Ed Wrote:

 

Hi Dave,

 

Thanks for the reports, I don't feel so lonesome {:>). 

 

 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.  :-)

 

 

 Actually, Dave you had a higher pressure ratio (manifold to ambient) at 10,000 feet with 26" boost than at sea level with 36".  At sea level the pressure ratio would be 36/29.92 = 1.20.  At 10,000 ft ambient pressure = 20.57 " hg , so the pressure ratio would  be 26/20.57 = 1.26.  Not much greater, but it was higher by approx.1.23" Hg. (1.20 -1.26 = .06 *20.57 = 1.234" Hg).

Ed A