Iridium is six times harder than platinum and withstands temperatures in excess of 4000 degrees. Platinum will withstand temperatures in excess of 3000 degrees. The electrodes on Platinum, and more so with Iridium, are smaller than standard plugs which means the plugs will fire even in extreme cylinder pressure conditions and with less voltage.
Does this mean you will get increased horsepower from swapping out plugs?
No, not if your current plugs are currently igniting your A/F mixture properly.
It means the smaller electrodes are able to ignite the A/F mixture in higher compression motors than stock without needing to narrow the sparkplug gap. Also, being of a harder metal than standard plugs, they will last longer before needing replacement. Thats why a lot of new cars can run the OEM plugs for 120,000 miles before needing replaced. Thats about it in a nutshell.
I've been running the NGK DCPR7EIX Iridium plugs on my carbed
'05 Night Train for about 6 years or so now.
Going to try the Denso IXU22 Iridiums this year. The Denso's have a .4mm electrode, vs .6mm for the NGKs.