This is why it sounds like potato potato

[Lone Wolf in Canada]

Thanx, Rensselaer, I believe you are correct. And ed in az for the web page,
I read a book that told me they move together, I just couldn't believe it. Thought there would be more vibration as one post here said. But then, a single cylinder moves up and down solo and it feels smooth.
Hope this question informed others also.

[Turbota]

Since this thread is something titled "Potato Potato" .... I can tell you one thing. If you use a 2-1 pipe on your Harley. you can pretty much forget about the "Potato" sound.

In fact, my bike with an RB Racing pipe don't even sound like a Harley anymore ... What does it sound like? ... I have no idea.

[ctilson]

Here is a page on Harley sound from a how H-Ds work article.

http://science.howstuffworks.com/harley3.htm

[dmraco]

Originally Posted by xl1200r

Techi person, here I go...

The pistons move sort of together, they are actually 45* apart on a common crank pin. What was said is true, they alternate which piston is firing. So while on piston fires and forces both down, the other has the intake valve open and is drawing in fuel (4 strokes...Down(intake), up(compression), down(ignition), up(exhaust).

What makes the "patato patato" sound is that the engine is "odd-fire", in that the spacing between the firing is not equal. You get cylinder 1 to fire, the crank turns 315*, then cylinder 2 fires, but the crank has to turn another 405* for cylinder 1 to fire again. This is due to the 45* piston angle and the common crank pin. This means that there's almost a 30% difference in the amount of time it take for each cylinder to fire from the other cylinder firing.

I wish I could find this animation of the whole thing happening - it's be much easier to understand when you see it.

Try this:

Hope that helps.

That is extremely helpful!!! Learn something everyday!

[kiwinz]

Garden Diagnostics :
Tomato Tomato /poss exhaust leak or tappet
Zuchini Zuchini /poss intake leak or clogged exhaust
Carrot Carrot /small end clearance excessive

[Jimbo999]

My old AJS twin would hop on idle due to both pistons moving up and down
together. Smoothed right out on acceleration. It was kind'a cool in a way.
Sitting at a stoplight the forks would slide about 1".

[juddster]

I heard the the reason why the pistons rotate so close together is because the engine is based loosely on an old 10 cylinder bi-plane engine; the engine is 2/10ths.

[sprtrjl]

I like this explanation:

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.

[MadMax25]

XL1200R wrote: 'The sentence in the link is worded poorly, but what it's getting at is that a 45* V-twin that makes 70hp has more "actual" power than a 360* parrallel twin making the same 70hp.'

Not wanting to start a torque vs hp debate, I present the following...
OK... notwithstanding crank HP vs rear wheel Hp
actual HP... real HP... bunch of crap...
70 hp is 70 hp... you can quote me on that...BUT
What does the hp curve look like going up through the RPM band?
Where does the torque peak?
With HD, the torque tends to peak early in the RPM band.
There is a sensation of power in that early peaking torque.

Torque = force times distance.

Force is the power of the explosion above the piston pushing it down.
Distance is the amount of offset the crank pin has from the center
of rotation of the crankshaft.
That's it... that's all... no magic involved.

[dave76]

Also remember Harleys engine is basically an airplane Radial engine with just two cylinders. Just check out some old photos of WW2 B17s and the Navys Hellcats. Thay were all radial engines, with cylinders spacing 45° appart.