MKI / MKII S type 240 340 & Daimler 1955 - 1967

Ignition leads.

 
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Old 06-26-2019, 02:17 AM
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Default Ignition leads.

Hello all,

I purchased some spark plug caps off E-Bay recently (the suppression type). I thought I'd check the resistance of each of the 6 and to my surprise found the resistance varied wildly from 220 K ohms to about 6 K ohms. I'm not an expert in the field of ignitions systems, but it seems to me I have bought some junk (Chinese? made, excuse the pun) and the correct resistance should be around the 5 kOhms mark.

So I have ordered some NGK ones which I believe will be of better quality. I have bought two sets, one set being the suppression type, the other non suppression.

I think the suppression type are designed to work with copper-core leads, and the non-suppression type with the more modern silicon leads which have inbuilt resistance, but I could be wrong.
Also, I have never understood why some resistance in the leads/cables gives a better spark, rather than zero resistance.

If anyone can confirm the correct set-up. i'd be very grateful for advice.

Cheers & beers

pete
 
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Old 06-26-2019, 12:11 PM
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Here's what I know, others may disagree with me or have something else to add.

The higher resistance in the wires, caps and plugs, the stronger the spark, but the system has to be designed to over come any extra resistance in the system.
The system has to over come any higher resistances and thus produces a stronger spark _ bigger gaps can be run too, but that can also cause other problems like pre-ignition/knocking/pinging.

The problem with our cars and any car that's still running points is that the points only stay closed so long in between the 6 lobes on the distributor cam.
The longer the points stay closed, the more current flow there is through the coil and the hotter the spark.
Coil designs are optimized to allow as much current flow as possible with out over heating the coil to achieve the strongest spark available in a given system.

You can use what ever you want to get the resistance up, but too much resistance and you will get no spark at all, or misfires.
I've set mine up just to keep the radio quiet, I use solid copper wires, non-resistor caps and NGK 5K OHM resistor plugs.

I never liked using modern resistor wires in cars with points, mainly because the wires were designed for cars with electronic ignitions.
There is simply too much resistance in the wires, especially the longer runs.
They will work, but as the wires age, you will get a misfire.
I find 5000 ohms in either the plugs or the caps is plenty.
You could probably run both provided you use solid copper wire.


I'm running a Bosch 3 ohm coil that does not require an external resistor, in case you're wondering.
 

Last edited by JeffR1; 06-26-2019 at 12:20 PM.
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Old 06-27-2019, 04:41 AM
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Thanks very much Jeff. That info will help a lot.

regards

pete
 
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Old 06-28-2019, 01:31 AM
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Hi Pete,

Jeff is pretty well on the right track, hopefully I am not disagreeing with Jeff too much, but adding something extra.

Resistance in the HT side of the ignition does not apply until the spark is initiated, voltage is required to initiate the spark, and there is no current flow until the spark jumps the gap on the plug, the current flow in the conductor (HT lead) induces a magnetic field which obviously comes and goes every time the spark is initiated, this is the RFI inteference that we are trying to remove.

When the spark initiates current flows, the coil only holds a certain amount of energy which is voltage x current x time, by adding some resistance we reduce the current flow and the energy in the coil is dissipated over a slightly longer time so a longer spark is achieved and by reducing the current we also reduce the RFI, but the voltage at the plug after spark initiation is also reduced. So more resistance gives a slightly longer but not stronger spark.

HT Leads come in basically 3 types, solid core, carbon coated core, and spiral wound. These can come in EDPM (rubber) coated or Silicone Coated.

Solid core transfers the best voltage/ current but the most RFI

Carbon core are formed by coating kevlar or similar strands with carbon and coating with the insulation (silicone or rubber), these are more flexible and have goood suppression, but do break down over time as the carbon burns away.

Wound wire leads have a similar Kevlar or Nylon core over which is wound a coil of wire, these create an inductive resistance, and are wound in several ways which make them better or worse, a well made spiral wound lead has many wore windings per inch, and is more expensive to produce, so cheap leads are not the best leads, this type give good RFI suppression and good spark characteristics, but good quality leads are expensive and working out the "snake oil" from the best is not as simple as you may think.

The difference between Silicone and EDPM leads (this only refers to the outer insulating cover) is that Silicone has better flexibility and is better at higher temperatures than EDPM, but generally for the use we all have for them either will suffice.

So in summary, avoid the Carbon Coated core type leads, good quality wound core leads are probably the best, but the higher cost really is not worth the gain for Classic car use. So Jeff is using the best solution in my opinion, and you can use either resistance plugs or suppression caps, but not both.

I have summarised this somewhat and may have lost a little detail by doing so, but hopefully have not changed the context too much.

I hope this helps.
 
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Old 06-28-2019, 03:34 AM
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I'm surprised to hear that Kevlar is still being used, that stuff breaks down into a crumbly mess after a while.
I imagine the process happens faster with the heat in the engine bay too.

When shopping for wires, I don't remember them listing what exactly they're made with either _ always lots of marketing hype though.
 
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Old 06-28-2019, 03:45 PM
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Originally Posted by JeffR1 View Post
I'm surprised to hear that Kevlar is still being used, that stuff breaks down into a crumbly mess after a while.
I imagine the process happens faster with the heat in the engine bay too.

When shopping for wires, I don't remember them listing what exactly they're made with either _ always lots of marketing hype though.
Unfortunately yes they do, carbon impregnated leads should be changed every 12 to 15000 miles, may be a few years in a classic, but almost annual on a daily car ! Not a lead that I would choose.

There is lots of hype re marketeers, especially when they want to sell "snake oil" wound leads, the number of turns and the size and type of wire determines quality not the mere fact that they are wound, with a solid copper core and resistive cap or plug you at least know exactly what you are getting.

With a car that had sensitive ECU etc, I would go with a good quality wound lead as these give the best suppression together with good spark quality, but on a classic, I would always stick with copper cored leads.
 
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Old 06-29-2019, 04:59 AM
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Hi all,

A good friend loaned me a set of leads (with factory distributor cap) today. The leads (all bar one) checked out with about 2 to 3 K Ohms resistance.

Upon further inspection, 5 of the leads were Magnecor, and one Bosch. The Bosch one was about 6 K Ohms resistance.

So I latched onto the Magnecor US website, which gives a product code of 967334 for the XK engine. Price is US$136.65 plus postage (which is a killer from US to Australia).

Given there seems to be a lot of rubbish on the market, the old adage "you get what you pay for" obviously applies here (as usual).

So I might engage in a bit of experimentation here, with the made up Magnecor leads, and the standard copper core leads with the NGK 5 K Ohm plug caps.

I'll file a report in a couple of weeks.

Cheers & thanks for everyone's input.

Pete
 

Last edited by redtriangle; 06-29-2019 at 05:04 AM.
 
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