A "shorting plug" just shorts two leads together by directly connecting them with a zero resistance element.

A zero resistance R16/1 (which is a shorting plug) yields the slowest rate of spark advance with RPM and the least total advance. I didn't try to buy a zero ohm R16/1 at the dealer (may not be available in the US since it was never included with a US spec car), but it's easy to fabricate one for temporary use by just cutting a 1-2" section of 10 or 12 gage wire, strip both ends and fold the copper strands back over the insulation, bend it into a U-shape, then stick it in the connector, and tape it back up as it was originally with the 750 ohm resistor so there is no obvious evidence of tampering.

If you want to get fancy, you could buy another 750 ohm R16/1, remove the cover, then solder in a thin piece of wire to short the little resistor. Being as how I will only need my homemade shorting plug for a couple of hours every two years, it's good enough for the purpose.

Removing the resistor (which is the "open circuit" condition) yields the quickest spark advance rate and total advance, but the difference between open circuit and the 750 ohm resistor on US models is not much.

Right now I'm running without a resistor, 87 PON fuel, and am not getting any detonation as long as the engine temp is below 90 and the ambient temp is below 70. Above these values I get a little transient detonation if I short shift and apply the throttle too rapidly, but can "drive around" this by shifting at higher revs and easing back on the throttle. There's no detonation at WOT and medium to high revs, and I suspect the transient detonation is just a result of lag in the vacuum advance pulling off when the throttle is opened quickly.

Duke