Question about the distinctive sound of engines (Janichol...

Been wondering about this for a long time.
All of the following engines should produce the same "pitch" (assuming all are four stroke engines):
a single cylinder engine at 8000 revs per minute,
a two cylinder engine at 4000 revs/min,
a three cylinder engine at 2667 revs.min,
a four cylinder enine at 2000 revs/min,
a five cylinder engine at 1600 revs/min,
a six cylinder engine at 1333 revs/min, and
an eight cylinder engine at 1000 revs/min.
However, this is clearly not the case: each of these engines has a distinctive sound to it.
Could anyone explain this, please?
Does the topology and geometry of the exhaust manifold play a role?
(I know that a two cylinder engine may be an odd one, depending of the relative orientation of the throws of the crank.)
Thanks, Hendrik

Keep in mind it takes half the power from each cylinder to turn a 4 cylinder (generalizing of course) engine at the same rpms as a 2 cylinder.

So matching it on rpm's is going to be way off on the sound right from the get go.

And dont forget the classic Harley Davidson idle sound "Potato Potato Potato Potato" from having two pistons on a common crank throw I believe.

Back to your posting, much of the sound you hear is when the exhaust valve opens correct?? and the overlap and consequent mixing of the 2 or more cylinders contributes to the sound heard.

Then theres the I believe Fletcher Munson hearing curves that determine what the human ear perceives versus what is physically taking place

DARN IF I CAN ANSWER YOUR QUESTION, but think the above has a part in it

John T

Hendrik - The fundamental "tone" (frequency) of an engine's exhaust is exactly as you describe. And the frequency for different engines with the same number of cylinders running at the same RPM will be identical as you mention.

HOWEVER engine displacement, exhaust port design, exhaust manifold geometry, exhaust pipe diameter/length and other factors generate all sorts of harmonics of the fundamental frequency. This causes engines with the same number of cylinders and running at the same RPM to have entirely different "voices".

Example: A straight 6 with with a center outlet manifold (Dodge/Plymouth flathead 6) sounds very different from a Continental straight 6 with its manifold outlet between the #1 and #2 cylinders.

End outlet manifolds, split manifolds, etc similarly result in dramatically different exhaust notes from otherwise identical engines.

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Incidentally as a kid I could distinguish just by sound IHC, Mack, White, Chevy/GMC, Dodge and Reo gas truck engines. Likewise Farmall, Allis-Chalmers, Minnie-Mo, Ford, etc. 4 cylinder tractors. Hearing these engines run at antique truck and tractor shows brings back fond childhood memories(!)

Each engine has its own sound, just like a voice. A dog can usually pick that sound out from every other similar engine.

This is cool. My only explanation is that the engineers within a manufacturer have guidelines and principles, and those design principles (like bore and stroke ratios, valve sizes, etc.) unintentionally make similiar sounding engines. I point to all letter series and hundred series four cylinder engines as an example. They all have a familiar sound. They all make a certian, unmistakeable tone when they have some load on them. Even the diesels have it. All the 6 cylinder gassers IH made sound alike, even though they come in many displacements.

I realize that my comment isn't that terribly relevant to your question. I just find it interesting that some manufactures engines all sound similiar but they are recgonizable from manufacturer to manufacturer. Especially since many different engines do have different sounds. I'm no engineer. I hope somebody who understands this subject much better comments. -Andy

The following factors are direct variables to the distinct sounds coming from each:
1) The bore size of the engine
2) The resonance of the block/heads (willingness to vibrate at certain frequencies
3) Fhe combustion chamber shape
4)The cam timing for exhaust open
5) The cam timing for exhaust close
6) The openin/closing profile of the cam or valve train
7) Multiple exhaust valves including the exhaust valve shape
8) Exhaust port computational fluid dynamics (shape)
9) Ignition timing
10) manifold runner geometry/pulse timing
11) Exhaust pipe dynamics
12) Internal surface texture of the entire system
13) The direction the pipe faces

A dog can hear the distinct NOTE of a particular engine from a mile or more away. My wife had 5 minutes warning when I came home due to the distinct sound made by my Jeep. (or several other vehicles we owned).
This is not the entire story, but covers several points. I hope this helpes! Jim

Not a big mystery at all. To double the RPM's means that the contents of the cylinder bore have to fill and vacate twice as fast. Although the "pop" is the same pitch for a one cylinder as a 180 degree two cylinder at half speed, the other sounds of air intake and exhaust will be different. Often the sound produced by air movement dominates.

