Two years ago I start the try and error way: the original copper wires, the fiber core w/pvc envelope, fiber core w/8 mm silicon envelope and plated copper core with 6 mm silicon cover.

The original ones was too cracked to be useful; both fiber wires working good on the initial setup, but develop some internal cracks after few months of use(engine sputtering). I replace it with the plated copper cores (I adquire it by the meter on a local VW bug supplies provider with a associated set of cups). Really, plated copper ones was the best for our old beasts (but not for the AM radio reception).

At end, since a year ago on my car the wires from distributor to spark plugs is plated copper, and the wire from distributor to coil is fiber (I carry a additional spare of that wire as a safety measure). The AM tune is good, and my engine work as a charm.

But, if I remember right, on some states of US, irradiating radio noise (a secondary effect of copper wires on some engines) won a traffic ticket if a police patrol detect it (happly, not on my country).
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Joaquin / Rojo 121 / Lima, Peru