Regarding iridium plugs from the Denso site as NGK doesn't post tech info:
Q. Do I need to set the gap?
A. Generally, no. The DENSO Iridium Power plug comes pre-set with a protective sleeve over the firing end, to protect the gap from accidental alteration. DENSO Iridium Power’s ultra-efficient firing power compensates for normally recommended gap settings that are smaller than the pre-set value. In the cases of vehicle modifications (nitrous, turbo-chargers, super-chargers, high power ignition systems, etc.), some adjustments may be desired. Or, if you prefer to remain consistent with factory specifications, you may adjust. However, please be careful not to place any stress on the fine center electrode during adjustment. You could accidentally break off the very hard, and therefore brittle, tip.
Denso iridium
FAQ page
