One should think of sports coils as 'high energy' coils rather than 'high
voltage'.
As Barney correctly point out, the output votltage will be exactly what it
takes to breakdown the compressed mixture and cause a spark. This voltage
will quickly drop to the much lower voltage required to sustain the spark.
The voltage on the primary side will be this output voltage divided by the
turns ratio (maximum of a few hundred volts, I believe).
Barney used to have some excellent charts of these voltages on his web site,
but I don't know if they are still there.
The high energy coil is able to store and then release more energy. This
energy can be used to provide a higher voltage at the spark plug (see note
below), a longer duration spark and a spark across a larger points gap. This
should provide better ignition of the mixture.
Also, at high engine speed conditions, there is limited time to 'charge' the
coil, thus a standard coil may not reach its maximum energy storage
capacity. Being able to store energy in the coil faster allows better
operation at high speeds.
The downside -- ie. why did manufacturers not use such coils as standard: a
high energy coil will reduce the lifetime of the points and manufacturers
have always been looking for ways to extend service intervals. A high energy
coil also puts a greater load on the electrical system, which may have been
a concern when generators were the norm -- however, this is just speculation
on my part.
Note: in a perfect system, one would not require more energy to create more
voltage before the spark occurs, however, losses through parasitic
resistances are quite significant, thus more energy corresponds to the
ability to create a higher voltage at the spark plug.
Simon
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