Originally Posted By: NMBurb02
Originally Posted By: martinq
Originally Posted By: NMBurb02
If you want a small sharp area that doesn't wear, you want iridiums.
The difference between iridium and platinum is very small but both are significantly better than the standard plug. You will find that iridium is usually priced significantly higher and this is usually only a single tip application. Dual-platinum should be significantly better than iridium but applications that would benefit are very rare I think.
What you'll also find is that the ceramic insulator gets layered with deposits that eventually will affect ignition performance. This can take a long while in a clean burning engine.
I find that the best value (by far) is a single-tipped platinum and I'd change those out every 2-4 years to keep the insulator and threads fresh. This will vary by application as I've found some OEM standard plugs that are $20 to $30 without a platinum option.
Quick example for one of my applications (RockAuto prices):
NGK 7133 $1.61 (std)
NGK 7084 $2.42 (plat)
NGK 3903 $6.45 (irid)
I believe the main performance difference between iridium and platinum are the service lives of the plugs. Plats are meant to last 60k miles or less, whereas iridiums go as long as 100k. At the prices you listed, it doesn't make economic sense to go with iridiums unless the plats will only last 30k miles and you plan to keep the car for 100k+ miles.
That being said, spark plugs aren't a major investment, unless you replace them frequently or have an engine that requires a gaggle of them (like the '98 MB C280 I owned at one time - 2 plugs per cylinder, for a total of 12!).
That's it, only 12? I have 16 in my Hemi Magnum and I change them every 40,000km (25,000mi) whether she needs them or not, and I shell out the big bucks: either Bosch Platinum +4 or NGK Laser Platinum. Gran tit, I am running significantly higher horsepower and a turbo setup, but any spark plug SHOULD be a major investment.
And, if you have an S/C Grand Prix ('06 GT) which I think you have, then I would definitely suggest you run Autolite Iridiums or NGK Laser Iridium in that beast or you are going to have some issues later on down the road - I have one myself.