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Speaker 1: Audible dot com is the leading provider of digital audio

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Speaker 1: Go to audible podcast dot com slash stuff brain to

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Speaker 1: get a free audio book download of your choice when

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Speaker 1: you sign up today. Welcome to brain Stuff from how

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Speaker 1: stuff works dot com where smart happens. Hi Am Marshall

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Speaker 1: Brain with today's question, what gives a Harley Davidson motorcycle

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Speaker 1: it's distinctive sound? There's no denying that a Harley Davidson

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Speaker 1: motorcycle has a unique sound, especially if the mufflers have

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Speaker 1: been removed. Even with the mufflers on, however, it sounds

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Speaker 1: different from other motorcycles. The reason for the sound has

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Speaker 1: to do with the way the engine is designed. In

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Speaker 1: a basic four stroke gasoline engine, a piston goes through

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Speaker 1: the intake, compression, combustion, and exhaust strokes every two revolutions

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Speaker 1: of the crank shaft. When your lawnmower is idling, you

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Speaker 1: can hear the pop pop pop pop pop sound of

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Speaker 1: the individual strokes. What you're actually hearing is the sound

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Speaker 1: of the compressed gases in the cylinder escaping when the

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Speaker 1: exhaust valve opens. Each pop is the sound of the

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Speaker 1: exhaust valve opening one time, and it happens on every

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Speaker 1: second revolution of the crank shaft. Now think about a

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Speaker 1: normal two cylinder engine. It has two pistons. The pistons

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Speaker 1: are times so that one fires on one revolution of

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Speaker 1: the crank shaft and the other fires on the next revolution.

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Speaker 1: So one of the two pistons fires on every revolution

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Speaker 1: of the crank shaft. This seems logical and it gives

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Speaker 1: the engine a balanced feeling. To create this type of engine,

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Speaker 1: the crank shaft has two separate pins for the connecting

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Speaker 1: rods from the pistons. The pins are a hundred eighty

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Speaker 1: degrees apart from each other. A Harley engine also has

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Speaker 1: two pistons. The difference in the Harley engine is that

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Speaker 1: the crankshaft has only one pin and both pistons connect

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Speaker 1: to it. This design, combined with the v arrangement of

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Speaker 1: the cylinders, means the pistons cannot fire at even intervals anymore.

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Speaker 1: Instead of one piston firing every three hundred sixty degrees

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Speaker 1: on the crankscheft, a Harley engine goes like this. The

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Speaker 1: first piston fires, the next piston fires at three hundred

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Speaker 1: and fifteen degrees. Then there's a four hundred and five

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Speaker 1: degree gap, and then a piston fires. The next piston

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Speaker 1: fires at three hundred fifteen degrees. Then there's a four

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Speaker 1: hundred five degree gap, and then this cycle continues. At idle.

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Speaker 1: You can hear the pop pop sound followed by a pause.

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Speaker 1: So it's Papa, Papa, pa pap. That is the unique

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Speaker 1: sound you hear. It's caused by that gap in the timing.

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Speaker 1: For more on this and thousands of other topics, visit

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Speaker 1: how stuff works dot com. Audible dot com is the

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Speaker 1: leading provider of digital audio books and spoken word, with

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Speaker 1: over one thousand titles to choose from. Audible lets you

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00:03:15,240 --> 00:03:18,320
Speaker 1: listen to your favorite books anywhere, any time. Go to

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Speaker 1: audible podcast dot com slash stuff brain to get a

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Speaker 1: free audio book download of your choice when you sign

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Speaker 1: up today.