by Dan Masters, firstname.lastname@example.org
Many Triumph owners opt to replace their standard coil with the Lucas Sport coil, to get a hotter spark. Often, though, they neglect to bypass the ballast resistor, used on some of the later models. Failure to do this will negate the benefit of using the hotter coil, as the Sport coil is designed to be used without a ballast resistor. What is the difference between the two types of coils?
Basically, a non-ballast coil is designed to produce full spark output with 12 volts on the input (+ terminal). A ballast coil is designed to produce the same spark output, but with only 6 to 9 volts on the input.
With a non-ballast coil, the input to the coil is the same, 12 volts, whether the engine is running, or being cranked by the starter motor. With a ballast coil, the starter relay 1 by-passes the ballast resister when the starter motor is spinning the engine, and applies the full 12 volts to the coil. Since the coil is designed to provide full spark with reduced voltage, the application of the full 12 volts produces a much hotter spark, which is an aid in starting. After the engine starts, and the starter motor is off, the coil voltage is dropped to the lower voltage, and the coil output is the same as for a non-ballast coil. The reason the ballast type coil is not run at the full 12 volts, for a hotter spark, is to prevent damage to both the coil and the points.
With a non-ballast coil, power is applied to the coil directly from the ignition switch, via a white wire. Power to the ballast coil comes from the ignition switch to the resistance wire, and then to the coil. When the starter relay operates*, power from the battery, via a brown wire, is routed through the contacts of the relay, via a white/yellow wire, to the coil. This shorts out the resistor wire, by placing 12 volts on both ends of the wire. With the same voltage on both ends, no current flows, so no heat is generated. The current flow is shunted around the resistor wire. This bypassing of the resistor wire places the full 12 volts on the coil.
As for the modifications required to switch from a ballast coil to a non-ballast coil, it couldn't be simpler - just run a wire from the most convenient white wire you can find (probably at the fuse box), directly to the + terminal of the coil (of course, all the rules of good wiring practice should be used). No need to remove the resistance wire, because it will now be constantly bypassed, just as it was before when cranking. This will work with any model Triumph, but there is an easier way with the '74 - '75 TR6. The resistor bypass wire from the starter relay to the coil (white/yellow) can simply be lifted from the relay terminal and moved to the fuse box terminal where the white wire is attached. The relay is located very close to the fuse box, and unless someone has modified the car with additional accessories, there is a spare terminal by the white wire that can be used.
The ballast resistor will need to be bypassed for any coil that is designed to be used without one, whether it is a high performance coil, or just a replacement, for example, using a coil from an earlier model Triumph. In the TR6, the ballast resister was used only from '74 and later. As I understand it, the modifications made to these cars to meet the environmental concerns made them hard to start, so the ballast type coil was used to get the extra hot spark during starting, since starting places the most stringent demands on the ignition system. Other models made the changeover in roughly the same time frame.
Bypassing the resistor is only one of the changes necessary to utilize the full advantage of the Sport coil, or any high output coil for that matter. Unless you also increase the spark plug gap, the plugs will continue to fire at the same coil secondary voltage as before, give or take a little, and will not give the hotter spark that you paid for. The voltage rise at the output of the coil secondary, although rapid, is not instantaneous. As the voltage rises from zero, as soon as it reaches the value high enough to jump the plug gap, it will. Since the plugs were firing at 20,000 volts with the old coil, that will fire at that voltage level regardless of the maximum voltage potential of the new coil. As soon as they fire, and the arc begins, the voltage drops to zero. By widening the gap, the voltage must rise to a higher value to jump the gap. Ideally, the plugs should be gapped to the widest setting that will still allow them to fire under all engine operating conditions. Usually, the coil manufacturer will give recommended gap setting, but these should be used as the starting point, and adjusted from there as experience dictates.
For a more detailed explanation of ignition systems in general, see Inductive Discharge Ignition Overview.