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.