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On 5 Nov 2002, at 1:15, Dave Hall wrote: > Sorry to hear current reasoning is that resonance doesn't come into play > with coil and capacitor in ignition systems, since it is a happy consequence > that maximum power is available in the system if so. The problem with this theory is that to match a resonance you need TWO things to both have the same natural frequency. With the Kettering LC circuit you have exactly one resonance which will move around if you change either L or C, but they will always have their resonance, which always matches itself (of course) but there's nothing else to match. In the end the C is chosen so that the dV/dt is "slower" than the dr/dt as the points open, otherwise you'll get a spark across the points after the opening. Such a spark will obviously ruin any spark intended for the plugs, as well as pit the points. The dr/dt depends on the dist cam profile and the rpm, so at higher rpms (where the dr/dt is larger) you can tolerate a dV/dt that is also larger. Thus race cars can use a smaller C which will give a faster rise time, because these engines never spend any time at idle. This also explains why it's a good idea to set your dwell to 45 deg because this is where the cam profile (assume it's a rolled up sine wave) has its steepest slope. If you want to get more energy stored in the coil you would be tempted to increase the dwell, but in most dists this puts you on a flatter part of the cam, which may lead to arcing at the points, so people like Mallory came up with dual point dists which have 2 sets of points, set not quite 180 deg apart from each other. They are wired in parallel and each one is set up to open/close at the "sine wave zero crossing." Since they are not quite 180 deg apart, however we can get more dwell while still opening the gap at the fastest slope part of the cam. - ******************************* Jim Adney, jadney@vwtype3.org Madison, Wisconsin, USA ******************************* ------------------------------------------------ To unsubscribe, E-mail to: <type3-off@vwtype3.org> For more help, see http://vwtype3.org/list/