Mick
It’s pretty simple.
Most of the things that can go wrong with a plug will result in the corresponding magneto having to work harder to make the plug spark. If, for example, the secondary coil voltage has to build to (and these are hypothetical numbers) e.g. 25,000 volts rather than 20,000 volts before the plug sparks, the coil is working harder, the coil is therefore working hotter, and the coil will therefore fail sooner. (By the way, on most standard GA piston engines, the coil is built into the magnetos.)
If you take a bog standard massive electrode plug, the gap is slowly getting bigger and therefore the voltage in the secondary has to build a little more to jump the gap. If left long enough, you can feel it in the engine. That’s why you gap massive electrode plugs.
Swapping to fine wire plugs solves at least the gapping problem, at least for 1,000 hours. (And please: Do yourself a favour and instruct your engineers not to go anywhere the fine wire plugs with a gapping tool unless they check with you first. If you’ve purchased Tempest fine wires, Tempest would rather replace or repair them for you, than have them wrecked by attempted gapping. And no: I have no direct or indirect pecuniary or other interest in Tempest or any seller of Tempest plugs. And yes: Your engineers might have successfully gapped old-style ‘dual fine wire’ plugs in the past, but they ain’t the same as the new iridium and other fine wire plugs.)
The internal resistor issue that is the subject of the discussion (for those capable of intelligent discussion) is the potentially more insidious problem. What’s undeniably happening is that when you measure the resistance of the internal resistor of a certain brand spark plug, using an ordinary, low voltage multimeter, the value becomes higher and higher as the plug gets older.
Now, the manufacturer of that brand plug says that the design and composition of the internal resistor is such that a measurement with an ordinary multimeter is
meaningless. The manufacturer says that its internal resistors only work when exposed to the high voltages present in standard ignition systems.
The real world experience is this: When that brand plugs with resistors that measure very high or open circuit on an ordinary multimeter are replaced, the engines to which the new plugs are fitted run better.
Now it may be mere correlation rather than causation, but it’s an amazing correlation: The known symptoms of bad plugs – poorer engine performance and ultimately magneto problems – seem to go away when the plugs with very high or open circuit resistors as measured on an ordinary multimeter are replaced with plugs that maintain a constant resistance below 5,000 Ohms as measured on the same multimeter.
Your first-hand, real-world experience seems to be consistent with everyone else’s!
