I agree that a big factor (negative one at that) is the capacitance of the plug wires. It slows the buildup of voltage at the plug, making the system more susceptible to fouling. And having a resistor in the plug has a (slight, in my opinion) benefit in that regard (is that current "shaping?"). But any stress in the magneto is due to high voltage - the least stress would be to ground the plug wire and the most stress is with an open plug wire. The current during the spark event has
essentially no effect, as the coil is current-limited by its own inductance. The most effective "improvement" might be to implement a secondary air gap in the plug to prevent current flow until voltage has built up to some value. A trick from the automotive dark ages. I can't imagine that removing the resistor will cause any distress in the magneto, although it will increase the rate of erosion in the plug, and the distributor rotor air gap). Given that, I think the advice to check the resistance (a 2,000 Ohm max limit is probably as good as any) of any plug that is removed for service is good, and I will do it from now on.
Gary
The
person at Aircraft Magneto Service told me the spark plug resistor isn't there so much for RFI suppression as it is for current limiting and shaping. He said the ignition harness acts as a capacitor, and the harness + resistor + spark plug form an RC network and thus affect the shape of the current pulse(s) inside the magneto. He said removing the resistor not only leads to rapid spark plug wear, but also stresses the magneto and associated components. At least that's what I think he was telling me.....
I have no intention of substituting a 0 ohm resistor in the spark plug....i was just wondering if any of our Reno racers really do it and see any difference.
-isaac