There is no denying that a Harley-Davidson motorcycle has a unique sound, especially if the mufflers have been removed! Even with the mufflers on, however, it sounds different. The reason for the sound has to do with the way the engine is designed.

If you have read the HowStuffWorks article How Car Engines Work, then you know how a basic four-stroke gasoline engine operates. A piston goes through the intake, compression, combustion and exhaust strokes every two revolutions of the crankshaft. When your lawn mower is idling, you can hear the pop-pop-pop-pop sound of the individual strokes. What you are actually hearing is the sound of the compressed gases in the cylinder escaping when the exhaust valve opens. Each pop is the sound of the exhaust valve opening one time, and it happens on every second revolution of the crankshaft.

In a two-cylinder, horizontally opposed engine, the pistons are timed so that one fires on one revolution of the crankshaft and the other fires on the next revolution -- so one of the two pistons fires on every revolution of the crankshaft. This seems logical and gives the engine a balanced feeling. To create this type of engine, the crankshaft has two separate pins for the connecting rods from the pistons. The pins are 180 degrees apart from one another.