OK more research inspires more questions:
From the Options wiki page:
battleInstantGrenade When enabled, grenades are "hot". This means that when primed to 0 turns, they simply explode on impact, instead of waiting for the end of the turn.
How does this affect the timer? are all the values decremented by one?
ie.
0=explode on contact
1=explode at end of player turn
2=explode at end of enemy turn
3=explode at end of next player turn.
etc
or does it simply mean that 0=instant explosion and the other vanilla values are still true, i.e. 1=end of enemy turn ?