Engines have pistons that compress then decompress in an engine and whenever the piston compress it is actually compressing a type of fuel (idk what it's called) and the spark plug of that piston sparks and ignites the fuel. This launches the piston to the other side where another spark hits and it returns. The sound of a motorcycle running is actually all those sparks going off and it's just that some motorcycles have more sparks/more powerful spark plugs.

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In general if two equivalent size engines have differences in cylinder numbers the sound will be from smaller cylinders, moving faster.

A 650 twin will have 2x 325 CC pistons firing in a specific order. Some with a 90 degree offset, others with 270. This just means “if one cylinder fires, where is the other one”. This can also have an effect on the sound since the combustion of gas in the cylinders is what causes the noise.

So there’s two larger cylinders firing in a different order.

In a higher cylinder configuration like a V4 or inline 4, the firing order is different, but also there are much smaller cylinders.
Generally a 600cc will have about 150cc cylinders, but firing much faster to get the same power.

It’s the difference of “big pops slower, or smaller pops much faster” and a little bit of what order the pops happen in.

If you want to know more about Harley's, this is a good video.

https://www.youtube.com/watch?v=XeuXShFIgyc&t=118s

In Harley's case, 2 pistons are mounted at the same point on the crank shaft, so the firing has to be 45 degrees offset, since the cylinders are 45 degrees offset.

Harley also fires both spark plugs at the same time. These two factors lead to a different sound when comparing it to other bikes.

> A 650 twin

Ah, gotta chime in with this, talking about unique sounds...

Vintage Triumph 650cc Bonneville's parallel twin engine have an even different unique sound than same cc v-twins.

This from the two pistons traveling up and down together, while the firing order is 180 degrees apart. That is, one is on compression while the other is on exhaust.

Music to my ears, hah!

The question has been answered - It depends on cylinder size, number, offset, etc.

I also thought it is worth noting here that Harley Davidson in particular chose to keep inferior engine designs in favor of keeping the terrible sound. 'Merica...

Every time an exhaust valve opens in a cylinder, the residual pressure of the exhaust gases is suddenly expelled thorugh the exhaust, causing a loud "pop". In most engines, the pops are regularly spaced, creating a regular "tone". In the V arrangement, two pops happen close after one another, and then there's a longer delay before the next two. This causes the characteristic noise.