taken in whole from Cyclefish.com; "What Gives a Harley-Davidson Its Unique Sound" Posted by RoadCaptain August 4, 2011
Why does the Harley-Davidson V-Twin have such a unique rhythm??
Engine noise is produced by the compressed gases exiting the cylinders though the exhaust valves and then through the exhaust system where they are either muffled or amplified depending on your exhaust system and muffler setup. In a typical two cylinder motor the crankshaft has two separate pins for the pistons’ connecting rods and the spark plugs are positioned 180° from each other. Each piston fires on every other rotation of the crankshaft and with the sparks plugs at 180° from each other; this provides a smooth, even rhythm sound.
The Harley-Davidson two cylinder v-twin engine is designed differently. The H-D V-twin has a single pin connecting the pistons, which was originally done to reduce manufacturing cost. To make this unique configuration work, the spark plugs were positioned in a 45° arc. So after the first cylinder fires there is 315° of rotation before the second cylinder fires and then 405° until the first fires again, and so on, and so on, giving the non-symmetrical distinct potato-potato-potato sound. This sound is so tied to the brand that Harley-Davidson even applied for a trademark on it.