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[jbust3]

A piston goes through the intake, compression, combustion and exhaust strokes every Two (2) revolutions of the crankshaft. When a single cylinder (Four-Stroke) like a lawn mower is idling you can hear the pop-pop-pop-pop-pop-pop-pop sound at regular (steady) intervals. It's the 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. That happens on every other (2nd) revolution of the crankshaft.

Normally if it's an engine with two cylinders, the pistons are timed so that one fires on one revolution of the crankshaft and the other fires on the next revolution. (One of the two pistons fires on every revolution of the crankshaft) This gives the engine a smooth balanced feeling. To get this type of engine, the crankshaft has two separate crank pins for the connecting rods from the pistons to connect to. The pins are 180 degrees (exactly half way around a circle, since there are 360 degrees in a full circle) apart from each other.

A Harley engine has two pistons. The difference in the Harley engine is that the crankshaft has only one pin, and both pistons are connected to it (the single pin) through their connecting rods. This combined with Harley's (unique Degree of the "V") arrangement of the cylinders means that the pistons can't fire at even intervals. Instead of one piston firing every 360 degrees, a Harley engine does 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 so on...
So at idle you can hear the pop-pop sound followed by a pause. So its sound is: pop-pop-----pop-pop-----pop-pop-----pop-pop-----pop-pop. That is what makes the Harley, the only bike that can sound like it does. There is a patent, on the design of their V-Twin so others can't duplicate it. That design is also the unique sound you hear! Plus, it being found to be the Optimal degree for the duration of the cylinder timing (the 45 degree V) to get maximum power, with much less (Rated) Horsepower, than the (actual) power to the rear wheel, actually is.

"Even with a very quiet exhaust system, the uniqueness in sound is still there." I prefer to say it as: Potato, Potato, Potato, spoken relatively fast, does quite a good job of defining how they sound. I don't think ya can get much closer. Say it out loud, and you'll see what I mean. The Po in Potato being the power.

Now, for you to figure, since many will say: Yes, but my Honda has a single pin crank too. Why isn't there a way for me to get that same sound with the right pipes?

Yes, some of Honda's and others use a single pin in their V-twin's But read the above again. And then look in your manual. The degree of the engine, and therefore, the piston timing, Cam and Valve timing and much, much more is quite different. As well as the stroke, head design, exhaust design and much more.

That's the reason why the sound will always be different, regardless of the pipes installed.