In a message dated 9/30/2010 10:47:06 A.M. Eastern Daylight Time,
SBoese@uwyo.edu writes:
Lynn and Bill,
Data recorded (including among
others: RPM, EGT, coolant temperature, and oil temperature) in
flight under SAG conditions compared to data recorded in flight
under spark plug misfire conditions (by disabling ignition coils) suggest that
the SAG event is possibly a preignition condition rather than
spark plug misfire due to fouling.
Examination of new and used spark plugs indicate that
the preignition may occur because of a gradual effective
heat range change of the spark plug with continued use as a result of
decreasing thermal conductivity of the copper core of the center electrode as
well as corrosion between the center electrode and the insulator at
the insulator tip. The corrosion appears to
be accelerated by the use of leaded fuel compared to the use
of 87 octane automotive fuel.
Let us review...............
We take an engine from a car that usually operates below 2,200 RPM, and at
partial throttle for 99.5% of its life, and put it in airplane and run it
at 6,000 RPM and wide open throttle for hours on end, and even a cold street
plug is boiling cement and rounding electrodes? Now who could have guessed that
would happen.
The overheated plug boils the cement near the tip of the plug,
thermally disconnecting the electrode from the ceramic, and allowing the
electrode temperature to run away. This could lead to preignition (an ignition
event that occurs before the planned ignition). Unless the mixture is well rich
of ideal or well lean of ideal, the pre-ignition would lead quickly to
detonation followed by the apex seals getting stuck in the muffler.
The high performance Mazda street plugs (turbo plugs) have shielding on the
shell ends similar in appearance to some aircraft plugs. They are not aircraft
plugs. They will not survive hours at full load wide open throttle. The heat
range is cooler than most street plugs it is true, but nothing like a racing
plug.
Real racing plugs do not have that shielding. They have a welded ground
electrode or a fine wire ground electrode stuck through the side of the
shell. They have porcelain filling the shell all the way to the end and may have
a fine wire center electrode. They are as cold a heat range as is possible to
produce. They are orders of magnitude colder than the coldest street plug.
More later.
Lynn E. Hanover