Ignition Leads

[andrew34ruby]

Adding suppression in the form of resistance in the leads, or resistive plug caps, has the disadvantage of less reliability. Rather than going through the resistance, the high voltages involved (thousands of volts) will not reach the plugs but will short circuit through the slightest crack in the coil neck, crack in the distributor cap, or crack in the rotor arm. With the addition of the slightest bit of condensation (damp) the high voltage might short circuit by tracking across the surface of the distributor cap, or short circuit through the slightly porous insulation on the older HT leads.
As resistive plug leads were introduced in the 1970s and became old in the 1980s the insulation became porous and the problems of not starting due to damp weather became worse.
To me, there seems a big advantage in copper leads and plug caps. Resistive leads with good insulation are ok when new.

As for coil resistance, there are plenty of options. Standard 12v coils of the 1960s are 3 ohms. 12v coils designed to use with a ballast resistor are generally 1.5 ohms. And 12v 'electronic ignition' coils fitted as standard to cars in the 1980s into 90s were 0.75 ohms. Standard 6v coils are probably around 1.5 ohms.
Aftermarket electronic ignition systems can be designed to use any resistance of coil chosen by the designers.

Me, I use 12 volts, with a 3 ohm coil, points and condenser. This was good enough for Austin throughout the 1950s, 60s and 70s, so it's good enough for me! Regarding radio suppression in those decades, I believe the carbon sprung contact in the distributor cap had some resistance in those days.

[Mike Morley]

I have contacted Power Spark Ltd, and they say, categorically, that using copper ignition leads WILL damage their electronic ignition modules.

[Colin Bromley]

I have a powerspark kit in my Triumph Herald, with silicone leads, it works fine. Earlier this year I tried to fit one to my 1930 Standard Nine special, which had copper leads, it was fine to 2500 rpm, then had a dreadful misfire... for various reasons, I went back to points, before selling the car. With the points fitted, it reved happily again.

[gezparton]

Josie and I have driven to John O Groats, Lands End, 4380 miles around the UK coast, several trips to Brittany, Scottish Austin 7 event in Guildton, the centenary event in Morton in Marsh and around the Goodwood circuit all with Powerspark electronic distributer and copper leads. So far no interference on the radio. (not fitted! )

[Graham Barker]

From my research so far, it seems that Powerspark units are quite different from Accuspark units.
On the subject of coil resistance, I think that Powerspark require less coil resistance. Their literature requires a minimum of 1.5 ohms. I asked them directly whether my existing coil with 1.1 ohms would be OK. They replied saying that because my engine runs 6 volts, the resistance of 1.1 ohms would be satisfactory.  From this I deduce that the requirement in their literature for 1.5 ohms must relate to 12 volt systems and because of the relative rarity of 6 volt systems they haven't bothered to mention the reduced resistance requirement. Their literature is very generic and email responses from them are very short, so I haven't bothered to enquire further on that.
I have watched a video of a man testing five different brands of electronic distributors. He classified them into two groups according to coil resistance requirement. He treats Powerspark and one other as having low coil resistance requirement and tests these with a low resistance coil. He treats Accuspark and two others as requiring a higher resistance coil and accordingly tests them with a different coil. 
Incidently, he advises that Powerspark are the cheapest and not his preferred unit but doesn't give his reasons. However he couldn't fault Powerspark on his test rig.
Powerspark were adamant in requiring resistive ignition leads in their response to my email querying the need for these, but didn't give any explanation. I see that Gez runs his Powerspark with copper leads. Perhaps he is using spark plugs with sufficient resistance to meet the Powerspark need for lead resistance. 
The Powerspark suggestion to use silicone ignition leads appears to be a reference to their own brand of ignition leads which are of the graphite core type.
I am awaiting the local Powerspark supplier to receive a shipment of the graphite leads, then will install the Powerspark distributor with the graphite leads and use my existing 1.1 ohm coil. I will let you know how it goes. 
Thanks so much for the many well considered replies to my post.
Graham

[gezparton]

Sorry I have made a mistake Josie has a Accuspark distributer ! Doh!

[Graham Barker]

Gez, thanks for correcting that. It helps make the distinction between the two types of distributors clear.

[Graham Barker]

    I have now installed the Powerspark distributor and Powerspark graphite (Silicone) leads exactly as recommended by Powerspark. It runs well, idles better and appears to have a little more power. A problem that I had with backfire on overrun is eliminated. 
The wiring  needs to be tidied up and the "notes to self" removed. 
Following the advice of others, I will keep the old distributor under the driver's seat just in case.