www.jec.org.uk

Hi guys seasons greetings

A quick question with hopefully simple answers
I have a 3.4 mk 2 with a 123 ignition system. It has a lucas coil at present which looks fairly old I wanted to get a replacement/spare, which type should I get and any recommendations for suppliers

Many thanks in advance of your help

Richard

A Lucas sports coil always seemed OK with a standard Dissy or one with an electronic trigger fitted, what does 123 recommend on their website?

Brian

Hi Brian

I had a look at their site the only recommendation is that the coil should have an impedance of greater than 1 ohm

I was thinking of getting a lucus sports coil do you use the one without the ballast resistor

Cheers

Richard

Hello Richard,

The ballast coil has an impedance of 1.5 ohms and non ballast 3 ohms (Designed for 12 volts). Unless the 123 limits current in some way then I would go for a standard 12v coil. I don't know that a sports coil will give any benefit?

Alec

Hi, I fitted a standard Lucas coil when i fitted the 123 ignition a year ago with no problems at all on my 3.4. For anybody thinking of fitting a 123 I would say go for it as it transformed my car.
It starts first time every time and ran so much smoother.

Hello Jim,

yes, but what advance curve do you now have, does it match the original Jaguar distributor?
The other point I would make if you car did not start first time before, your old system must have been below par.

Alec

Hi Alec , The 123 was set up on Novatechs, Slough , Rolling road. They also tweaked the carbs. I drove in with a pit pony and drove out with a race horse, it was that impressive.they did a great job. Jim.

I use a Lucas Sports coil with the 123 tune on my Midget - no ballast resistor.
Works a treat, so much smoother and revs much better.

Thinking about one for the Jag.

Dave

Hello Dave,

you should not feel any difference between a sports coil and a standard coil, unless you have highly tuned high compression engine. It is the spark plug that determines the voltage, not the coil.

Alec

Hi Alec,

My electrical theory is a bit sketchy now, how does the plug control the voltage (I assume your refering to the high tension side)?

I'm using a coil pack and Iridium NGK plugs theoretically to give a better spark for the notoriously difficult to light LPG. In practise its easier for the ECU to control a coil pack, and the Iridium lasts far longer on gas.

Merry Christmas

Brian

Hello Brian,

when the coil is fired the voltage rises until it reaches a level at which the plug fires, that is the maximum voltage attained as the coil voltage then collapses. Obviously this is for conventional coil based ignition systems. I know very little of modern ignition systems.

Alec

The output from coil packs and coil on plug is stated as much higher than a conventional coil, plus the coil has to build and fire 6 times in the time a coil pack or coil on plug has, beyond that I'm a bit sketchy so I asked a friend from the Marina forum who works with high voltages and his response was:

Hi Dad, well I think he may be partly correct, the spark plug will control the maximum voltage as it acts as a spark gap so if the gap is increased the potential voltage has to rise higher to skip across the gap. The max voltage will depend on condition around it. For example if the gap is in a vacuum the voltage will be higher for the same gap or if in humid environment the voltage will be lower before it arcs over. (Humid air conducts better).
Hence setting spark plug gap correctly is important.
I think a sports coil will deliver more current and a stronger spark at the plug with more current.
Also the rotor arm and cap introduces a gap.
With coil which is capable of more current the plug gap becomes less important if it varies as the extra current will ensure a good spark.

It is an interesting subject. At work we have power supplies which may be 10kV and only 1mA these will arc across 12mm of air or we have 10kV at 100mA these still jump 12mm but the arc is thick and strong and more like a plasma arc.

To increase the voltage to high tension or high voltage you will have a primary and a secondary winding and the ratio of the 2 will be how much the voltage is increased by.
The coil could for example be capable of producing 15,000V but the plug gap may only allow it the fly up to 10,000V.

To re cap the volts allow electricity to travel further but it is the current the has the power.

Not sure what the coil puts out but I do have kit at home to measure it!
I've scope and HV probe which can measure 40kV
I suspect with a standard coil plug gap is more critical and a sports coil ups the current or is more efficient or the copper winding wire thicker producing less copper loss.
I hope this helps,

Cheers,
Paul (Son)

Lesley understood that as well so I think it was a good explanation.

Nowt like going to the experts.

Best wishes

Brian

Hello Brian,

the point about plug gaps is valid and some people increase the gap over standard, but this gives a less concentrated spark and increases plug erosion. It doesn't, however give a better running engine.

The current a spark plug draws is quite surprising and apparently is in three phases. I would have to dig out the book that I read about this but I know it surprised me (By Alan Bywater, the owner of AJ6 Engineering?)

If a sports coil does draw more current then that would be to the detriment of the points, if used, so I suspect that they are purely a higher ratio primary to secondary so would have the same rated power?

Alec

PS, I just replaced the plugs on my newly acquired X Type, plug gap .0512"

Thanks for the replies I notice ken Jenkins has coils in stock so I will give him a ring tommorrow

Cheers and happy new year

Rich

Hi Rich,
I am tempted to get a 123 for my 3.8 as the 123 fitted to my Midget has improved things. I think the old dizzy was quite worn.
Has your 123 made a difference to your 3.4? Would you recommend them?

Thanks

Dave