[KIWI]

I ordered a few parts from MWR and one of them were 4 new spark plugs.
I got the NGK BKR6EIX-11 Iridium Spark Plug as my car is still NA, now my problem is i checked what ones i put in my car 50000km ago and they were NGK BKR5EIX-11.
I don't know much about spark plugs but what is the main difference between the two? Apart from the heat range of the two anything else different?
The car is mildly modified (mods listed in my sig) and i have had the timing advanced via the unichip to run the higher octane fuel.
Will using the NGK BKR6EIX-11 Iridium plugs instead of the original ones cause any major issues like rough idle or poor performance or even any sort of lean/rich issues?

 

[plebian]

If you get a problem with fouling go back to the 5's, but the 6's are colder plugs, colder plugs usually tend to be better for the engine because of better heat transfer.

 

[KIWI]

Thanks Plebian, when you say fouling you mean they are covered in black carbon/soot like substance?
The original ones were black, but my current ones a few months back had a slight brown hue to them, was told this is normal

 

[Ken Branch]

A slight brown hue is what you want, probably the right heat range of plugs for you.

 

[carlbecker]

This may help:
http://www.ngksparkplugs.com/tech_support/spark_plugs/faqs/faqread.asp

for reading plugs.