Spark polarity

[Kalee20]

Is the polarity of a traditional, Kettering-type, ignition system critical?

Ignition coils generally have the two smaller terminals marked CB and SW. Internally, one is commoned with the HT winding and the LT winding; the other is just the LT winding. (The HT winding other end , of course, goes to the big HT terminal).

But, I'm thinking, for best results, shouldn't the spark plug get a negative pulse, because the central electrode will generally be hot and so ready to emit electrons? I which case, the polarity of the LT winding depends on whether the vehicle is positive earth or negative earth.

Swapping the SW and CB terminals will change spark polarity, but also give a slightly different spark voltage due to, one connection the coil will act as an autotransformer and the other, it won't. But as the ratio is 100:1 ish, it probably won't make much difference.

Am I right, then, in thinking that for best results I ought to 'scope the HT feed to the distributor (energising with 100mV of DC not 12V!) to be sure of getting an initial negative kick, before the damped oscillatory decay?

[Ed_Dinning]

Hi Peter, from what I remember of the theory, the autotransformer "tap" went to the CB terminal, so that at points opening there was a "lift" of about 400V on the tap, giving a little bit extra to the 25KV or so at the HT terminal.
I'm not sure if that is better than having a hot emitter spark plug tip or not.

Ed

[Murphyv310]

It makes no difference. Think of the double ended coils in a lot of today's cars and the 2CV, on each firing stroke there is a wasted spark in the opposite cylinder of the opposite polarity.
So each cylinder plug has the opposite polarity to the other.

[Kalee20]

Thanks Ed and Trevor!

Ref the 2CV, yes I know it has a double-ended coil and no distributor, but many times I've heard a 2CV being started and running for some seconds on one cylinder before the other decides to play ball, and I'm wondering, is it always the same cylinder which is reluctant to fire and if so is it the positive or the negative spark one?

Of course, if it's being started, it won't have a particularly hot electrode in the spark plug anyway.

[Murphyv310]

Hi.
I had three 2CV's including a 6 volt 1960 AZL, never had the issue of starting on one cylinder. The plug gaps though were fussy too big and they misfired so possibly if the gaps are too wide then there might be an issue.
The 1.8 Rover has two coils only and uses the wasted spark set up. What you can see though is erosion on the side electrode on two plugs and erosion on the tip of the other two but on the 75 the plugs will do 60,000 miles plus (recommendation to replace at 60,000 miles) so if there was an issue then surely misfires would show up long before that.
There is also a lot of positive earth cars that have been converted to negative and no one bothers with the coil. I'd not worry if the coil was wired wrongly perhaps one of those spark EHT testers would be a way of seeing the energy difference on polarity reversal.... assuming there is any.

[boater sam]

Years ago when I used an Allen electronic tuning rig you could see a difference in the trace if the coil was in reverse. The recommendation was to have it the correct way round.
Whether it made a difference to the running I don't know.
This was prior to the 9v coil and ballast resistor set up.