How Do Motorcycle Engines Affect Sound?

Motorcycles have every different number of cylinders that you can imagine: singles, twins, triples, fours, and at least a couple of sixes. Single-cylinder engine? Generally speaking, an uninteresting sound, like a lawnmower. British twins fired at 360 degrees—an even firing order—and made a kind of droning sound. Lovers of British twins like that sound.

Possibly a more interesting sound is made when you move those cylinders apart to make a V-twin. Now you have a difference in the firing order so there is a quiet time, then a cylinder fires, a shorter quiet time, and another cylinder fires, which is why V-twins have more like a V-8 sound. Triples usually have their crankpins at 120 degrees, and they produce a wonderful musical sound, which I have enjoyed since the first time I heard a Kawasaki H1 in 1969.

Most inline-fours have a so-called "flat" crankshaft with their crankpins at 180 degrees so the cylinders in the middle are at top dead center when cylinders one and four are at bottom dead center. As a result, the engine fires twice per revolution and the firing interval is always the same. On a high-revving engine, a flat-crank inline-four produces the famous GSX-R screech. MotoGP rider Valentino Rossi liked a 90-degree crankpin angle better than a 180 so that's what Yamaha gave him in 2004. A 90-degree-crankpin inline-four makes a throatier, more V-8-like sound.

BMW's touring six is one of the smoothest engines there is, and it also produces a very smooth sound, like that of a Jaguar inline-six or a Merlin V-12. They all have a quite civilized purr. The Honda Gold Wing is a flat-six, the aim being to have enough cylinders that they can cancel each other's shaking forces, leaving the engine smooth enough for touring.

Engine sound also has a component that is determined by how fast the exhaust valve opens. A two-stroke is an example of an engine whose exhaust is released very rapidly. This produces a very steep wave front, which people call that “ring-ding” two-stroke sound.

A four-stroke can approximate that by having very fast-lifting valves, but if you are trying to comply with a federal sound regulation and your styling department has told you exactly how big your mufflers can be, you may have to redesign your camshaft so that you release the exhaust more slowly, resulting in a less-steep sound front that turns out to be easier to muffle.

Mufflers usually take the form of a stack of quarter-wave tubes, each of which cause noise to be self-canceling within a certain range. By choosing which quarter-wave tubes are placed in the muffling system, you can vary the characteristic of the sound. I believe a great deal of science goes into this.

When you hear a crisp, slightly sibilant, but sharp in the upper ranges sound from a stock muffler and you find it pleasing, it is the result of acoustical engineering. They know what you want, and they aim to give it to you. Otherwise, the nature of the engine itself determines the basic sound.

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for Cycle magazine and, since 1992, for Cycle World. Kevin’s unparalleled experience and knowledge of the sport were—and continue to be—prompted by a lifetime of curiosity. His willingness to share that information with anyone who is willing to listen is likewise unique.

Kevin’s greatest strength lies in his ability to present complex subjects in simple terms with clarity and, often, humor. In this video series, shot in his home shop, Kevin draws upon his vast historical references to address modern-day questions. As Kevin has written, “Emotions bring us to engineering, but engineering then becomes a special way of confronting reality.”