I have just changed my NKG Racing -9 plugs only because my HC's were getting to high , i made the mistake of cleaning them and i think this has cause damage to the plugs i can't moan because they have been in the car for the last 5yrs ,in that time they have seen atleast 15 track days loads of rd use miles awell (and off the rd for one yr), these had to be one of the best plugs for the cos , expensive but worth everypenny
i will add that they haven't not skiped a beat on boost /off boost perfect

i have heard that they don't make this plug any more

marco