indian036 wrote:One of the tricky things about this is the use of the term "resistance". This generally refers to current restriction in a steady current DC situation.
When it comes to a varying current and voltage situation, other things come into play. This applies whether it is a regular change such as the 50Hz sine wave from a wall socket, the wild variations of an audio signal from the stereo or a pulsing situation from an ignition system.

In this case the correct term is impedance, though confusingly, it uses the same units, the Ohm. (Ω) Impedance is a combination of resistance, capacitance and inductance, though not a simple sum. To add to the confusion, impedance changes with the frequency of the varying current! This gets even weirder the more the changing current varies from a sine wave.

When we check a plug lead with a multimeter, we are measuring resistance, not impedance. However, it is satisfactory for checking whether something is way out of whack. As people have found, resistance generally increases with the lead's length. If one lead's resistance is strange compared to others and their lengths, it needs replacing. I'd replace the lot in case another one is going to go strange soon.

As Rick G says, an impedance balance between the source (coil) and the load (spark plug and leads) is essential for maximum energy transfer. The coil will always have an impedance because it is just that - a coil of wire which has inherent inductance, some resistance but little capacitance. The trick is to have a matching amount in the plug/lead combination. Pretty much universally, its either impedance in the lead and not the plug, or in the plug and not the lead.
If you have impedance in neither or both, it won't match the impedance of the coil.

Will this cause a problem?
In the short term, usually not. Longer term, it can, as Rick says, lead to coil failure. Not cheap on our toys!
With the less-than-optimum power transfer, as components deteriorate with age and use, the spark may not be as strong as desired, leading to misfires and less efficient combustion. 

Conclusion: Follow the manufacturer's recommendation of lead and plug "resistance". They've worked out what works best for the correct impedance, which in the situation in question, we can't easily measure with normal equipment such as a multimeter. They've also worked it out for the frequencies that will apply in the circumstances.

In an emergency, to get home, don't worry about a short term mismatch, but don't think that because it seems alright it will stay that way long term.

Bill

Point-Seven-five wrote:At least I can't be called a HALF wit.

Dai wrote:Just to state the obvious and for completeness: if you replace the HT leads with non-resistive ones, fit 5KΩ resistive plug caps (very commonly available) and continue to use D7EA/X5DC spark plugs.