spark plug heat range advice

[chris-mr2]

Interested to know what heat grade plug is recommended for a meth injection setup, im currently running a 8 grade instead of a standard 6 grade and my plugs elctrodes are very clean and slightly white whilst the rest has a sooty coating.

Im aware allot can affect spark plugs but am keen to find out if others run standard heat grade plugs or colder ones like me