copper is better??!!!!!!
Iriduims can be used to well over 50,000 miles even for a weekend autox-er. I didn't really notice them begin to age until around 20-30,000 miles. Many cars are fine using them upwards to 100,000.
Hey guys, the search button is your friend. These questions has been asked and answered many years ago by people with experience. There is a lot of guessing going on here rather than correct information.
If you can't find it on this site, check another evo site.
this what I found.
http://forums.evolutionm.net/evo-eng...ead-plugs.html
100hp increase is a rough rule of thumb.
With a proper tune, it is not necessary to get a colder set of plugs unless you are around 375-400whp on GST's dyno.
They will also foul sooner.
Whitrice is still on the stock heat range plugs and he has over 400whp....