So im an auto parts store worker, probably a cliche one at that. I'll admit I'm not a guru by any means but I know enough to help out the average joe. So I decided to tune up my 86' 300zx turbo. I get a healthy discount so buying the most expensive plugs wasnt a problem. Best we offered happened to be iridium. Well, the day after I changed them I was talking with a customer and he was mentioning that a colder plug is best with a turbo engine. Well im getting a little pre-ignition, metal on metal sound. Dosnt sound nasty but it's not purring. Eh so im putting that info out there. Any input, recommendations, whatever.