Which coil?

[Mr Spigot]

I run a Lumenition Magnetronic ignition system on a 1275 Midget engine with LCB 1.5" exhaust and a 1.75" HIF44 carb. I am looking to get a Pertronix Flamethrower coil but not sure whether I should get 1.5 or 3 ohms? I currently run an old Lucas coil which works but not sure of it's provenance and wanted to have something reliable!

[svenedin]

I suggest looking up the instructions for your system. As far as I know, you must use a 3 Ohm coil. I use a Petronix 3 Ohm coil with Lumenition Optronic. The Petronix coil will need a new bracket as the diameter is larger. You can buy oil filled or resin versions. The resin version is said to be more resistant to vibration and so better for mounting on the dynamo.

Stephen

[Mr Spigot]

Thanks. I have tried doing some research and some say 1.5 and some say 3.0 ohms for a system without a ballast resistor. Thanks for mentioning the bracket, I will have to get a new one as well. Mine is already mounted on the bulkhead so hopefully not affected by vibration.

[geoberni]

I run a Lumenition Magnetronic ignition system on a 1275 Midget engine with LCB 1.5" exhaust and a 1.75" HIF44 carb. I am looking to get a Pertronix Flamethrower coil but not sure whether I should get 1.5 or 3 ohms? I currently run an old Lucas coil which works but not sure of it's provenance and wanted to have something reliable!

Well as Stephen said, check your Instruction leaflet.
I would imagine a 3 Ohm as that's standard for a 12v system.
1.5 Ohm Coils are for Ballast systems. Ballast systems are wired differently, to provide a higher spark for starting that it delivers when running.

As to an 'Old Lucas Coil' it's probably more reliable that anything more modern.... Just carry a spare coil anyway.

[Mr Spigot]

Thanks. I wish I had an 'instruction leaflet' !! The Lumenition system was fitted about 25 years ago and I've had the car for 35 years and it has had various modifications over the years. From what has been said, the 3.0 ohm one seems the best bet. I will be keeping the old one as a spare anyway.

[geoberni]

Well fitting Instructions are available to download from https://autocar-electrical.co.uk/produc ... -ignitions

This is for the 25D4:
https://autocar-electrical.co.uk/images ... s-V3-1.pdf

It basically says if you have a Ballast System connect it up before the Ballast, which is what I would expect.

The 3 Ohm is not 'the best bet' it's simply a case of being the correct one to use...
If you fit a 1.5 Ohm Coil to a non Ballast system.... you'll almost certainly cook it because it's designed to run on 9v and only have 12v for the few seconds it needs to start up.

[Mr Spigot]

Thanks. That is very helpful and confirms what is required, just what this forum is so good at.

[svenedin]

This is like to me trying to explain a leading shoe or a trailing shoes of the brakes. I don't understand it terribly well and I have to think every time.

There is so much that is not helpful about coils. So, early cars like my father's Austin 7 were 6 volt and they had no starters. Then they had a starter but by that stage many had been converted to 12 volt. Then the idea came that via the ignition circuit (the ignition switch) the ignition could be with the full enormous cranking amps (which is enormous at 100 amps plus obviously via the solenoid). The problem is that when the battery is cranking it cannot maintain 12 volts so the idea was to use a 6 volt coil and a separate circuit. When the engine is being cranked it will directly run the coil but in normal use it will go via ballast resistor. So actually what is happening is you run a 6 volt coil with half the resistance, 1.5. Ohms. There is no advantage to doing this because the Morris is not set up for this. I tried this, you end up with a cold coil (that's a good thing) and a very hot resistor.

Only when you have a specific starting circuit can it work. This is designed to give the full (reduced) cranking voltage to the coil but when the starter motor is not engaged it gets normal battery voltage.

Berni will explain this better. He worked on aircraft.

Stephen