BMWs when in Eco Pro mode, but given that this car doesn't have that mode, it's likely a selectable option for Comfort mode.
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      04-30-2014, 11:17 AM   #16
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Interesting background info, nice post. Wish he would have addressed the piped in sound part head on but not surprising. The sound is growing on me, seems to have more of a classic I-6 rasp.

BTW, only the single set of cats?
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      04-30-2014, 11:24 AM   #17
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Originally Posted by spool twice View Post
hmmm is it me or is the exhaust is missing a true x-pipe/crossover pipe? I see an H-pipe, which is probably why it sounds like 2 inline 3's put together. I'm willing to bet a true center x-pipe will give it a nicer/smoother/warmer overall tone, like a true i-6 engine. For thosse complaining about the sound (me included), it appears to be a simple fix.

It looks like an H-pipe to me. I don't know the ins and outs, but wouldn't an X-pipe configuration be more suitable, especially as they tend to aid top end gains?
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      04-30-2014, 11:33 AM   #18
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I love the raspy sound and its uniqueness, the belches on downshifting could be contagious but I hate the canned fart noise during up shifts - that is not the sound of a german race car.
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      04-30-2014, 11:33 AM   #19
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Long term reliability of complex M3/M4 exhaust

While I am gobsmacked by the engineering described here, you've got to wonder how underbody corrosion will effect the reliability of the open/close exhaust gate system. The cost to replace this muffler won't be small, and you really need the exhaust system working 100% for the car to run properly. In North America, we use way more salt than in Europe, and exhaust systems pay the price.
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      04-30-2014, 11:35 AM   #20
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I'd take out that middle silencer and de-cat and keep it in sport + all the time
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      04-30-2014, 11:41 AM   #21
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Great technology and I'm sure it's leading edge stuff. I'm just not sure I'm sophisticated enough to appreciate it. I prefer the old fashion deep growl that turns heads. Perhaps this will grow on me.
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      04-30-2014, 11:50 AM   #22
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Originally Posted by spool twice View Post
hmmm is it me or is the exhaust is missing a true x-pipe/crossover pipe? I see an H-pipe, which is probably why it sounds like 2 inline 3's put together. I'm willing to bet a true center x-pipe will give it a nicer/smoother/warmer overall tone, like a true i-6 engine. For thosse complaining about the sound (me included), it appears to be a simple fix.

so you know more about the exhaust system of the new M3/M4 than these engineers at BMW AG in Germany? please specify what you studied, what you do for living and for how long. or are you just a street tuner?
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