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The following link is to NGK's site. It was a great read on spark plug performance.

An excerpt regarding modified engines-
"Q: Do I need to set the "gap" when installing a new set of plugs?

A: Maybe. A spark plug part number might fit hundreds of engines and, although the factory will typically set the gap to a pre-selected setting, this may not be the right gap for your particular engine. Insufficient spark plug gap can cause pre-ignition, detonation and even engine damage. Too much gap can result in a higher rate of misfires, loss of power, plug fouling and poor fuel economy. It is always best to check the gap against the manufacturer's specifications.

Another consideration that should be taken into account is the extent of any modifications that you may have made to the engine. As an example, when you raise compression or add forced induction (a turbo system, nitrous or supercharger kit) you must reduce the gap (about .004" for every 50 hp you add). However, when you add a high power ignition system (such as those offered by MSD, Crane, Nology) you can open the gap from .002-.005".
If you have any questions, please contact the NGK Sparkplugs Tech Staff here. The manufacturer of your vehicle, the company that produced the aftermarket products you've used and/or your mechanic are all additional sources of gapping information if you've modified your vehicle. "

Technical Overview to better understand your sparkies...

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