How to obtain octave "overtones"?

[benro2]

Hi,

I have been looking all over for the answer to my question for a few years now but no one seems to know! I thought I would try the exhaust experts :) Apologies for not having a Mustang but they're a little expensive in Australia :(

Basically, I'm wondering what causes the exhaust note to be raised by an octave in some cars. A good example would be the Ferrari F360 vs F430 - the F360 has a high-pitched exhaust that has octave overtones compared to the F430.

I know that it isn't necessarily the flat-plane crank of the Ferrari as I have heard an old Chevy (?) V8 produce this sound.

I have also heard some V12's (Pagani Zonda) and V10's (Porsche Carrera GT) have these octave overtones.

Is it possible in a straight-6? I have noticed that some straight-6's have very deep exhaust notes - even deeper than 4-cyl's, while others have a much higher pitch (Toyota Supra), although it's not really an octave overtone, just a higher, flat pitch. Some have a mixture of in between with octave undertones (Nissan Skyline GT-R). Are these differences for the same reason as the Ferrari V8's and other engines I have mentioned above?

Any information would be much appreciated! Furthermore, if there are any good books on exhaust design, please let me know where to find them! Amazon doesn't seem to have anything useful.

Thanks in advance :)

[GerRrg281]

what the heck is an octave overtone?

i think the answer to your question is that the engines are constructed to handle completely different things.

ferraris may have v10's or v12's or whatever they have and the stroke is a lot shorter than an american built V8 (or so i figure) which allows them to rev up higher?

i think its just the basic layout of the engine that will produce its own unique exhaust tone. Plu with the help of certain resonators or mufflers, catalysts or just race pipes (no cats)....all will drastically affect the tone of the exhaust.

but its just like an old muscle car has a lot of camshaft overlap giving it that low loping idle of a classic muscle car.

as ferraris most likely have a shorter stroke, producing less vacuum, plus the unique grind of the cam and the ability of higher redline RPM's without having things fall apart. Plus clearances are very minimal in new exoctic cars giving them greater durability.

as old muscle cars have large clearance/tolerances. which does not allow it to as high.

could be wrong, but thats my take on the question. its open for interpretation.

[Nino]

There are a lot of things that effect the sound of the exhaust, include stroke, number of valve, type of crossover pipe, mufflers, tubing diameter and length, cam specs, compression ratio, etc, etc. I'm not sure what an octave overtone is either.

[benro2]

Sorry for the huge delay, but I didn't have it set to email me when I get a reply!

Anyway, to clarify what I mean by an "octave overtone", perhaps the best way would be to show a couple of samples of exhausts with and without octave overtones.

The best examples that come to mind would be the Ferrari 360 Modena and F430. The 360 has a shrieking, high-pitched exhaust that sounds like two "notes" - one low note and another that is an octave above, while the F430 sounds like a single, low-pitched note. Have a listen here:

360 Modena w/ octave overtones:
http://www.youtube.com/watch?v=uXN7OAUZePk

F430: "regular" exhaust tone without (m)any overtones:
http://www.youtube.com/watch?v=FuTf_1Oo1Qg

Do you hear the difference?

Yes, I know these are flat-plane V8's, *however* I know that it is possible to get the overtones on a cross-plane V8 - I have heard it in person on an old-school American muscle car, and I couldn't believe my ears when I heard it, as I'd never heard anything like it before from any sort of muscle car/cross-plane V8! It really sounds awesome, like nothing else, arguably better than the 360 above because you still get the cross-plane burble in there too.

BTW, this American V8 I heard must have been something special as it was in a race called the "Tasman Revival" series where a lot of the notable race cars from the 60's-80's are brought together. It had side-exiting pipes and was BY FAR the loudest thing there - literally ear-piercing even though I was a good 20-30m away!!

I have also come across 180 degree headers and this is *almost* there, but it's still not the same thing.

