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Info on coils and ignition


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On a recent thread about coils, people have mentioned that a note on the topic might be useful.  Here is my attempt which covers all the ignition system and which I hope will fit the bill. Please let me know if you see any glaring errors. This has been updated from the original to include a bit more detail including coil polarity, and to correct a few typos.  

Coils 2.pdf

 

Edited by RobH
Updated to issue 2
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Just read it through, very informative read. 

Gareth

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Magnificent work Rob,

This will help many folk, yours truly included, understand just what does happen,

when you flick the starter.

Many thanks.

John.

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Thats was just what was needed, thank you,   is there a way to keep that as a locked document on the forum please?

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Rob,

Unless I missed it you seem to have not made any mention of changing a car from positive earth to negative earth and the need to change the connections to the coil terminals. I know it’s drawn correctly on the drawing, but some people may still be confused as to why it has to be done (I am one of those people …).

I have a collection of old coils and used one for my TR3A.

It is an old Lucas “Red Top” sports coil with just “CB” and “SW” marked on the connectors. (From, I guess, a positive earth car, with the "CB" terminal actually ending up connected to the positive side of the battery when the points were closed.)

In my hurry to get things going I connected my negative earth car up to the terminals as they are marked, not thingking any more about it.

The car has gone for almost 1000 miles like that without much problem. It’s only last week when I decided to connect up my timing light to see what was actually going on that I discovered my error.

Oddly, the timing light did not flash properly.

Eventually I changed the coil terminal around and the strobe gun worked as it should (I had tried turning the ferrite pickup around on the lead, to the opposite of what was expected but to no avail.)

 

I didn’t actually notice any difference in performance when I made the change, just the correct functioning of the timing light.

I’d be grateful on your thoughts about connecting the coil the right way, or does it not really matter.

 

Excellent article, by the way.

Charlie.

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Ah yes that was an oversight - thanks for that Charlie.  I will update the screed in due course to include it. 

Coil polarity does make a bit of difference but as you have found - not a lot on a well adjusted car. It's something of a bonus that your timing light showed that - I wonder if it is intentional by design?

As a rule of thumb, it is usually reckoned that coils marked 'CB' are for positive-earth systems while those marked '-' are for negative earth but that may not be true in all cases.

Having the 'wrong' polarity coil fitted will invert the coil output voltage. By design the voltage applied to the spark plug centre electrode should be negative and the reason for this is to ease electrical breakdown of the plug gap.  Hot surfaces produce a cloud of electrons and the hottest part of a spark plug is the centre electrode.  Breakdown is easiest if these electrons can be repelled from the centre and attracted across to the earth electrode and this requires a negative-going pulse with respect to earth. (Do not confuse this with the car battery polarity.  The HT needs to be negative going for both positive and negative-earthed cars

A positive going pulse from the wrong sort of coil - or from the right type connected the wrong way round - will find it harder going and a higher voltage will be required to start the spark.  However, most coils used with a well-adjusted distributor have sufficient voltage capability in hand except perhaps at very high revs, so this may not be apparent during normal driving.  

If you are in doubt about the coil polarity the 'pencil lead' test is a good way of checking. An orange flare should be seen to go from the pencil towards the spark plug.

 

 

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3 hours ago, RobH said:

On a recent thread about coils, people have mentioned that a note on the topic might be useful.  Here is my attempt which covers all the ignition system and which I hope will fit the bill. Please let me know if you see any glaring errors:

Coils.pdf 3.6 MB · 40 downloads

A very useful, and well written document Rob, so thanks. Agree it should be locked to the top of the forum menu.

Colin.

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Another excellent bit of work which I've downloaded read and filed away!

Thanks Bob

 

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Many thanks Rob, that's gone straight into my Useful Stuff file. Now I just need to remember I have it! 

Thanks again,

Nigel

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Excellent work Rob, May I suggest you offer it for inclusion in TRaction.

Bob.

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As I said in the screed Ian, that data came from a web search so maybe I hit on the wrong site and that is old gen  - I know manufacture of those has been moved around a bit over the years so the spec may have changed at some time. Thanks for the comment -  I will check again and correct if necessary.  

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Apologies to all who have recently downloaded this - I have made a few additions, tried to improve the clarity here and there and corrected some typos  (and probably made some new ones in the process!).  It is now at issue 2 replacing the original attachment in the first post. 

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