Exhaust tonality

Here is a question that has bothered me for years, decades even, and nobody has ever given me a satisfactory answer:

Why do different engine configurations produce different sounding, readily identifiable exhaust notes?

For instance, there is no confusing the nasal bark of an in-line six cylinder engine with the mellifluous sound of a V-8. A flat-opposed six cylinder engine (Porsche, Subaru) has kind of a "soft-edged" sound compared to the sharp edge of an in-line six. Ditto 4-cylinder engines. An in-line 5-cylinder engine sounds just... wrong when I try to relate it to any of the more usual configurations--kind of a blend between a 6 and an 8. It's not likely that many of you have ever heard a straight-8 engine, but It doesn't sound like a V-8 or a straight 6.

These are all engines that fire at even intervals through the 720 degree crankshaft rotation of one combustion cycle, so all that should matter is how many firings occur in a given time. A 4 cylinder engine turning 6,000 RPM should sound the same as a 6 cylinder engine turning 4,000 RPM should sound the same as an 8 cylinder engine turning 3,000 RPM. In other words, each of those engines is producing 200 explosions each second at even intervals and passing them on to the exhaust system.

Now, one prominent exception is the old 305cc and 250cc Honda twins from the 1960's. The 305cc engine had a single-plane crankshaft (that is, the pistons went up and down in unison) while the 250cc engine had a 180 degree crank where the pistons worked in opposition. Due to the uneven firing intervals mandated by the 180 degree crank, the 250cc engine had a completely distinctive sound that could never be mistaken for anything else -- but this could be attributed to the uneven distribution of firing impulses.

So the question remains: why do different engine configurations with even firing distributions have noticeably different, readily identifiable exhaust sounds?

tanstaafl.

At a guess, I'd say that it has more to do with header/manifold configuration. The header pipes are of different lengths to time the pulsing exhaust gases and I'll bet that it has a lot to do with the sound -- how those exhaust pulses are interleaved. Manifolds, obviously, are less well tuned and probably have the same effect, if different.

It probably also has a lot to do with the bore of the cylinders and the width of the exhaust path. I wonder if you fitted a wider but otherwise equivalent ehaust pipe if it would change the note?

But, like I say, it's just a guess.

I'm told that auto manufacturers take great care in tuning the note of the exhaust system in sports cars, as it's an essential part of the ownership experience. I seem to recall this being a very particular thing with the Mazda Miata, for example.

Didn't you used to play the trumpet, Doug? You of all people should know how tubing affects sound...

Great question. I've often wondered how a Ford and a Chevy engine of similar size and configuration can sound different. Obviously mufflers and headers can make a difference but excluding the impact of those, the actual engine configuration apparently makes a difference.

Googling I found this:

Basically, it's firing order that gives it the cadence.

1) Essentially no common automotive engines have two cylinders firing at exactly the same time.

2) A four cylinder engine has a 1342 firing order with one cylinder firing every 180 degrees of crankshaft rotation. A four cylinder engine would be perfectly balanced only if it had infinitely long connecting rods. Only in that case would the velocity of a piston as a function of time be described by a pure monochromatic (single frequency) sine wave. Only in that case would the forces generated by the acceleration of the pistons near the bottom of their stroke exactly balance the forces of the pistons near the top of their stroke. With finite length connecting rods the piston velocity contains harmonics (multiples) of the fundamental frequency of the sine wave describing the motion of a piston. Engineers would call these Fourier components. Others often refer to these as secondary vibrations. Unlike the fundamental, some of these do not balance out, so there is some residual vibration.

3) A straight six has a 153624 firing order with one cylinder firing every 120 degrees of crankshaft rotation. A straight six is perfectly balanced.

4) A V8 can have various firing orders. But, there is still just one cylinder firing every 90 degrees of crankshaft rotation. A V8 is two four cylinder engines arranged at 90 degrees to each other, sharing the same crankshaft. In that sense a V8 shares the imperfect balance of a four in the plane of each bank. In addition, there is a certain component of fore-and-aft rocking vibration which arises from the fact that though each connecting rod shares a crank journal with the corresponding rod on the opposite bank, they of course are offset on that journal by their thickness. The off-beat of the V8 exhaust note arises from the fact that, no matter what the firing order is, the firing on a given bank is not even. Since in almost every case the exhaust is fed into one exhaust manifold for each side, and then into one pipe for each side, there is an off-beat. If you combine the exhaust from the outer cylinders on one side with that from the inner cylinders on the other side, you get a nice even sound. Think of the
bundle-of-snakes exhaust on an F1 car or an old GT-40. They gave a nice ripping canvas sound. Alternatively, think of the nice honking sound a big four barrel carb on a V8 makes when you floor it. The intake manifold on a V8 connects the intake ports in a manner identical to the way a bundle-of-snakes exhaust system connects the exhaust ports.

5) A V12 is commonly arranged like two straight sixes with 60 degrees between them. One cylinder fires every 60 degrees of crankshaft rotation. Think of it as evenly splitting the 120 degree firing interval of each bank taken separately.

6) I'm not familiar with V6s. I assume that a 60 degree V6 should fire every 120 degrees and sound like a Corvair. The few 90 degree V6s made by casting 3/4 of a V8 are pretty raggedy

Cars with carburetors (vs EFI) can add variation to the pressure waves produced by introducing air&fuel unevenly. They can also change the evenness by firing the spark plug on the exhaust stroke (wasted spark).

I was surprised that I couldn't find a website that offers "sheet music" or waveforms of different engines. I did find several articles on how Harley Davidson motorcycle are designed to produce its distinctive sound.

I also found new sounds for the empeg. http://www.autospeak.com/library.htm

At a guess, I'd say that it has more to do with header/manifold configuration


I don't think that is the case.

For example, take a drag racer (A-modified, AA fuel, funny car, your choice) where the exhaust system consists of eight equal-length straight pipes coming right out of the cylinder head. The sound is unmistakably V-8. I have heard other engine configurations with similar exhaust setups, and they are readily identifiable by their sound.

AcuraSquirrel... where are you when we need you?

tanstaafl.

All I can say is the rotary engine on the le mans car I watched sounded like sick dog combine with an impact gun .
Other than that Im not that sure everything sounds the same to me......at 8000+ rpms.

I have to second Tony, They do spend time and effort tuning the exhaust to produce a particular sound. Sometimes they add extra resonators to produce a given tone. Luxury cars are alomst silent, others well....