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Spark plugs are made to operate in certain heat ranges ie certain combustion chamber temperatures. There is a trade off between giving good cold start performance and not causing detonation. A spark plug that gives good cold start performance also has more potential to cause knock.
Car manufacturers probably spend millions designing just the combustion chamber so if your car is close to standard the original spark plugs will probably be the best choice.
However if you have modified your engine your combustion chamber could be operating at higher temperatures which means your spark plug is hotter which could mean it is causing pre ignition.
As a general rule NGK recommend using a spark plug that is 1 grade colder when the horsepower of the engine is increased by around 75bhp.
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