Spark plugs!
I don't care what kind you buy , Just make sure that the gap is set correctly! This is the single most thing that will effect any performance that you hope to gain!
The biggest mistake that I have seen is that most people change their sparkplugs with a filthy engine! What good is changing your sparkplugs when you foul them just by inserting them in the holes!! Clean your area around the sparkplugs before removing them or putting new ones in!!
After setting and checking your gap , Do not let them touch the sides of the hole when you insert them!! The slightist nudge to the electrode can effect your gap setting!
A lot of people complain that they have a misfiire and that they have just changed out the sparkplugs , wires and cap and rotor. when all it was , was a fouled or incorrectly gapped sparkplug that they did themselves by incorrectly installing something as simple as a sparkplug!!
Sparkplugs simple , engine can't run without them! your performance ,well what performance without them?
Rule of thumb. bigger gap , normally aspirated engine. Smaller gap, higher compression , forced induction. the bigger the gap the more stress that is placed on the ignition system! keep your Gap at the propper setting!