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On 13 May 2003 at 1:02, Per Lindgren wrote: > I know what happens when the points break the circuit, it triggers the > coil to "send" the voltage to the spark plug. What I wonder about is the > significance of the points gap, or dwell angle, which is a result of the > points gap. In other words, what will a smaller or bigger gap cause? A > longer duration of the spark? I just don't get this... It does not affect spark duration. Think of the cam as a sine wave wrapped around a circle. The wave goes up and the wave goes down, but the place where the slope of the wave is the steepest is in the middle, halfway between the peaks and the valleys. When the points open there had been current passing thru them. This current wants to keep flowing due to the inductance of the coil primary. This current continues to flow into the capacitor (condenser,) building up the voltage across the points, which are just opening as this is happening. The secret here is for the points to open "faster" than the voltage can build up. If the voltage builds faster, the points will arc across, and the voltage rise across them will stop. This will burn the points AND give you no spark. So the best time to open the points is when the cam is rising the fastest, which is when the dwell is 45 deg (or 50% of the way up the sine wave.) We then choose a capacitor that gives a voltage rise time as fast as possible (faster rise times produce better sparks) but not so fast that the voltage rises fast enough to arc across the points. The energy stored in the coil depends on the current in the coil, but this current takes some time to build up, so there is a good reason to let it build up as long as possible, this means letting the dwell be as large as possible. In this case we'd like the dwell to be as large as possible. Of course if it got to 100% the points would never open and there would be no spark, and if we even let it get CLOSE to 100% we might occasionally have a problem. SOOOO, what generally happens is that we compromise on a dwell that is just a bit more than 45 deg, and you can see that VW likes to see 48-52 or so. This is pretty typical for most cars. Everything here happened on the primary side of the coil, but when the voltage changes there it is mirrored and amplified on the secondary side, which is where our spark actually comes from. -- ******************************* Jim Adney, jadney@vwtype3.org Madison, Wisconsin, USA ******************************* ------------------------------------------------------------------- List info at http://www.vwtype3.org/list | mailto:gregm@vwtype3.org