Interesting debate how a Vulcan V-twin is different than a HD V-twin

[ubernomad]

I'm a very curious sole. I love to learn about things. One of the things I've been digging into lately is why/how the unique sounding Harley Davidson V-twin engine sounds so much different compared to the sea of Japanese "metric" V-twin engines. Google searches turned up all kinds of info, correct or not, but I stumbled across this discussion which had me thinking about how the 1500/1600 Vulcan engine operates compared to the Harley Davidson engine. Both are single pin cranks. Both are fuel injected. Both a narrow angle V-twins.

So, for those who care to share.....why doesn't the Vulcan V-twin engine sound anything like a similar H-D v-twin?

http://www.justanswer.com/motorcycle...ato-thing.html

It's not the potato potato thing that makes me curious. That's a simple explanation.

My question is -- and no one has ever been able to provide a really good answer:

The HD V-Twin, while at idle, has a noticeable 'miss' or a 'pause' every few cycles.

We love the idle sound and cadence. We agree it's unique. Other single pin crank V Twin bikes don't have the HD cadence at idle. We don't understand why.

Whatever you want to call it: Skip. Misfire. Pause. Lope. Cadence.
Why does it do it.
And HOW does it do it.

If it is a "designed-in" random idle misfire to get that cadence - how does HD get away with it.

Listen to any Harley at idle. Carb, FI, stock or custom pipes, doesn't matter. At idle, every few rpms, it "skips" a beat, sounds irregular. I'm not talking about the natural uneven firing rythm due to the single throw crank. I'm referring to the "SKIP". The resulting rolling sound is wonderful.

What causes it. I can understand sloppy carbueration on the older models, an occasional 'hiccup' at idle.

Even the modern Harley FI motors, the ones supposedly so carefully computer controlled... how can they have that idle 'skip' or hiccup. To me, seems impossible. Or implausable.
My 2011 103" Street Glide is fuel injected, yet idles like an old carbuerated model. I love it. But with modern fuel injection, just don't understand how it can happen. Properly running fuel injected engines simply don't misfire at idle.

But mine does, and so does every other Harley on the planet.

A designed-in idle misfire? To create the unique rolling idle sound with the occasional skip?
Just can't see any way an idle misfire could sneak past EPA. The HC ('unburned') emissions would blow off the scale.

I grow weary of tired old explanations like "it's the pushrods" or "the unique HD valve angle", it's all a bunch of nonsense. Those things by themselves don't make an engine occasionally miss a beat at idle. It has to be something associated with fuel and or ignition delivery at IDLE, maybe combined with some weird cam overlap profiles, to produce the irregular sound. Something is causing the engine to skp a beat, to produce the Harley stutter step idle quality. So, what is it.

Answer according to the "expert":

the cause of the miss at idle....and occasionally when riding....is the result of the way the entire engine is designed, not just the common crank pin.

the problem comes from the "siamesed" intake system. this does two things. first, it causes the front cylinder to run leaner than the rear and occasionally results in an upset of the air flow to the rear cylinder, resulting in a lean mis-fire.

it causes the front cylinder to run leaner than the rear due to a combination of the valve timing. it was designed this way to account for the front cylinder receiving fresh cooling airflow and the rear cylinder receiving warmer airflow off the font cylinder. most people mistakenly assume the rear cylinder runs hotter than the front on a Harley. it does not. it was designed to run cooler.
the issue exists in fuel injected bikes, but is controlled by the ability of the fuel injection to offer a different air/fuel mixture to each cylinder.

the lean mis-fire will always exist in 4 degree V twin Harleys. even fuel injection cannot compensate for the occasional air flow disturbance to the rear cylinder.
basically....the air is drawn into the common intake and then must either go right or left, in uneven intervals and, during valve overlap there is another airflow component which is exhaust from front to rear or rear to front. occasionally, the airflow to the rear cylinder stalls.
the fuel injection can only provide a fuel injector pulse for the anticipated air flow when the intake valve opens, but it cannot compensate when the airflow gets disturbed at that moment and only for a fraction of a second.
this effect is more evident on older models with points triggered ignitions idling at about 600 rpm. an added affect in that system is that when the cylinders fire, the crank is accelerated and the advance flyweights open up and advance the ignition, then the flywheels coast down to the next power pulse and the flyweights pull in, retarding the spark.
the issue, although still prevalent with electronic ignitions, is less noticeable due to a constant advance input at idle and by setting the idle rpm to 1,000 rpm.

I'm trying to wrap my head around how these engines operate from a engineering/design standpoint.