I read this on the NGK website:

"A rule of thumb is, one heat range colder per modification or one heat range colder for every 75-100hp you increase. In identical spark plug types, the difference from one full heat range to the next is the ability to remove 70°C to 100°C from the combustion chamber."

I'm currently running BKR7Es, one range cooler, and I'm seeing EGTs between 1300-1400F while cruising. If a step cooler plug can remove 125-175F, then that should also correlate to a drop in EGTs, right?

The only thing I'm worried about is fouling the plugs out at idle.

as long as you dont drive like a old lady the plugs run fine. but they do have a tendency to get fouled if you do a lot of "putting"

they did help the WOT detonation i was having, but i never saw any decrease in EGT's

in the end i pulled them & settled for 1 heat range colder. the car seemed to start nicer when the engine was cold.

if i was to do a track event or something similar, i might consider tossing them in short term, but IMO they were a bit too cold for a street application

my egt's at cruise only reach 1100 max and that is when I am cruising at very high rpms.

Mines right at the #4 cyl.

I've got one in the DP too, and IIRC, it reads ~75F cooler.

Maybe I need more advanced timing, who knows.

now I can try running some actual 91 octane in the car and see about getting more timing then I have now for it. Also just did my first 6k rpms launch with the setup and all I can say is DAMN 7k comes fast in each gear. Now to turn the boost up past 15-17psi.