In a message dated 9/5/2005 12:31:18 AM Eastern Daylight Time, wdleonard@gmail.com writes:

         Regarding SAG - my analysis is that susceptibility to SAG is
        proportional to combustion chamber pressure at ignition and
        the condition of the plugs.  The more pressure - the harder it
        is for the spark to jump the gap, so if your plugs are getting
        coated with lead/carbon, I would expect to see more SAG with
        More manifold pressure.  Tracy and I have found the same thing
        (unturboed).  At high power setting (higher manifold pressure)
        the problem becomes more aggravated.  Reducing throttle
        setting helped - but, replacing (perhaps cleaning the spark
        plug center ceramic cone) is the cure.  I must have a fortune
        in little-used spark plugs setting around waiting for me to
        have nothing else to do but clean spark plugs {:>).

         Ok, that fits except for the fact that at increasing altitudes,
    the MAP at which the SAG occurred was lower (eg. 36" at s.l. but
    only 26" at 10k').   So something else is involved but I don't
    know what.  Time to order more spark plugs.  :-)

You folks are not old enough to remember this, but, you used to take your plugs to the local gas station where there was a Champion plug tester/cleaner. This thing had a high voltage source and a window in it so you could watch the plug gap while increasing air pressure around the plug. When dirty, the pressure could not be increased much at all before the arc would go out.
 
You then let out the pressurized air and pushed a button that would sand blast the plug tip.
You then repeated the test to find that full available pressure would not stop the arc.
 
In the rotary there is another problem. The mixture is moving past the leading plug at the speed of a rifle bullet. The fact that it lights at all is amazing. The trailing plug is in a little room by itself, with the fuel air mixture filling in around it much like a piston engine. This is much more likely to work than the leading plug situation.
 
When you loose a leading plug, you know right away. About a 15% to 20% power loss. You can loose a trailing plug and not notice it. About 5% power loss.
 
In a Kettering style system, the coil is charged up to 12 volts and to fire, the primary voltage is removed (electronically or by a set of points opening) to allow a field collapse an a high voltage (based on windings ratio) to be generated.
 
It will soon be the case that there will be no vehicle left on earth that uses the Kettering system, because of the pollution and/or fuel consumption problems.
 
The field collapse system in electrical terms is very slow. The secondary (high) voltage develops slowly, and this allows time for a path of flow to develop around the plug gap.
This what you are feeling in the SAG. Once a path of carbon is established, there is not sufficient voltage left in the parallel path to jump the plug gap.
 
A high energy system that has a very short rise time, eliminates the problem.
 
Also a very small plug gap can help the problem. I use .010" to .012" on both leading and trailing plugs with an MSD on both systems. One set of plugs per year. No problems up to 10,000 RPM.
 
Lynn E. Hanover