MSD crank duration?

[Paul Ko]

Does anyone know how the MSD 6A and similar boxes determine the
advertised "20degree crank duration" of the spark?
>From what I am guessing, the units measure out the timing between two
spark triggers to determine the "total time" between the sparks.  Then,
knowing the "total time", which equals 90degrees crank duration on a V8,
it holds the spark duration to 22% duty cycle, or 20/90 = .22

Well, if the MSD unit is used on a 4cyl, that 22% duty cycle will be
40degrees crank duration.  I'm using the units on the trailings,
direct-firing the trailing plugs.  The trailings fire once/rev, so the
MSD is hold a 80degrees crank duration on it!!! That is dangerous.

Can anyone confirm my suspicions?



-- 
Paul Ko
pko at mindspring.com
http://www.mindspring.com/~pko