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I used to be the OEM copper plugs preacher. They do an excellent job, but you have to be watching the performance. Cleaning possible but not economical due to the low price for new plugs. Happy every one year or less, gap everything that you put in. Small price to pay.
Then came the V8. 94 GMC Sierra with a 350. American classic work truck, terribly picky about spark plugs and the fuel economy reflects it every tank. I tried about a dozen different plugs in the truck with varied results. Copper plugs last about a month or less before power and fuel economy suffer. Platinum plugs started out with less power and fuel economy and just got worse, slowly. Groovy and multiple elecrode plugs not any better than copper stock plugs in either raw power or fuel consumption.
Then came my buddy with the firebird. Rip roaring LT1 straightening curves and burning rubber everywhere he went. I helped him change the stock exhaust manifold and ignition system for super street components. During the swap, he took out his perfectly good optispark distributor, plugs and wires. I asked him what he was putting back in for plugs and he said no change, just new. So I kept his year old plugs to try in the GMC. They're NGK iridium tr55ix.
Buddy back on the road, now sounds like the dukes of Hazzard, thanks to the x pipe and flowmaster 10 series muffler. So I took out the existing plugs, I think they were Delco pro platinum super plus or something. His were pregapped in the box, checked them anyway. Still dead on after a year of rip roaring. Installed plugs and took a test drive and the difference was evident in the idle smoothness and the butt Dyno.
Those plugs are still in the GMC going on 4 years now and I still get a thrill out of the gas pedal. When it's really cold and the iac sticks, I might get a little rough idle while the engine is still cold. Once the O2 sensor warms up, the idle smoothness comes back. No complaints about the plugs from me at all.
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