Closer gap, are you (they) refering to gasoline as a starting point?
I have no reference to gasoline, but the reference in the Audi service book, which is not consulted in this matter yet!
The reference to the "wartime" gap was a reality in those days, with weak ignition coils and bad insulation on all hightension cables, bad condensers in the primary circuits, and you name it...
Accumulators dipping the tensions when cranking made it worse.
DC Dynamos could not keep up with the frequent fanning...
Now in the Audi, I made a series of gap settings from 0,3 mm upwards to 1,0 mm, 0,1 mm at a time.
There was still some "cracking" at 0,6 mm.
The refered sound is exhaust sound.
1.0 mm was giving absolute wonderful sound and power results.
All flags "clocked" in "downwind" to the incomming gas-mix stream!