In a nut shell, Steve's excellent data gathering (how many of use have access to a scanning electron microscope or spectrum analyzer {:>)) and analysis indicates a possible alternative reason for SAG other than fouling.
It basically boils down to a theory of pre-ignition based on the deteorating heat removal capacity of the plug.
There is a copper core (the photos showing the two cores) in the center of the ceramic cone which is responsible for conducting 90% of the heat away from the center electrode. Based on Steve's' electron microscope viewing of two cores (one new and one having been used in a 100LL environment), it appears that the plug using leaded gasoline has corrosion occurring on the boundary between this copper core and the surrounding (hot) elements. This layer of corrosion hinders heat removal from the plug leading to parts of the plug becoming hot enough to trigger pre-ignition.
This SAG tends to happened at high power settings and reducing power tends to make it go away (for a while). Steve's theory is that the plug encounters max heat and temperature at high power settings and that reducing the power permits the plug to cool off enough for the pre-ignition to go away (for a while).
So we now have two theories - one is the fouled plug and one of the overheated plug causing SAG. Preignition generally leads to detonation which does not appear to be happening (at least no one is finding the typical detonation damage to the rotary engine). A possible reason is the quenching effect of the less than optimum shape of the rotary combustion space may tend to delay or prevent the onset of detonation damage that one would expect from continuous preignition caused by a too hot plug.
I believe Steve intends to see if he can find some colder plugs (the ones that conduct heat better) and see if that extends the time between SAG. Currently I find that roughly 25-30 hours is about the max I can get using 100LL before SAG sets in, whereas Tracy and others have well over 150 hours using unleaded Mogas. So if the colder plugs extend the time before SAG, I would say that pretty much would suggest Steve's theory is correct - if Not, then perhaps the fouled Plug theory still has merit or it could be some other phenomena happening.
Steve, my apologies if I did not present your case as well as you could
Ed
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Sent: Thursday, September 30, 2010 9:59 AM
Subject: [FlyRotary] Discussions of SAG
I understand that there was a presentation on SAG at the meeting. Can someone go over the alternative theory with the ones of us that were not in attendance? It looked like someone had a scanning electron microscope photo that they were showing! Wow! Are we getting sophisticated or what!! Was this an explanation of the cause, or ideas for prevention, both??
Bill B