What gives a Harley its distinctive sound?

A Harley Davidson motorcycle emits a very distinctive sound because of the unique way its engine works. Find out more in this podcast from HowStuffWorks.com.

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Episode Transcript

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(00:29):
Brain with today's question, what gives a Harley Davidson motorcycle
it's distinctive sound? There's 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

(00:50):
to do with the way the engine is designed. In
a basic four stroke gasoline engine, a piston goes through
the intake, compression, combustion, and exhaust strokes every two revolutions
of the crank shaft. When your lawnmower is idling, you
can hear the pop pop pop pop pop sound of
the individual strokes. What you're actually hearing is the sound

(01:13):
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 crank shaft. Now think about a
normal two cylinder engine. It has two pistons. The pistons
are times so that one fires on one revolution of

(01:36):
the crank shaft and the other fires on the next revolution.
So one of the two pistons fires on every revolution
of the crank shaft. This seems logical and it gives
the engine a balanced feeling. To create this type of engine,
the crank shaft has two separate pins for the connecting
rods from the pistons. The pins are a hundred eighty

(01:56):
degrees apart from each other. A Harley engine also 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 the pistons cannot fire at even intervals anymore.

(02:18):
Instead of one piston firing every three hundred sixty degrees
on the crankscheft, a Harley engine goes like this. The
first piston fires, the next piston fires at three hundred
and fifteen degrees. Then there's a four hundred and five
degree gap, and then a piston fires. The next piston

(02:38):
fires at three hundred fifteen degrees. Then there's a four
hundred five degree gap, and then this cycle continues. At idle.
You can hear the pop pop sound followed by a pause.
So it's Papa, Papa, pa pap. That is the unique
sound you hear. It's caused by that gap in the timing.

(03:00):
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