Can anyone tell me why...

[sparkybean]

HT leads have resistance? Surley its better to have a low ohm reading across the lead to get a nice beefy spark, correct? What dangers are there in using ordanary copper wire?

Cheers.

[MK_Bob]

I'm guessing that voltage is more important for jumping the spark plug gap than current?

Since: Voltage = Current x Resistance, by increasing the resistance, the voltage is increased.

Something like that?

[Tanguero]

The resistance is there to limit the peak current. At high voltage there is negligble drop across the lead resistance, but enough resistance to prevent the high current you would otherwise get with what is effectively a short circuit (when the spark is present) with a low resistance lead.

[Pigeon]

No, the peak current is limited on the primary side mainly by the primary winding inductance and resistance. The peak current available on the secondary side is the maximum primary current divided by the turns ratio, which works out orders of magnitude less than what the HT lead resistance would limit it to.

The resistance is there to damp oscillations to reduce radio interference.

You are "better off" without it as you get a more powerful spark but the difference is insignificantly small.

[Tanguero]

Pigeon said:

No, the peak current is limited on the primary side mainly by the primary winding inductance and resistance. The peak current available on the secondary side is the maximum primary current divided by the turns ratio, which works out orders of magnitude less than what the HT lead resistance would limit it to.

The resistance is there to damp oscillations to reduce radio interference.

You are "better off" without it as you get a more powerful spark but the difference is insignificantly small.

Fair point - live and learn