This is why it sounds like potato potato

[Maxanimal]

Originally Posted by 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.

I doubt a ten cylinder radial engine would be spaced 45 degrees.

[havi0412c]

some cool reading, speaking of radial engines and such. it also has an animation of a 5 cylinder radial engine. The radial spacing of the cylinders seems to depend on how many cylinders there were. this article states that 4 stroke engines usually have odd numbers of cylinders to run smoothly, hence the talk of 7,9 and 5 cylinder engines. cool article.
http://en.wikipedia.org/wiki/Radial_engine

[Don Burton]

Originally Posted by Maxanimal

I doubt a ten cylinder radial engine would be spaced 45 degrees.

You're correct. A ten cylinder radial engine would have 36 degree spacing between cylinders.

[Don Burton]

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.

You've given a good explanation which I'd like to elaborate on. The cylinders are odd fire as you have explained. The pistons neither go up or down together nor do they oppose each other. The odd fire is due to the two connecting rods riding on one journal with the cylinders spaced 45 degrees apart as you have said.

Also, some have thought that the simple 45 degree arrangement gives more power. Well, thats almost the opposite of correct if you're talking about peak HP at high rpm. This is an inherently unbalanced motor and that lack of balance along with the long stroke ensures that it could not live as a high rpm, high peak output motor. With that fact in mind, H-Ds have always had fairly broad powerbands due to the valve timing of the camshaft along with combustion chamber design. A 13,000 rpm machine can't make any power at 3,500 rpm where our carbed 1200 Sportys put out max torque. H-Ds can be designed to make power down low as they don't have to (and can't) make power at 13,000 rpm. Put "bigger" cams in the Sporty and it will lose some of that power band and raise it at the peak which is still relatively slow at around 7,000 rpm. Low speed torque (twisting force) is what makes these things fun to ride.

[oakies]

Nobody but Harley would use such an outdated, inefficient design in a modern engine..Hence the potato-potato sound...That we love

[dave76]

Originally Posted by Don Burton

You're correct. A ten cylinder radial engine would have 36 degree spacing between cylinders.

Yeah you guys are right I forgot about all the multiple cylinder arrangments. I do know the B17s were a Curtis Wright Cyclone 9 cylinder not sure of the cylinder spacing. I think the B24s and B25s had 14 cylinder engines. The B29 was the crazy one with 28 cylinders. Question is how weird was the timming on those engines?

[Don Burton]

Originally Posted by dave76

Yeah you guys are right I forgot about all the multiple cylinder arrangments. I do know the B17s were a Curtis Wright Cyclone 9 cylinder not sure of the cylinder spacing. I think the B24s and B25s had 14 cylinder engines. The B29 was the crazy one with 28 cylinders. Question is how weird was the timming on those engines?

A nine cylinder radial engine would have 40 degrees between each cylinder. It's simple. Just divide 360 degrees by the number of cylinders unless there's another set of cylinders behind the first set.

[Folkie]

Originally Posted by oakies

Nobody but Harley would use such an outdated. inefficient design in a modern engine.

I know; great, isn't it?

[wilbur]

Originally Posted by Don Burton

You're correct. A ten cylinder radial engine would have 36 degree spacing between cylinders.

Can you picture this in an XL frame?

[SPURS-DarkRider]

great animation and explanation, i learned something new today.