I did a quick measure of the spark duration with two coils compared to one. These numbers are variable depending on the coils used and the plug gap. However, the duration with two coils in parallel with both gaps firing is approximately 1/3 the spark duration for a single coil in my testing. So, that makes the spark duration extremely short for the single polarity MSD output firing two coils in parallel. Only 10 to 15uS or possibly a little longer with high efficiency coils and a narrow plug gap. Fred
Quote:
Originally Posted by Fred Winterburn
Folks,
There was a thread I read here from a few years ago entitled 'MSD ignition exposed'. In that thread at least one member was firing a twin plug engine with a single MSD ignition box. I did a quick test today and I certainly do not recommend doing this. It works, but not very well, and that is provided the coils are in parallel with the MSD output and not in series. In series the impedance is doubled and the energy output from both coils is so low that it isn't worth considering. Here are the test results. By the way, some of you may be surprised at how low the voltage output is with MSD given the advertising. The unit I tested was a 2009 model MSD 6A. It is basically the same design that they used in the 1970s. The output is completely dependent on battery voltage as it does not use an oscillator type power supply to charge the discharge capacitor. Instead, the emitters of two power transistors in parallel, are connected directly to the battery by the thick red wire. When triggered 'on', the transistors attempt to charge the 1uF discharge capacitor in one half cycle through a massive transformer (at least that's how the circuit appears to operate) The spark duration is short too (about 35 to 40 uS) and much shorter than you would expect for a 1uF capacitor which should give a spark duration of at least 100uS with most CDis. The time between individual sparks is almost exactly 1mS, which I think is far too long after the first spark to be of any use (my opinion).
Results of quick test using two more or less matched coils.
1500rpm (6cyl)
-Maximum voltage at calibrated spark gap 28kV with single coil (4 sparks separated by 1mS).
-Maximum voltage attainable at calibrated spark gap 20kV with two coils in parallel. Still able to produce 4 sparks.
2000rpm (6cyl):
-Maximum voltage at calibrated spark gap 28kV with single coil (3 sparks separated by 1mS)
-Maximum voltage attainable at calibrated spark gap 20kV with two coils in parallel. Still able to produce 3 sparks.
4000rpm (6cyl):
Maximum voltage at calibrated spark gap 26kV with single coil (2 sparks separated by 1mS).
Maximum voltage attainable at calibrated spark gap 19kV with two coils in parallel. Still able to produce 2 sparks.
6000 rpm (6cyl):
Maximum voltage at calibrated spark gap 24kV with single coil. ( 1 spark)
Maximum voltage attainable at calibrated spark gap 16kV with two coils in parallel. Single spark.
The above testing was done at 12.5V applied. Charging voltage near 14V would make the values higher. What I didn't check for, but perhaps will do another time, is check how much shorter the spark duration is with two sparks plugs firing at once. Certainly the energy of each spark is considerably lower than if the MSD 6A were driving a single coil. One thing that did stand out on the scope was the massive voltage overshoot in comparison to the actual attainable gap breakdown voltage when two coils are used. So despite the lower energy, the spark plug wires and coil secondary windings are still taking a beating from voltage stress with the CD output split between two coils. Fred
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