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Tech. Advice: Industrial Engines
V-Twin Magnetos
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<blockquote data-quote="Magnetoman" data-source="post: 88301" data-attributes="member: 2806"><p>I have a thick notebook of magneto measurements I've made, but the offset of a V-twin magneto makes it easy to get mixed up. However, think about it the following way:</p><p></p><p>Irrespective of anything else, we know that one cylinder will get full output because the points open just as the magnetic flux lines have been compressed the most and and are in the process of flipping direction through the armature, resulting in the maximum time rate of change of flux (Voltage = -d(flux)/d(time)). </p><p></p><p>The other cylinder will receive a smaller output because timing for it has to be set too early, when the flux lines are still being compressed, or too late, when whatever flux is left over is still flipping direction through the armature. Clearly, the situation isn't symmetric so it makes a difference if the second cylinder is timed for the wrong one of these, i.e. too late or too early. So, which is best for the second cylinder? Rather than dig through my old data I can give you the answer. It's not because my memory of the data is so good, it's because I remember finding that Lucas did it correctly. The '1' and '2' stamped on the KEV magneto housing, coupled with the timing imposed on the spark by the cam, ensure that #2 gets the largest voltage possible. </p><p></p><p>But, to directly answer your question, #1 sparks, then more than 180-deg. movement of the armature later #2 sparks. That means the second spark comes "late." After that it is less than 180-deg. before #1 sparks again, but since #1 is set at the optimum point the flux concentration is maximum and in the process of flipping so the less than 180-deg. motion doesn't affect the output of #1. Again, since it's easy to get mixed up with 'early' and 'late' the easiest thing to do is simply copy the timing Lucas built into the KVF.</p><p></p><p>The Bosch ZEV design from before WWI has asymmetric armature and internal pole faces that are offset by 45-deg (engine) to give the same, maximum output to both cylinders on a V-twin. However, by sometime in the 1920s engineers had figured out that this more complicated structure wasn't needed in order to get a large enough spark to the weak cylinder so designs became symmetric.</p></blockquote><p></p>
[QUOTE="Magnetoman, post: 88301, member: 2806"] I have a thick notebook of magneto measurements I've made, but the offset of a V-twin magneto makes it easy to get mixed up. However, think about it the following way: Irrespective of anything else, we know that one cylinder will get full output because the points open just as the magnetic flux lines have been compressed the most and and are in the process of flipping direction through the armature, resulting in the maximum time rate of change of flux (Voltage = -d(flux)/d(time)). The other cylinder will receive a smaller output because timing for it has to be set too early, when the flux lines are still being compressed, or too late, when whatever flux is left over is still flipping direction through the armature. Clearly, the situation isn't symmetric so it makes a difference if the second cylinder is timed for the wrong one of these, i.e. too late or too early. So, which is best for the second cylinder? Rather than dig through my old data I can give you the answer. It's not because my memory of the data is so good, it's because I remember finding that Lucas did it correctly. The '1' and '2' stamped on the KEV magneto housing, coupled with the timing imposed on the spark by the cam, ensure that #2 gets the largest voltage possible. But, to directly answer your question, #1 sparks, then more than 180-deg. movement of the armature later #2 sparks. That means the second spark comes "late." After that it is less than 180-deg. before #1 sparks again, but since #1 is set at the optimum point the flux concentration is maximum and in the process of flipping so the less than 180-deg. motion doesn't affect the output of #1. Again, since it's easy to get mixed up with 'early' and 'late' the easiest thing to do is simply copy the timing Lucas built into the KVF. The Bosch ZEV design from before WWI has asymmetric armature and internal pole faces that are offset by 45-deg (engine) to give the same, maximum output to both cylinders on a V-twin. However, by sometime in the 1920s engineers had figured out that this more complicated structure wasn't needed in order to get a large enough spark to the weak cylinder so designs became symmetric. [/QUOTE]
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