NGK give general guidance on the best grade of spark plug for your engine
For every 50-75 bhp over the factory power output, go one grade colder.
The recommended spark plug for your engine (factory power output) is BKR6EIX
The next grade colder is BKR7EIX
Why are colder spark plugs better?
The more power a cylinder makes, the more heat it makes, which increase the likelihood of detonation.
A colder spark plug removes more heat reducing the potential for detonation.
Can I go toO cold?
Absolutely. Spark plugs have an optimum temperature operating range. Too hot can cause detonation. Too cold and the spark plugs are not hot enough to burn off residue from the combustion process ie fuel & oil leading to weaker sparks and even misfires.