You want to run the hottest plug you can, too many people jump straight down to 103s and 104s... no reason for that most of the time. If you have a 3.4 with headers there's no reason for you to be even as cold as a 104. I run AL605s in my car on the 3.2/3.3 pulley. I had 104s in there for a couple months and they misfired at low rpm and I had trouble starting the car sometimes... and I got ****ty MPG. Swapped out to the 605s and EVERYTHING was better. MPG, throttle response, power, starting, etc... I did back the timing off 1 degree just because... but I probably could have gotten away with keeping everything as it was.
