My two cents worth!  The prop unstalls a bit and loads the engine more as
speed increases. The situation is more pronounced at low elevations if the
engine is running over lean (lean of peak EGT). At elevation the engine is
closer to best power if the mixture is not changed. Best power is just rich
of peak EGT. So at elevation the loss of power is reduced as the prop
unstalls. Actual power is reduced also but the effect remains the same. Look
at the cement around the center electrode of the damaged plug. If there is
evidence of cement boiling up and escaping around the electrode then the plug
was very hot indeed, but this may not be evidence of detonation. If the
insulator is normal in all respects except for a missing piece, then there
may be other explanations. The plug may have lost the insulator long before
it started to act up. The unshielded electrode can become very hot and start
preingniting the mixture.

How much was gone? A long run at high power on a plug with too high a heat
range will often swell the electrode and pop off a piece of insulator. We run
ice cold NGKs with fine wire electrodes gapped at .012" I have yet to foul
one out even while destroying an engine. If the engine is still running as
before, I would say that little or no detonation occurred. Detonation is the
fuel air mixture igniting on it's own long before the plug fires. The rotary
will not tolerate this but for a second or so at high power settings. I mean,
as in, the front cast iron cracks in two through the oil passage when the
offended rotor housing suddenly gets much longer than it should be. I was
lucky to save the distributor.
Street plugs are great for street cars. They are a very hot heat range to
keep them clean while lugging around the town. If you want to hammer around
at full throttle above 4,500 RPMs for hours on end, I would look for a much
cooler plug.
Lynn E. Hanover

 
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