I have been doing alot of searching and i am trying to figure out the best plug for my car.

My car only has a downpipe, stock cat, and turbo XS catback for mods and in the upper RPM range I am having a hesitation/breakup. Everything else is still factory up the the air filter which I will be changing this week. However I discovered the car had the magnecore wires which I have read mixed reviews with.

Then i thought man, these cars boost out of the factory at what, 19ish PSI peak. And with the exception of a downpipe and catback wouldn't that free up maybe a few more PSI? Here we are at 5,000 feet about sea level and have 91 octane.

Is the factory boost overkill for our octane or is the factory ECU tuned to handle that?

I have bought three sets of plugs to play with. The NGK 7EIX iridiums are my favorite because they run smoother and seem to have more of a response but they break up between 5000-6000 rpm range.

The NGK BPR7ES's seem to work fine but it seems that the car has a slight loss in power. Should I go 8RES's? to be safe in these colder weather conditions making things lean out a bit? Thanks for any input sorry for the huge post.