Well, the primary side of the coil will reach (routinely) 250 to 300
volts, and can reach 450 volts. Dry air at atmospheric pressure only
holds off 30 volts per mil, so a .010 gap can be broken down by a 300
volt pulse, under some circumstances. The capacitor in the circuit
prevents the full voltage from building up until the points are open
enough to stand off the primary voltage. True the additional close
time will allow the coil to charge more fully, and too much will cause
it to overheat, but you really need the gap to make sure the energy
stored goes to the plugs, not to the points.
The coil is an autotransformer; primary voltage will be secondary
divided by turns ratio, which is usually near 100 for a
points-operated coil. So if it takes 25kV to break down the plug gap
(typical for a .030 gap in a 9:1 engine operating near its torque
peak) the primary voltage will be 250 volts. I have seen 450 volts on
the primary with a plug disconnected, high-quality wires, and nowhere
for the coil energy to go -- it toasted the transistor switch in the
primary circuit.
Donald.
> Date: Wed, 12 Jul 2000 12:22:30 -0400
> From: George Richardson <gprtech@frontiernet.net>
>
> Although you're correct about the gap decreasing as the block wears, the
> high voltage spark can't ever jump across the points. They're only
> connected to one end of the high voltage winding of the coil. The other
> side goes through the rotor to individual spark plugs, which unless the
> rotor, cap, plug wires, or timing is bad, is the only place the high
> voltage spark can go.
>
> What does happen is that as the gap closes, current runs through the
> points and coil for a longer period of time. The most immediate effect
> is that the points and coil run hotter. Sometimes with plastic mounted
> points, this causes them to melt. It can cause excessive arcing across
> the points from the low voltage winding of the coil. And it can result
> in less high voltage spark to the plugs.
>
> "Donald H. Locker" wrote:
> >
> > I beg to differ Chip. As the rubbing block wears down, the gap will
> > _decrease_, until it reaches the point where the point doesn't open
> > sufficiently to stand off the inductive kick of the coil and the spark
> > goes across the points gap instead of the sparking plug gap.
> >
> >
>
> --
> George Richardson
> Wyvern - '57 Triumph TR3, TS15559LT -
> http://www.merlingroupinc.com/tr3.htm
> Griffin - '71 Triumph Stag - undergoing restoration
> Pikachu - '75 Triumph TR6 - undergoing repair
> Kitty - '83 Jaguar XJ6 Vanden Plas - Daily Driver
>
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