The Harley Sound

Harleys have a unique sound, a symphony to those who know the breed. The idle is an irregular, lumpy rumble; utterly unmistakable. And it all comes about because of an engine design that no modern engineer would use on a bet. But we love 'em, and wouldn't have it any other way.

Before you can follow this explanation, you must understand a couple things. First of all, if you don't understand about the four cycles of power generation (suck, squeeze, bang, and blow), you probably won't be able to follow this explanation.

Then there are a few more things about the design of the engine you should understand, and a couple of terms.

The angle of the V of the cylinders is 45 degrees.

The crank rotates counterclockwise when viewed from the left side.

"TDC" means Top Dead Center, that is, the piston is as high as it can go, and is about to start down. A piston is at TDC twice in each power cycle, once on the compression stroke (at which point a spark fires the fuel,) and once on the exhaust stroke, when it is blowing out the burned gas.

"BDC" means Bottom Dead Center.

While most engines have a crankpin for each piston, a Harley as only one crankpin, shared by both pistons. That means the pistons go up and down ALMOST together. Because of the V shape of the engine, one piston will run ahead of the other by the angle of the V, or 45 degrees. In this case, the rear one leads the front by 45 degrees of crankshaft rotation, or about 1/2 inch of travel in the cylinder.

OK - let's walk through a firing sequence, beginning with ignition in the front cylinder.

The front cylinder fires, pushing the piston down on a power stroke, spinning the crank. The rear piston is already 45 degrees down its bore on an intake stroke, being pulled by the crank, sucking in a load of fuel. Make sure you can mentally visualize this or you're gonna get lost.

For the sake of simplicity of explanation, I am saying that the spark fires the fuel mix at TDC. In fact, it fires it before TDC by up to 50 degrees of crank rotation, depending on the engine and the rpm level. This is "timing." There is an article to explain timing in this section. Also, valves actually open and close before and after TDC and BDC.

When the front piston reaches bottom dead center on its power stroke the rear piston is 45 degrees up on its compression (squeeze) stroke, pushing the fuel it pulled in up into a shrinking chamber. In other words, it's getting ready to fire, right?

The front piston, having delivered its power, is now starting up on its exhaust (blow) stroke.

At this time there is no power stroke going on in either cylinder. The engine is being turned over by the intertia of the spinning flywheel.
The rear piston arrives at its firing point on the compression stroke and a spark fires in the rear cylinder, 315 degrees (360 degrees - 45 degrees in the V) after the front did. That puts the rear cylinder into its power stroke, and the front is on its intake (suck) stroke, being pulled down by the crank.

The front cylinder goes down on intake, then back up on compression, then fires 405 degrees (360 + 45) after the rear cylinder did.

So here's the firing order.

Front Bang, rotate 315 degrees - Rear Bang, rotate 405 degrees - Front Bang, rotate 315 degrees, - Rear Bang, etc.

So there it is. The engine sounds like it does because the firing order is irregular, it kind of lopes along, as it has since 1903. And probably will until the next century, if the traditionalists have their way. I hope so.