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!
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:
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.
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.
Moved to Lounge for better exposure.
In terms of 2 stroke (specifically RC methanol fuelled engines) many examples exist where the exhaust is a tuned pipe, I believe somewhat along the lines of Helmholtz or quarter wave resonance. I believe the pressure reflex, as in reflex speakers is used to provide backpressure to the engine. These tapered tubes are used in some cars, at least partially (eg 'megaphone' conical pipe exhaust used on some racing Minis)
Much of the 'note' is due to the length and CSA, as you'd expect.
Consider the sound and length of exhausts used in Mid engine cars verses front engine cars.
However, I never heard a 911 that sounded good, yet a Impreza WRX is nice - both Boxers, short pipes verses long large diameter back box.
So many factors involved ��
That's about all I know - many of these modern cars have shuttered exhausts, which changes the game all.over again lol
Much of the 'note' is due to the length and CSA, as you'd expect.
Consider the sound and length of exhausts used in Mid engine cars verses front engine cars.
However, I never heard a 911 that sounded good, yet a Impreza WRX is nice - both Boxers, short pipes verses long large diameter back box.
So many factors involved ��
That's about all I know - many of these modern cars have shuttered exhausts, which changes the game all.over again lol
Last edited:
Hi,
Tuning two strokes is about creating negative pressure to help
with the engines exhaust scavenging. However such tuning
can cause poor scavenging at lower revs, so you get things
like "powervalves" that cause back pressure in the exhaust
and change the tuning to be more effective at lower revs.
FWIW 2 stroke tuning is way more complicated than 4 stroke.
rgds, sreten.
Ahhhh.
Yes I think I mixed 4 and 2 stroke
I know back pressure is required for 4 stroke (something I read once about using oversized exhausts and losing back pressure, hence screwing up carburetion. Again Minis. The result being a rich running engine requiring needle change to compensate)And yes 2 stroke is touted as a black art of sorts - I once looked into it and sought explanations for certain things. I found few folks who either knew anything, or were prepared to explain.
Yamaha made a 50 cc 2 stroke road race bike that, at full throttle and top speed, you could not hear the exhaust note since it was above the hearing range.
You could hear the chain, engine mechanicals and wind rush as it went by and that was about it.
Exhaust tuning is about air mass and how to control it since this also pulls the charge out as well as affecting the incoming fuel.
Not mentioned is whether an engine is over square or under, vs square meaning stroke and bore dia. is equal
This probably accounts for sound characteristics more than anything else
The distance from the ground and surrounding panels of the chassis would also contribute to the final output you hear
Does it really matter at the end of the day what it sounds like ?
Regards
David
You could hear the chain, engine mechanicals and wind rush as it went by and that was about it.
Exhaust tuning is about air mass and how to control it since this also pulls the charge out as well as affecting the incoming fuel.
Not mentioned is whether an engine is over square or under, vs square meaning stroke and bore dia. is equal
This probably accounts for sound characteristics more than anything else
The distance from the ground and surrounding panels of the chassis would also contribute to the final output you hear
Does it really matter at the end of the day what it sounds like ?
Regards
David
Of course it matters
I used to have a lotus7 clone. For them the sound is an integral part of the experience. Also the layout (and ways of getting around type approval) meant that you could play a lot with tuning, and there were a lot of engine options. I used to be quite good at looking at dyno plot and being able to tell fairly accurately what the exhaust config was and how long the inlet runners were.
I used to have a lotus7 clone. For them the sound is an integral part of the experience. Also the layout (and ways of getting around type approval) meant that you could play a lot with tuning, and there were a lot of engine options. I used to be quite good at looking at dyno plot and being able to tell fairly accurately what the exhaust config was and how long the inlet runners were.
Are you quite sure that's how it worked? Because for the exhaust fundamental to be above the audio band you're looking at 1,200,000 firing cycles per minute, which is crazy even if it was a four cylinder...
The octave is the first overtone. Open pipe acoustics... "In an open pipe the fundamental is at L = 1/2 wavelength and the first overtone is L = 1 wavelength (2nd harmonic)." http://openstudy.com/updates/4dd1140752f48b0b5389e7eb
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