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iridium and platinum are a waste of money. platinum is a very poor conductor so material transfer from electrode to ground is reduced for longer life But performance is not near what an iron or copper plug (NGK) will be.
NKG TR-55'S. Installed some the day before yesterday. Accidentally cracked the porcelain putting one in. I had a miss at higher rpms. Found the problem, now she runs great. Smoother idle. Also tossed in a zip tie mod as well, the car seemed stronger by the butt o meter. The air intake hole in the stock housing is ridiculuously small. It looks designed to restrict air.
NGK TR55s are definitely the way to go, but make sure you gap them at .050, not .060. It will give you much better throttle response, and better low end torque with the lower gap. (trust me on this one, I've been playing around with all kinds of different plugs and different gaps ever since I got my first LS1 powered vehicle back in 1998)
I, too, was running the +4's and the car ran great. I'm switching to LS6 heads, along with a new cam, longtubes, etc., and am now going to use Delco 41-985 Iridiums (factory recommended), as Bosch has no recommended plug for the LS6. I thought that very strange.
Ed