Re: Ignition Coil

["David Kirk" <david@domain.elided>]

Dear Mike and other respondants.

The ignition coil thing is something I discovered whilst trying to get my
Rev counter working properly. The engine currently runs fine, cold starting
and all. I have tried shorting the ballast resistor and notice no change in
either running or starting performance. The problem is the rev counter.

At present the rev counter is extreamly internittant in operation. Most of
the time it reads zero.

I have had the thing to bits and checked it out internally and can see no
problems.

I have reset all the ignition timing using strobe lights and oscilloscopes
to set the dwell angle and pride myself it getting it all spot on.

So why doesn't the rev counter work all the time?

Is it perhaps that the coil is 12v rated and not 6v?. 

My thinking this far is this.

The rev counter works by passing the feed line to the coil through a
torroid, thus making a simple pulse transformer. Every time the points in
the distributor open and close the current flow changes in the primary of
the ignition coil and thus, as the current changes, it exites a current to
flow in the secondaries of the rev counter.

Now, if this is all true, and everyting was set for use with a 6v coil,
then when using a 12v coil the current flow should be less, and as such,
perhaps not enough to exite the required amount in the rev counter!

I have previously tried increasing the number of turns that the coil feed
wire makes through the torroid primary (initially only one, and I tried 2).
Thinking about this now I think I need to decrease the number of turns (not
possible as it has only one as standard) ) or increase the current in the
ignition coil - by fitting a 6v coil and using the ballast resitor (like
the way it was made to begin with).

If anyone has managed to follow my irrational and unlogical thoughts enough
to comment upon my thinking, they would be greatly appreciated.


Dave Kirk

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From: Mike Wattam <101714.1343@domain.elided>
To: David Kirk <david@domain.elided>
Cc: TSR stag digest <stag-digest@domain.elided>
Subject: Ignition Coil
Date: 12 March 1998 21:52

Hi David

Ref your query about ignition specifications;

All Stag coils should be 6V.  While the engine is running, the external
ballast resistor (Mk1) or a resistor wire built into the loom (MK2) cuts
the battery voltage by approximately half, so that a 6V coil gives
sufficient high tension current to keep the spark plugs happy.

I should imagine your car has a 'Sparkrite' coil fitted because it has (or
once had) Sparkrite electronic ignition fitted.  Check this out first.  You
may find this coil has a LT to HT conversion rate higher than standard
coils - that is it may be capable of a higher HT voltage than normal.

The ballast resistor/resistor wire is short-circuited during starter
operation via the ignition switch, so 12V flows into the coil and doubles
the normal output to give what is potentially a huge spark, thus aiding
cold starting without thumping lots of pollution out of the tailpipe end.

In practice however, two things happen which make this theory less than
highly effective.  The coil will only put out enough volts to give the
minimum spark, so this feature is really only useful when the plugs are
worn out or sooted up and a very high voltage is necessary to bridge the
gap.  Second, if the HT leads are old, the plugs bad or the distibutor
cap/rotor arm is less than perfect, a lot of tracking across to earth
occurs and nothing gets to the plugs.  In fact a very high potential
voltage will increase the possibility of 'tracking' under adverse
circumstances.

If you use a 12V coil with the resistor still connected, the maximum
achievable voltage may not be enough under adverse conditions as mentoined
above.  REmove the resistor circuit and you will get sufficient volts for
normal running and for normal starting, but again in adverse conditions it
may not be enough.

As a footnote, let's dwell for a moment on the subject of electronic
ignition.  If you think about the fact that the HT circuit will only
produce sufficient voltage to make a spark, the many claims that starting,
fuel consumption etc will be improved, is a complete myth.  Electronic
ignition is alleged to improve cold starting by giving out a very high
voltage, but the Stag with it's 6V coil does this anyway.  So, I believe
the only real advantage given by electronic ignition on the Stag, is to
give an excellent idle quality which can be difficult to achieve with
old-fashioned 'points' - this assumes your electronic ignition has a rotor
'paddle' to transmit current at the right time (some systems still use the
old 'points').

I've still got the standard system for the last 25 years, if I remember
right I am on my fourth set of points, leads and distributor cap.  I would
classify my idle as reasonable, and top end power is superb with loads of
grunt through 6,000rpm.

Mike Wattam
Triumph Stag Register

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