No more than one step colder . Some one went 2 steps , and said is was to cold of a plug

 

I think my misfire at idle could be the Copper plugs I'm using, so you may want to stay away from std coppers. I know some new car's like the Mitsu Evo8 have weird coils that don't like copper plugs.

 

http://www.cj-motorsports.com/91psi115af.jpg

I pulled that dyno run with about 30,000 miles on the original spark plugs that came in the car when we bought it... i had never even had them out once at that point.

If you do anything... one step colder but same make/model plug. keep the gap at stock too. For up to 400rwhp i would probably just keep totaly stock plugs... but 1 step cooler might be a good idea.

 

I know when I installed the Vortech SC they advised to change plugs if you have over 10,000 miles on OEM plugs. As hard as they are to get to, I would change plugs regardless of the miles. I went with NKG Iridium LFR6A1X-11. These are 1 step colder.

 

That's the same exact plug I use. Do you have any misfires or rpm drops at idle? Around ~600 rpm?

 

When you say "misfires" , are you getting a CEL?

To clarify, NGK LFR6A1X-11 s are Iridium , you mentioned copper,
So which are you actually having the issue with?

 

i have 8's but had spome misfire probs i would go w/7