Detroit's theory back in the days of muscle cars and points ignition
was that a resistor was needed in the coil primary circuit, to limit
current through the points. As the points open they tend to arc,
which causes the points to pit. Reducing the current through the
points with a resistor reduces pitting. The condenser controls arcing
too, but adding a resistor further reduces arcing. American engineers
were acutely aware of points degradation and worked to minimize it,
carefully calibrating primary resistors in their designs. You often
see this primary resistor attached to the coil, or as a resistive wire
hooked to the coil primary connection, or it can be internal to the
coil itself.
When starting the car, typically the resistor is bypassed by the
starting circuit. This applies full voltage to the primary circuit,
which is proportionately multiplied in the secondary for a hotter
spark during engine start. Once you release the ignition key from
start to run, the resistor drops back into the primary
circuit......the primary voltage is reduced to a safe level for the
points. Of course the secondary (spark plug) voltage is also
proportionately reduced, but the theory is that a cooler spark is okay
once the engine is running.
In my limited exposure to British cars, I can say I've seen this exact
setup on my '78 Midget, with a primary resistive wire, bypassed during
startup for hotter spark. The resistive wire in my Midget is about
1.5 ohms, which interestingly enough equals the rated primary
impedance on the coil they used. I understand they used a 6V
coil.....this gives the effect of doubling the output when in the
start mode (when the coil primary is seeing nominal 12V, twice what
it's designed for), but when the matched-impedance resistor drops in
(in run mode) the voltage across the coil primary falls to roughly
the 6V it was designed for. In this case they've obviously chosen the
resistor value to be roughly equal to the coil's internal primary
impedance, to cut the effective run voltage across the primary in half
(I wouldn't be surprised if they had a bunch of obsolete 6V coils
laying around, or got a good surplus deal, and this was a good way to
use em up). They're slugging the coil at twice it's rated voltage
during start, but that's not continuous, and the coil is probably
over-rated anyhow. But I wouldn't want to run that coil continuously
at double rated voltage (no resistor).....you could get some internal
coil breakdowns over time. Plus the points pit more without a
resistor (my Midget had the original electronic ignition replaced with
an earlier-type points ignition, so the resistive wire is good for my
points. But even the original electronic ignition system still used
the resistive wire setup so they could do the hot-spark-start thing
using a 6V coil).
Anyway, the primary resistor is used to protect the points (if you
have points), and/or as part of a scheme to boost the coil voltage
during startup. I like having a resistor in there for the points'
sake, but the coil secondary obviously then needs higher output (=
more cost) to compensate for the reduced voltage across the primary.
The resistor can be calibrated to give whatever voltage reduction you
want with a given coil, but the tradeoff is the coil's secondary
amplification must be greater, to get the same spark with reduced
primary voltage. You're balancing points' life against the cost of a
higher output coil. My Midget had the right idea to some extent (the
6V coil they used has double the amplification of a similar 12V coil),
but the problem is it's not rated to work at 12V input, only 6V. They
got extra output by using an under-rated coil but only banging it hard
during the brief startup periods. I don't really like that approach,
but it's cheaper than using a fully 12V rated high-output coil that's
designed around the higher startup voltage, and then basically loafs
at part-capacity during the lower-voltage run mode. They didn't want
to pay for extra capacity (that's only used in the start mode) in mass
production, but we probably don't mind for a one-off performance
upgrade to our personal vehicles. That's why you have all those
aftermarket high-performance coils available. Note that if you put in
a new coil with an internal resistor into a ballasted circuit you will
have to remove the existing resistor and forfeit the voltage jump at
startup (you may not notice the difference if the coil's output is
high anyway). The only way to have the hot-start feature is to have an
external resistor that can be bypassed during start. A
high-output/internal-resistor coil is basically for running the hot
spark all the time, not just when starting, and needs no external
resistor. Also note for electronic ignitions the total impedance of
the primary (coil primary impedance plus any resistors) may be
critical if the system is fussy.
Other resistors you may see are for various inputs to electronic
ignition systems. I've seen one of these lurking in my Midget from
the days when it had its original electronic ignition. These
resistors feed some logic within the control module. Typically the
module will also require a full voltage input as well. I don't know
anything about the electronic ignition you mentioned; it may or may
not need a resistive input. Some electronic ignition systems also
expect a certain total impedance in the coil primary circuit and might
be sensitive to changes. Perhaps the ignition manufacturer could
provide a schematic? The Bosch coil ad is probably trying to warn you
that the coil has no internal fixed resistor, and if you need one for
your system you'll have to provide it yourself in the circuitry. But
hey, if it's running okay, you probably lucked out and the coil
primary is within what the system can tolerate. It would be nice to
see a schematic of what the ignition system expects to see, and to
find out from Bosch what the primary coil impedance is. Then you
could know for sure what you have/need.
---Christopher Palmer <ctp@gbn.org> wrote:
>
> Whilst perusing the VB catalog, came upon their Bosch Super coils, you
> know...red and blue.
> Their blurb said that Bosch Super coils cannot be used with electronic
> ignition.
>
> The problem? I have a Pertronix Ignitor and a Bosch Blue coil...it
seems
> to run fine. I can't figure out why my coil would upset my
ignition. The
> Blue is only supposed to put out 17,500V.
>
> Anyone have some clarifying info on this??
>
> CTP
>
>
>
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