This subject (engine sounds) is certainly material for a doctoral thesis - just my way of saying that the "sound quality" of an engine IS affected by a great many many factors which cannot be adequately explained by just a short post. What is fundamentally underlying the original posters note here is that ALL of the engines noted at the stated RPM's DO have the same firing frequency (# of explosions per second for a 4 cycle engine {two cyles are another matter} which is determined by dividing rotational speed by 60 and multiplying that value by the # of cylinders divided by two).The engines sure sound differently,however. As an example of firing frequency, the one cylinder engine would be 8000/60x1/2 = 66 2/3 HZ (or cycles/seconds); the 5 cyl firing is 1600/60 x 5/2 = 66 2/3; the 8 cylinder firing frequency is also 66 2/3 HZ (1000/60 x 8/4 = 66 2/3). What IS different is the rotational frequency - even that (which we would call 1 st order speed) has a great influence on sound quality. There is also a great difference in the sound quality between engines with the same firing frequency but different bank angles (e.g. a 60 degree vs a 90 degree V-6) as the spacing between firing pulses is NOT uniform on a 90 degree V-6 like the "balanced" firing configuration on the 60 deg V-6. Other factors also such as primary and secondary forces & moments (couples) are involved here also.
I would add a clarification to Bob M's statement in that the tone of an engine (it's fundamental firing frequency) WOULD be somewhat the same for ALL of these noted engines running at the RPM's stated. You can, however, have a very different sound quality - see my comments above for a 60 vs 90 degree V-6. Sound quality will be significantly different when comparing a flat opposed 4 cylinder engine verses an in-line one - they can have the same firing frequency at the same RPM but the in-line 4 may have a two plane crank or crank pin offset which significantly affect sound quality. Another example would be to compare the Harley V-twin verses a twin cylinider opposed BMW - same firing frequency but markedly different sound quality; one is even firing and the other NOT. Ever hear an International Titan and compare that to a two cylinder John Deere - both are two cylinder configurations with the same fundamental firing frequency but that's about it. There's NO mistaking one for the other.

Nuff said. Just be happy when you can guess the # of cylinders correctly when you listen to the sound of an engine and appreciate/marvel at them ALL, two or four cycle, gas or diesel, etc.

Even engines with similar or the same bore and stroke and similar cam and cam timing can sound totally different. We had a race engine built by Parky Nall who at that time worked for Holman and Moody in Charlotte, NC. It had the most distinct sound you ever heard in a Ford engine. We had later engines built at the same shop by Tommy Turner with the same cam grind, same headers, and similar internal parts, but that Parky engine was the darndest sounding engine we ever had... and the fastest for a 355ci.

Thanks to all who responded!
Not saying I understand every detail of your learned explanations, but it is clear that there is much more to engine sound than firing frequency.
Plus side is that we often can hear that there is something wrong with an engine (provided you are familiar with it).

Thanks again, Hendrik

One little detail here. Firing cycles are not in hertz. Yes they fire so many times per second, but blow down time for a cylinder has nothing to do with cycles per second. Blow down is just an ordinary exponential time constant. It takes so long to blow down and that is that.

What it means is that a cylinder pops a pulse into the pipe. Separation time between pulses is the major factor in all exhausts. It is why you cannot turn a 3 cylinder engine at a higher speed and make it sound like a 4 cylinder, or a six, or an 8. It is why they all sound different. Putting more cylinders on the same pipe eventually causes the pulses to run together, to the point that an 8 or 12 has almost no distinguishable pulses at all.

I will NOT dis-agree or call into question how or what others use to note various engine physical or operational characteristics. In terms of describing engine sound quality, however, firing frequency (HZ) is at LEAST one of the terms used to characterize engine noise by engine and/or sound quality analysts. Cycles or Hertz are also basically the same term as each refers to a repetive action usually stated for a given period of time (CPM - cycles per minute, CPS - cycles per second, or CPD - cycles per day, et etc.) The term Hertz (HZ), however, is ONLY indiciative of the # of cycles per second. It is true that when looking at spacing between firing "pulses", the higher # of cylinders are hard for the human ear to perceive as a "pop". The human ear can usually notice the different sound quality for these engines running at the same rotational speeds. Sometimes it's much harder for the human ear to distinguish between engines of different cylinders but at the same firing frequency. With proper measuring equipment, however, one can ALWAYS objectively/repeatedly determine these firing or other sound quality differences.

