"Octave overtones" in car exhausts - how? Why? - diyAudio
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Old 12th July 2015, 11:38 AM   #1
benro2 is offline benro2  Australia
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Default "Octave overtones" in car exhausts - how? Why?

Hi all,

Apologies for REALLY long post! Also for being slightly OT - hence why it's in the car audio forum...

This is a subject that I have been researching and pondering for years and finding information and people knowledgeable on the subject has proven impossible! I've asked this question all over car forums and have never been able to get an answer. I've tried looking for various books on the subject - same problem. Then I thought - "an audio website! Surely someone there would know??" Let's hope so!

What I would like to know is – why and how some cars with the exact same engine (or engine configuration) can have what I call “octave overtones” in the exhaust?

To explain what I mean by “octave overtones” – basically you have the main exhaust pitch (as in, “note” of sound), and then on top of this, you have another note sounding an octave above. In addition to this, you might have some other undertones or overtones. These under/overtones are always fixed and present in every engine configuration, which is obviously why a straight 6 sounds basically the same as another straight 6, and a Ferrari flat-plane crank V8 sounds basically the same as another Ferrari V8. On top of these overtones, you may have “octave overtones”.

I have also found that each engine configuration produces its own overtones that are equal in interval (in musical note terms) to the number of cylinders. So for example a 6-cylinder will have a "6th" interval between the 2 main sounds, a 10-cylinder will have a 10th interval, and so on. So for a V12, you would naturally expect a 12th interval - but you might also get an "octave overtone" from the 2 x 6th intervals (as you're essentially joining 2 x straight-6 engines together). So potentially in the V12 you'd have your base "note", then a 6th, 12th, and an octave above the base note. How much each of these are accentuated has a very large effect on the quality of the sound. Most people tend to like octaves since the two notes are "in tune" with one another, and this is why having 2 cylinder banks produces a nicer sound than one. And why a V8 might be the most desirable sound - because you're *only* talking in octaves (essentially "8th's" in musical terms) and no other intervals (but if you listen carefully, I've found the firing order seems to have an influence on the undertones in various V8's - but I'll leave that for another time!).

Some of these under/overtones can be accentuated depending on exhaust manifold design (I assume) and other principles. A good example might be a Nissan Skyline GTR with its 2.6L straight-6 (RB26) vs the Supra 3.0L TT 2JZ straight-6. Since the GTR’s engine is fitted with twin-turbos that are non-sequential, in terms of the exhaust manifold, it is like two separate 3-cyl engines. This tends to accentuate the undertones. Compare this to the Supra’s engine, while although it is still twin-turbo, it is a sequential setup so the whole 6 cylinders are routed into the first turbo. This tends to accentuate the overtones. I have even found that on say a BMW N54 straight-6, a custom 6-into-2 manifold feeding into a single twin-scroll turbo will accentuate undertones as opposed to a custom 6-into-1 manifold feeding into a single, single-scroll turbo (which obviously accentuates overtones).

However, I have found that with some engines, you can have another overtone that is an octave above the main engine note. None of the examples in the above paragraph have these “octave overtones” and at first I thought it was only present on a Vee engine, and even then, only those where the exhaust pulses were evenly spaced so that you had say two 4-cyl engines with equal length manifolds sounding together. My logic told me that since an octave is a doubling in frequency, it made sense that two equal exhausts sounding together might produce these octave overtones since you now have double the exhaust pules. So this is why a Ferrari 355/360 has octave overtones while your average American crossplane crank V8 does not (well, not as much). I have found that 180 (or 360) degree headers on a crossplane V8 can provide some of these octave overtones, presumably because of the exhaust pulses being more evenly spaced, thus lending itself to being more like two 4-cyl engines “in concert” and thus a doubling of exhaust pulse frequency (i.e. producing an octave above in pitch).

I also thought these octave overtones were limited to V8 engines and above. To more clearly illustrate these overtones – compare the stock exhausts of a Ferrari 355 and 360 vs a 430 and 458. The former sound very high-pitched, almost F1-like in sound, while the latter two sound almost like two separate inline-4’s together, without much of these octave overtones. (This is another question in itself – how come the same engine configuration can sound so different? My logical explanation would be the exhaust header design – and the differences between say the 360 and 430 I am yet to figure out – so any information on this would be much appreciated!) Another explanation might be how the exhaust pulses are spaced. Are they pulsing two cylinders at once or do they have 8 evenly spaced pulses? I would assume if it were the latter, then you would get more of an octave overtone. Perhaps Ferrari designed their 355 and 360 as the latter and the 430 and 458 as the former. However this doesn’t really explain how aftermarket exhausts can make the 430/458 sound like the 355/360!

