Mmmm, not knowing much about the properties of re chambered heads I would say it would have to be something to do with the burn rate of the fuel charge.
Can you estimate where the plug was in relation to the smoke ring, can you count the turns out.
Unfortunatly not since ive cut the threads off the plug. Its the same head as you bought blaaster....but im not impressed, my compression actually dropped with it nd dunno but the performance is very much the same.