Quote:
Originally Posted by Lorenfb
"And by pure chance(??) that is EXACTLY what results from the use of 450 volts sv 300 volts applied to the primary does."
The key factor is the dv/dt of the spark pulse and not absolute voltage. Do a little research into
why a CDI was used versus an inductive discharge is the late '60s, i.e. because the rise time
(key for firing fouled plugs) of transistors back then were limited. Now days with COP OEM
ignitions systems the max spark signal is about 300 to 350 and NOT the unnecessary 450+ volts
of the MSD. With coil turns ratios of 100 to 1, 20KV to 30KV is all that's necessary.
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Having build numerous versions of both, mostly for a '63 T-bird, I am well aware of the short-comings of a simple transistorized "switch" to replace the points vs an actual CDI. PNP transistors with the current handling capability required were pretty rare, expensive, and "fragile". If I remember correctly a 300 volt POWER zener was used to prevent the inductive kick of the primary from exceeding the breakdown voltage of the PNP. That 300 volt limit required a specially build ignition coil.
The biggest problem I had, remember, was finding the correct ohmage and power resistor to limit the current flow that was reliable enough in the long term.
Once I converted the system to CDI I built several for friends.
I actually turned the Ford distributor cam down on a lathe so I could make use of the magnetic trigger Chrysler introduced later on.
And the rise time of the PNP transistors were most certainly not any worse than the "condensor" used to limit the inductive kick risetime and prevent voltage flashover at the points as they opened.
Not being aware of the actual design of todays "coil on plug" inductive ignition I would still bet that they apply a fairly HIGH voltage, inverter supplied, to the primary but regulate the current flow level by lowering the applied voltage once the design current flow is attained.