What gives Harley-Davidsons their distinctive sound?

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What gives Harley-Davidsons their distinctive sound? 
Some engines hum. Some whine. Others roar. But Harley-Davidson motorcycle engines rumble. Harleys produce a low, deep-throated, somewhat syncopated vibration many aficionados describe as "potato-potato-potato-potato." The sound is so distinctive - and so closely associated with the brand - that Harley-Davidson once sought to have it trademarked. (The company ultimately withdrew the application.)
What exactly gives Harley-Davidson motorcycles their distinctive sound?
First, it's important to understand that engine noise is produced by compressed gases escaping from an engine cylinder through the exhaust valve opening. In a normal two-cylinder engine, the crankshaft has two separate pins for the piston's connecting rods. With the spark plugs positioned 180 degrees from each other, each of the two pistons fire on every-other revolution to produce an exhaust noise that is even, smooth and balanced.
Harley-Davidson engines are unusual in that they have a V-shape and only a single pin to which both pistons connect. (This was originally done to simplify the engine design and minimize costs.) To make this arrangement work, the spark plugs had to be positioned in a 45-degree arc. As a result, after the first cylinder fires, the second has to wait until the shaft has completed 315 degrees of rotation. Then there is a 405 degree gap until the first cylinder fires again. This asymmetrical arrangement is what gives Harley engines their distinctive, deep bass rumble.
Learn to service Harleys at WyoTech in Daytona Beach
If you'd like to become a professional Harley-Davidson motorcycle mechanic, you can acquire the knowledge and skills you need to go pro at WyoTech in Daytona Beach, Fla.
The 18-month program combines general motorcycle technology career training with a Harley-Davidson specialty concentration. Along with "core" Motorcycle Technology programs, you'll study:
*Harley-Davidson engines
*Harley-Davidson engine controls
Students get hands-on experience in the disassembly, inspection and assembly of Harley's Big Twins engines. They also learn the measurements and adjustments critical to proper engine operation