-------------- Original message from Lynn Hanover <lehanover@gmail.com>: --------------
Steve,
I hope Lynn chimes in here. He has seen a lot of plugs under a lot of
conditions. They look pitted to me. There seems nothing wrong with your
conditions. The only thing on the list would be too lean.??
Someone is out there writing up a great response right now! :>)
Bill B
That plug has been too hot.
Stock Mazda plugs are rather cold in heat range by way of the guards around the shell. The electrode runs a bit hotter than normal. Plug heat ranges are determined by how far down the shell the porcelain touches. The longer the heat path, the hotter the plug.
Using car plugs in the aircraft application seems to be asking for trouble. In the car on the freeway you use 2,600 RPM and 1/2 throttle, maybe 24 to 28 HP. Not much of a challenge for a spark plug. But there is the
driver who will jump into his 95 twin turbo in LA and run out to LasVegas at full tilt and expect to still have an engine when he gets there. And usually he does. So, if I was to use a street plug in an aircraft, I would use the 93-95 twin turbo plug, whatever that is.
The aircraft application, less the 100LL fuel problem, is clearly a cold racing plug with a conventional gap or close to it.
Years ago when we started racing rotaries, Champion said to use the N-57G Fine wire center with gold paladium plating and a conventional ground strap. About 20% of the center wire is exposed in an end view.
The N-57G and similar are discontinued and the N-82 series replaced it. The shell is thinner and the porcelain is wider. Same fine wire center, no gold anything. 20% of the wire exposed in an end view.
Both worked fine even with points and a Kettering coil.
Poor folks race with Autolight AR2692 end style and small porcelain just like the N-57G. Full size center wire, conventional strap, but 100% of the center wire is exposed in an end view. Actually used by NASCAR and the local circle track people, and stocked by Autozone and others. About $5.00 a box of 4. They work fine. I raced them one weekend, no problems, but we were getting the NGK plugs for free so I stuck with them.
The NGKR6725-115 stamped on the body actually means that the heat range is 11.5 not 115. $25.00 each.
50% of the center wire exposed in an end view. Free for some Rotary engined competitors. Retracted gap (the whole thing is inside the shell. Fine wire center and fine wire ground strap. Ice cold. Long body so it is easy to install and remove in the rotary.
I grind off those eyebrow things above the plug holes so I can use any plug I like. And any 5/8" plug just goes in so easy. Most heat ranges are available in the 5/8" shells.
Rotaries do not like high heat range plugs. The housings crack across the leading plug hole. Over time a complete set of radial cracks form.
In the picture, left to right is the Autolight AR2592, Champion N-82, NGK R6725-115, and Champion