High performance Iridium racing plug for the 315, much improved over standard.
We have always used these plugs in our 315's with excellent results/
NGK official blurb..-
Designed specifically for the performance enthusiast, Iridium IX offers extreme ignitability, improved throttle response and superior anti-fouling.
 
Fine iridium tip ensures high durability and a consistently stable spark
Iridium alloy has extremely high melting point, perfect for high-performance engines
Trivalent metal plating provides superior anti-corrosion and anti-seizing properties
Outstanding acceleration, high fuel efficiency and durability
Ultimate design, technology and performance