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  #1  
Old 30th January 2006
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Default This is why it sounds like potato potato

The way I understand it, the V twin Harley engine's twin pistons move up and down at the same time, as they are connected to a single crankpin on the twin flywheels, but only 1 piston fires at a time. I realize this gives the classic "potatoe potatoe potatoe" sound loved by most.They have "knife and fork" connection piston rods... what does this knife and fork mean?
Does it not lose power with one piston firing forcing both pistons down?
Why have two pistons if they alternate firing?
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Old 30th January 2006
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Not sure this it true. The pistons move up and down opposite of each other, that is why the motor shakes. If they moved up and down at the same time, the motor would "hop." I do beleive what you are referring to is most HD motors will fire the plugs on the exhaust stroke as well as the power stroke. The newer motors are single fire for the most part??? Correct?? Can someone who really knows what they are talking about chime in??
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Last edited by Folkie; 14th March 2018 at 11:31..
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  #3  
Old 30th January 2006
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To be correct, both the pistons don't go up at the same time. As the previous poster said, it would hop rather then vibrate. Now, there are 4cycles that a piston goes through and only one of them is a combustion stroke. 1. intake 2. compress, 3. combustion 4. exaust. 2 of which are upstrokes, 1. compression and 4 exaust, and 2 of which are down strokes: 1. intake, 3. Combustion.

Both pistons alternate in the combustion stoke inorder to create a consistent powerband. I could explain more, but I don't have the time. I am sure a more techincal person will kick and and straighten things out. Also, feel free to correct me if I am wrong w/ something.
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Old 30th January 2006
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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.

Last edited by xl1200r; 30th January 2006 at 16:53..
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Old 30th January 2006
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Here's the animation:

http://www.sa750.com/vtwin/vtwin.htm
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Old 30th January 2006
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That's the one...there's a lot of good info on that site too.
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Old 30th January 2006
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Quote:
Originally Posted by Lone Wolf in Canada
They have "knife and fork" connection piston rods... what does this knife and fork mean?
To add to what has been posted, the knife & fork means that one rod has a slot that the other rod fits into like sticking a knife between the tines of a fork.
both rods and connected together to a single crankpin.
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Last edited by Folkie; 14th March 2018 at 11:20.. Reason: Fixed quote
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Old 30th January 2006
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Quote:
Originally Posted by Y2K View Post
To add to what has been posted, the knife & fork means that one rod has a slot that the other rod fits into like sticking a knife between the tines of a fork.
both rods and connected together to a single crankpin.
This makes it so both pistons can be right in line, rather than being offset.

Last edited by Folkie; 14th March 2018 at 11:30.. Reason: Fixed quote
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Old 31st January 2006
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my daughter had a two cylinder fiat bambina 500cc car
both pistons go up n down together so that you get power 360 degrees apart which makes sense for the power band
once it was running even at idle it was quite smooth
but the way the back end of that little car bounced up n down when the starter was cranking over was absolutely lewd!!!!!!!!!!!!!!!!
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Old 31st January 2006
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"a couple octaves lower" was the main point. All engines sound like crap with no exhaust on them - it's just the nature of it. Even my racecar with a 400 small block, all juiced up sounds like ass with just manifolds on it (can't run headers in my class).

There are some bad sounding big engines out there, just as there some really good sounding small engines.

Quote:
Originally Posted by nzsailor View Post
my daughter had a two cylinder fiat bambina 500cc car
both pistons go up n down together so that you get power 360 degrees apart which makes sense for the power band
once it was running even at idle it was quite smooth
but the way the back end of that little car bounced up n down when the starter was cranking over was absolutely lewd!!!!!!!!!!!!!!!!
As for the 360* crank thing, this would be the same setup the triumph twins use. I'm not sure why your fiat didn't hop around when running, but I understand those triumphs have a little shake to them since both cylinders are moving up and down together, even thou they are on different strokes.

As for the powerband thing, on paper the 360* firing order makes sense, but from what I've read it's not the case. According to that link, the 45* Single pin V-Twin makes the most "real" power per "rated"power. The scentence 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. I don't know the science behind this, just relaying some info I came accross.

Last edited by Folkie; 14th March 2018 at 11:31.. Reason: Fixed quote
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