What gives a Harley-Davidson motorcycle its distinctive sound?  

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 from other motorcycles. 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.

A Harley engine has two pistons. The difference in the Harley engine is that the crankshaft has only one pin, and both pistons connect to it. This design, combined with the V arrangement of the cylinders, means that the pistons cannot fire at even intervals. Instead of one piston firing every 360 degrees, a Harley engine goes like this:

* A piston fires.
* The next piston fires at 315 degrees.
* There is a 405-degree gap.
* A piston fires.
* The next piston fires at 315 degrees.
* There is a 405-degree gap.

And the cycle continues.

At idle, you can hear the pop-pop sound followed by a pause. So its sound is pop-pop...pop-pop...pop-pop. That is the unique sound you hear!