Herald Ignition Coil

[midgeman]

Hi

Can anyone recommend where to get a new Coil for my 13/60 as I have been told that some are not of a very good standard these days.
I don't mind paying a bit more for quality if that quality is known to other owners on here.

Also am I correct in thinking that the 13/60 uses a Standard 12v NON BALAST type Coil ?


Many thanks

Alan.

[Casper]

Yes, 12v non-ballasted.
Standard is Lucas GCL110
Lucas 'Sports' [gold] DLB105 is an alternative preferred by many.
Available from the usual suspect or Google them up.

C.

[piman]

Hello Alan/Casper,

there's little point paying extra for a 'Sports' coil on a standard engine. As it's the plugs that determine the voltage the higher voltage potential of a sports coil will not be used.

Alec

[Casper]

In what way do the plugs determine the voltage?  Surely it's a product of the turns ratio.

I was tempted to advise against using a coil with high output as it tends to find weaknesses elsewhere in the system.

C.

Edit.  I am aware that high compression, etc. may prevent sparking with a normal plug gap, and also that a larger plug gap can be driven by a higher voltage.  Which is why on my (mildly) increase CR I use a plug gap of 30 thou with a Sport coil.

[thebrookster]

My advice would be to avoid the Lucas Golf coils, unless quality has gone back up recently!

My big saloon had multiple issues with these, all solved once I changed over to a Bosch sports coil.

On a related but separate note, I have also had some good successes with the Bosch triple electrode plugs, our engines seem to like them! I know a few other members in here have had similar findings.

Cheers,

Phil

[RobPearce]

Quoted from Casper

In what way do the plugs determine the voltage?  Surely it's a product of the turns ratio.

Many people believe this, but they're wrong. It's a case of "a little bit of knowledge". They know that a coil is "basically just a transformer" and they know from school days that a transformer gives an output determined by the turns ratio. Add those two facts and 2+2=5. What they forget (or are unaware of) is that neither of these statements is actually true - they're both approximations.

The one that matters is the turns ratio approximation. It's the result that you get if you do the proper maths on a sinusoidal voltage applied to a transformer primary with a reasonably well-matched pure resistive secondary load. An ignition coil has neither a sinusoidal voltage nor a sensible secondary load. So the simplified approximation is utterly useless.

In the case of a coil, the primary gets battery voltage switched in and out, and there's an open circuit on the secondary. This means that the secondary does not conduct while the primary current increases, meaning the magnetisation isn't counter-balanced and the reduction in inductance doesn't apply. Hence you need to calculate the current build-up over the dwell angle based on primary inductance. Then the points open. Primary current drops instantly to zero (not possible) and there's no secondary current to control it. The maths picks up a whole load of divide-by-zero conditions until the secondary voltage spikes past the barrier. And the barrier in question is the arc threshold of the plug gap.

[Casper]

Thanks for that.  I was aware that it is far from a simple transformer and that the physics can be complicated (always struggled with hysteresis and stuff).  Presumably, if the arc threshold of the plug gap (coupled with things like the dialectric, affected by CR) is increased by increasing the plug gap then this will require a higher voltage to pass the threshold, and a standard coil may not be able to reach the threshold.
Thus, bigger gap, bigger spark, provided the coil can (is designed to) provide enough voltage?

C.

[RobPearce]

Well, not quite. Bigger gap means a bigger voltage needed to create the spark, which means more of the coil energy is used in making the spark so the spark duration will be shorter. On the other hand, the bigger gap does give more volume to ignite a potentially poorly homogenized mixture. But you're spot on as regards potential for a standard coil to struggle with making the "bigger" spark.