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
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