The Trigger Voltage :
1. Di466N - 3.8V / Di466C - 4.8V
2. Di622N - 3.8V / Di622C - 5.8V
3. Di622N MARKII - 3.8V / DI622C Mark II - 3.3V
4. Di866N - 3.8V / Di866 - 3.4V

This is the official range of trigger voltage of the Nissin flashes.