Could be the lenght of the actual ignition of the spark, or the wasted spark configuration or the current spark plugs used with the MSD.
I saw that same thing happening once to the Protege5 turbo, It was having heavy backfires and detonation when using an "X" brand of spark plugs. He didnt told me anyway about which sparks was working good with the MSD.
I dont know, I haven tested the MSD in my protege. this is only what Ive seen.
You can test the Electromotive crank trigger/ignition retarder too, its a similar unit, but you can retard timing on it and even to a full retard to spool up a turbo.