Then I heard the the “Hakosuka” GTR, which is a straight-6. Have a listen:

https://www.youtube.com/watch?v=EAXGuD_f3Kw

It almost sounds like a V12! But this is only a straight-6! How are these octave overtones possible in “only” a straight-6?

And then I also heard Alex Kelsey’s home-built rally car:

https://www.youtube.com/watch?v=1blrI7kt6m8

To my knowledge, this uses a Renault-developed racing 3.5L V6 which is based on the V6 out of the Nissan Maxima/350Z. I had a look at the manifold design and I couldn’t see anything different to a regular tuned V6 on a 350Z. As you can hear, Alex’s engine has the accentuated “normal” V6 overtones *and* octave overtones – making it almost sound like a V12! This is where I am perplexed – how can this engine sound like this with “only” 6-cylinders *and* with a Vee design?

Another example of octave overtones can be heard in the Lexus LFA and Porsche GT, as opposed to say the Dodge Viper and BMW E60 M5. The former two sound almost like F1 cars while the latter sound like big, almost low-pitched V10’s. I have never heard an aftermarket exhaust for either the Viper or M5 that makes them sound anything like the LFA or GT. However, take the Lambo-derived V10 in the older Audi S6 and S8. With a Kreissieg or Milltek exhaust, these can be made to heavily accentuate their “octave overtones” to sound like an F1 car, or like a Gallardo with an aftermarket exhaust. Have a listen:

https://www.youtube.com/watch?v=48SOTBftCb4

At first I thought that Kreissieg must have redesigned exhaust headers, however as can be seen below, this is simply a cat-back exhaust:

AUDI S8

So how is this possible? I am assuming that at least in these V10’s cases, it’s the firing order that is determining whether these octave overtones are possible.

However, what about Alex Kelsey’s V6 or the Hakosuka GTR? Do they use some unusual firing order that allows this rare octave overtone in “only” a 6-cyl engine?

So – my questions are:

1. How are these octave overtones possible?
2. What are they caused by?
3. In what engine configurations can they be produced?
4. Is it possible to modify any engine/manifold/etc to reproduce/accentuate these tones?
5. Are there any good resources available where I can learn more about this?

Thanks for reading and I'd really appreciate any help on the topic!

Last edited by benro2; 12th July 2015 at 11:48 AM.
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Old 18th July 2015, 05:08 AM   #2
benro2 is offline benro2  Australia
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No one has any comments?
Is there another forum I might better off posting in? Or any particular members I might be able to PM?
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Old 18th July 2015, 07:58 AM   #3
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Internet + wall of text = TL : DR.

Talking about fluid dynamics and resonances of mechanical devices will go down like a lead balloon in a car Audio forum that is mostly frequented by those that have trouble understanding the basics of dc wiring.
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Old 18th July 2015, 08:16 AM   #4
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Perhaps have a word with the mods and see if they'll move it to The Lounge ? Seems the sort of thing that might go down well there perhaps.........
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Old 18th July 2015, 12:11 PM   #5
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Firing order does make a difference for sure, which would explain why a Mercedes V8 sounds smoother and "revvy" compared to a more :trucky: sounding American V8.

Also, shorter stroke vs longer stroke makes a difference also.
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Old 18th July 2015, 05:49 PM   #6
sreten is offline sreten  United Kingdom
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Hi,

The exhaust is a transmission line system and can be tuned.
The tuning is often about pulling as much air as possible out
of the cylinder within the exhaust valves opening timing.

The higher the "state of tune" the exhaust has, the
more liable it is to overtones of the tuning frequency.

1. How are these octave overtones possible?
Basic acoustics.
2. What are they caused by?
Basic acoustics.
3. In what engine configurations can they be produced?
Any highly tuned engine.
4. Is it possible to modify any engine/manifold/etc to reproduce/accentuate these tones?
In theory, yes, but you'd be compromising the basic tuning.
5. Are there any good resources available where I can learn more about this?
None that I'm aware of.

rgds, sreten.
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Old 18th July 2015, 06:16 PM   #7
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OEMs spend a fortune on getting engine note right whilst still meeting emissions and noise specifications. But the most gain can often be had from the inlet. Nothing beats the roar of a set of weber carbs IMO But meeting noise specs on track days can be hard then.
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Old 18th July 2015, 06:25 PM   #8
sreten is offline sreten  United Kingdom
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Hi,

Yes, in tuning, pushing as much gas into the engine as possible within
the inlet port opening timing is more important than exhaust timing.

rgds, sreten.
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Last edited by sreten; 18th July 2015 at 06:28 PM.
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Old 18th July 2015, 06:26 PM   #9
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A Harley sounds nothing like a Kawasaki.

Muffler technology may be a place to look for relevant info.
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Old 18th July 2015, 07:31 PM   #10
Pano is offline Pano  United States
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Moved to Lounge for better exposure.
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