It appears that I am in a topic surrounded by engineers here but I will try my best. In regard to the original question, the reason for a different "sound" between a 4 cyl, 6 cyl, V8, etc. is 75% crankshaft and 25% exhaust pipe period! If we study the geometry of a 90deg 4 rod journal crankshaft rotating in a 90deg block then we see that there is only 2 possible firing orders for this congiguation (unless 2 cylinders were fired together, we will ingnore that experimental stuff). One firing order would yeild XOXXOXOO on the right bank and OXOOXOXX on the left bank. This is the firing order for a 390 Ford, the Clevlend engines used the other possibility. The reason that there are many listed "firing orders" for V8"s is because the manufacturers number their cylindres differently. It reality there are just the 2 possibilities.
When we look at the firing order layout above we see an uneven rythymn in each bank. This is what creates the distinctive V8 "sound". Headers enhance this sound because the tubes are of equal length dumping into the collector, keeping the uneven rythymn going all the way through the exhaust. The pulse rythymn gets somewhat lost in the cast iron manifold of unequal length. This uneven pattern is eliminated with the use of 180deg headers on a V8 (seems to be a craze these days because the Ford GT uses them). This gives a sound of 2 large 4 cylinder engines side by side. 180 deg headers were tried for a while in the mid 70"s but died out. We didnt see any real power gains using them.
I have used the V8 configuration as an example but this holds true for all engines. Another example is the "Zoomies" used in demolition cars. If they are all pointed straight up you hear just lound noise as compared to a dragster where you hear mostly 1 bank of cylinders with an eneven rythymn.
This can be played with by using a sequencer type program available many times free on th web.
Just carefully chart out the pattern as I did above for your engine and place the pattern in the sequencer then play it. If you want to simulate a dual exhaust type engine just change the tone slightly for each pipe. Most sequencers are designed for music so the sound would be like a slow engine. You could set it for 64th notes at 200 beats a minute or faster or maybe there are other programs out there to do this.
One other example was the old 6 cyl GMC of the 50s. Split manifolds were a fad for a different sound.
To the poster that went to Parky Nall, you spelled it correct, I used to work for him. He never worked at H-M but knew John Holman since the 50"s when he did work for Bill Stroppe. I must have left before you came to him as I dont remember a puller engine. I dont know why it sounded strange as you say. His engine of choice then was a 351 windsor with 351 Boss heads, his "Windsor Boss".
Hope this helps.

well, no one has stated the obvious:

There is no sweeter or tone pure sound than a well tuned F-20, with a 90 degree exhaust on it, under load.

Makes the New York Philharmonic sound like a grade school band!

LOL!

Tool...., that was NOT a puller engine, it was a dirt track late model engine. I was a 351 with "Boss" heads and the high exhaust port plates. Did Parky use H-M's dino rooms, because although I wasn't at the H-M shop until later I was sure the engine was picked up there? I'll see the car owner at a local b'ball game tonight and will try to remember to ask. Pound for pound (or cube for cube) that was the hottest engine we ever had. We later had a 383 that Tommy built that I don't think would have out run that 355. This was the days of the old NDRA and when they came to our local track for a show, they said they had never seen a Ford that fast. BTW, I'm [u:100b38868d]pretty[/u:100b38868d] sure this was in '79.

Update for Toolmaker.... talked to the LM owner that I crewed for at a b'ball game tonight. I was wrong... over 30 year and I guess my memory has faded. Parky had his own shop and we didn't get engines from Holman (Tommy Turner who built for the Wood Bros.) until after Parky quit building... something like he got married and moved to Fla.? Bernie Piper in Denver had been our builder and he lost his shop in a divorce and put us onto Parky. The one engine he built for us was a standard Cleveland 2 bolt main block that Parky made 4 bolt caps for and pinned the cylinders to strengthen them, and had the heads modified for high port plates. Ended up cracking a cylinder wall after running most of the '79 season. Anyway, it was one heck of a ground pounder engine.