[T3] ignition wire resistance

[Jim Adney]

On 15 Oct 2011 at 0:30, Dave Hall wrote:

> I think all the wires I've had in my air-cooled VWs have been stranded
> copper.  The '59 Beetle had them too, but the caps were unsuppressed (no
> resistors there) and made a radio impossible to use.  I think in '61 VW
> started using red carbon-filled leads, which were notorious for misfires
> when the lead core became broken.

I had a '62 beetle, but I don't know what it came with. I think I 
replaced the SP wires with a set that used "TVRS" wire, which was the 
carbon core stuff. One of my manuals talks about different SP end 
connectors to be used with the carbon core wire. From that I assumed 
that VW used that stuff for some period. I know that my '68 
squareback came with copper core wire.

> I suppose modern leads have different cores, but what's wrong with the
> original material?  

I used to be able to buy SP wire with stainless steel conductors. 
That was good stuff, but the extra resistance of SST is irrelevant in 
the face of the 1000 Ohm lumped resistance in the SP connectors.

The carbon core wire seemed to be more like a graphite-saturated 
string surrounded by rubber insulation. It was fragile and you could 
break the circuit by bending the wire sharply or pulling on it too 
hard. The whole idea was to introduce some resistance that would damp 
the high frequency ringing that tends to broadcast in the AM band. 
The lumped resistor works just as well while being more durable.

> Jim, is the spark plug wire you supply copper cored or 'self-suppressing'?

It's copper core. It's good stuff, made by Belden, which just happens 
to be in the town where I grew up.

With carbon core you have to worry about how well the core is holding 
up as well as how good the insulation is. With metal core, the only 
concern is the insulation. OTOH, with the metal core wire you need to 
check the resistance of the SP end connectors to make sure they 
haven't gone open.


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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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