Morris Minor Owners Club

At one time I did have a 1.5 ohm coil with an external ballast resister. The ballast resistor was in a ceramic housing and it did get rather hot. The coil did not. I think it just made things more complicated than necessary with no noticeable advantage so I reverted to a 3 Ohm standard resistance coil. My coil is the one recommended by Distributor Doctor. Pertronix 3 ohm resin (said to be less prone to damage from vibration than oil filled coils). Expensive but very reliable. It has a wider diameter than a standard coil and needs its own larger diameter bracket.

Stephen

As with condensors, rotor arms and point sets, modern coils can sometimes be poorly made. Anything the Dizzy Doc recommends is likely good but, outside of that, a known-to-be good, vintage coil is probably the safest bet. That proposition is the reason for the old adage used on here, 'its never the coil'.

svenedin wrote: ↑Fri Jan 24, 2025 3:43 pm At one time I did have a 1.5 ohm coil with an external ballast resister. The ballast resistor was in a ceramic housing and it did get rather hot. The coil did not. I think it just made things more complicated than necessary with no noticeable advantage so I reverted to a 3 Ohm standard resistance coil.
Stephen

That's exactly what I's expect. There's a reason Ballast Resistors are big chunky things designed to have good airflow around them, cooling for energy dissipation.
The reason the Ballast Coil/Resistor system was developed was so that in normal running, the Spark Plugs effectively had a 3 Ohm Coil providing their spark.
But on operating the Starter, the Ballast Resistor is by-passed, thus providing the Plugs with the spark from a 1.5 Ohm Coil, ergo, bigger spark on turning it over.

The Ballast Resistor is a Current Limiter.
As I mentioned earlier, 12v applied to a 3 Ohm Coil will result in 4 Amps current flow, resulting in 48 Watts of 'power' for the spark.

Using the correctly wired Ballast Coil of 1.5 Ohms, that will be the same when running with the Ballast Resistor in Circuit, a total of 3 Ohms.
But when the Resistor is bypassed, when using the Starter motor, then 12v to the 1.5 Ohm Coil gives 8 Amps current and 96 Watts of 'power' to the spark.

Without the complication of measuring ohms etc, I always understood that when a ballast resistor was used it was wired in conjunction with a 6 volt coil and the ballast resistor was bypassed while the starter motor was cranking possibly supplying 8 - 9 volts to the six volt coil. This provided an excellent spark for starting and compensating for the low battery voltage available due to the load from the starter motor. The ballast resistor simply returned to the circuit as soon as the starter motor was not drawing current thus providing a restriction to the 6 volt coil. I have had several vehicles over the years using a ballast system, but yet to see it used on any of the BMC products I have owned.