the 411 on spark plugs: http://www.motorsport-developments.c...AF272_tech.pdf
Iridium is preferable because the thinner center electrode diameter allows for a higher quality spark and because iridium degrades much slower than copper it allows you to run a larger gap (or just replace your coppers constantly). Also the AZFS22C is the proper heat range for a regular Zetec but I don't think it is the proper heat range for a SVT. http://sparkplugs.com/results_app.as...1&AAIA=1385580
also helps out a lot for the specs for different plugs and they have some of the best customer support I have ever worked with.