Posted for Rick titsworth <rtitsworth@mindspring.com>:

 Can you further explain or site a known/scientific reference source for your
statement below?
 
 "High altitudes increase the voltage required to jump the gap on the plug"
 
 Commonly understood engineering/physics is that increases in air
pressure/density impeeds a spark.  Thus, as air pressure/density is reduced
(increased altitude) a spark will normally be EASIER to initiate.
 
 Reference: "...The Physics Fact Book" @
http://hypertextbook.com/facts/2000/AliceHong.shtml - "The dielectric strength
of air is approximately 3 kV/mm. Its exact value varies with the shape and
size of the electrodes and increases with the pressure of the air...".
 Additional factual references are cited at the link above.
 
 Additonally/empirically, I've seen weak coils that would "spark" a plug out
of an engine cylinder, but that would not create a spark under the pressurized
conditions inside a "compressed" cylinder.  e.g.  high pressure increases the
voltage required to create a spark.
 
 Why would High altitudes (low pressure) require MORE voltage???
 
 
 Rick
 - always open to learning something new, if true.