Anyone know what I'm on about yet? :p

[stangcoupe58]

[QUOTE=GerRrg281;35163]what the heck is an octave overtone?

i think the answer to your question is that the engines are constructed to handle completely different things.

ferraris may have v10's or v12's or whatever they have and the stroke is a lot shorter than an american built V8 (or so i figure) which allows them to rev up higher?

i think its just the basic layout of the engine that will produce its own unique exhaust tone. Plu with the help of certain resonators or mufflers, catalysts or just race pipes (no cats)....all will drastically affect the tone of the exhaust.

but its just like an old muscle car has a lot of camshaft overlap giving it that low loping idle of a classic muscle car.

as ferraris most likely have a shorter stroke, producing less vacuum, plus the unique grind of the cam and the ability of higher redline RPM's without having things fall apart. Plus clearances are very minimal in new exoctic cars giving them greater durability.

as old muscle cars have large clearance/tolerances. which does not allow it to as high.

could be wrong, but thats my take on the question. its open for interpretation.[/QUOTE

This post may be old but had to respond (Music and it's tones are my thing lol)... Octave is kind of like the tone of a particular "tone" that you hear, like the bass is songs. Although bass is rendered in (db) which determines how much that note will "Push" and how loud, he's referring to that bass sounding "Higher" instead of low boom boom, you get a higher paino like tone .. Like changing the sound "Pitch".

He is wanting to uniquely change the "pitch" of his exhaust by tuning the sound up and more of a European GT v8 car as opposed to the Typical American low rumble throaty high rpm tone.

[benro2]

Hi stangcoupe58, thanks for answering - you raise some excellent points!

I've done some more thinking since the last time I posted and have had a number of thoughts:

- The burbly sound of any engine (ie: not a "smooth" tone like a Ferrari or 4-cyl engine) is caused by unequal length headers and/or uneven angles of rotation of the crankshaft in between exhaust pulses (such as in a crossplane V8). A good example of this might be with a WRX - you can lose the burbly sound by fitting equal length headers, and then it pretty much sounds the same as an inline 4-cyl. Or have a listen to a Supra with an "RPS Manifold" (plenty of examples on youtube) - that uses unequal length headers and gives it a nice burbly tone. A lot of people pay big money for this header even though it's 10 yrs old, prone to cracking, and causes boost creep problems due to the bad position of the wastegate.

- I *thought* that these octave overtones were only present on engines with 8 cylinders or over, however I recently found an NSX that also has this sound! Have a listen here (http://www.youtube.com/watch?v=iYYEtb1crS8&list=FLUJzYVY3GCmZPtPy5DiBZtA).

- That got me thinking, why is that V6 any different to other V6's? It's relatively highly tuned, yes, but compared to today's engines, not *really*. So it must be to do with how the exhaust has been (re)routed. What about if the header(s) make all the exhaust pulses evenly spaced, but are also evenly spaced between one another between cylinder banks? Obviously firing order comes into play here, but this would essentially mean you have 2 x inline 3-cyl engines, and with an octave meaning a doubling of the frequency, evenly spaced exhaust pulses between banks *should* give the octave overtones. Flat-plane cranks *are* just like 2 x inline 4-cyl engines, and I would have to study the crankshaft to see how each bank fires, but I know they are partially designed this way so that the exhaust pulses are evenly spaced.

- The "bundle of snakes" headers (AKA 180 degree headers), as seen on a GT40 for instance, can give these octave overtones to some degree, although not as much as a flat-plane crank. The idea of these headers is to provide the same evenly spaced exhaust pulses as a flat-plane crank - and due to the different firing order of a cross-plane crank, you need to cross over 2 pipes from either cylinder bank. Sill works with my theory!

- I'm sure there are still other factors at play here, such as the points that stangcoupe58 raised - all of which I will have to study :) Now what I'd like to find out is - is it possible to get these octave overtones on an inline 6? I own a TT Supra and I *love* the sound of the RPS manifold - but something that screams like that NSX may sound even better! So I need to look at the firing order and figure out how to route the headers so that it works like 2 x inline 3-cyl's (so would probably need to go twin-turbo rather than big single)... but this is all just theory :) Sounds feasible? I've never heard of it being done before... however I have a feeling that it explains why some inline 6's have a very low pitch and other emphasize the higher overtones - mine already emphasizes the overtones so I may not be able to get it sounding any better - and mine is still a twin turbo. Food for thought!