Date registered: Aug 2009
Vehicle: '72 250C, '74 280C, '85 500SEL, '81 300CD.
Location: Tucson, Az.
Mentioned: 3 Post(s)
Quoted: 53 Post(s)
The numeral in the plug's listing is for heat range. For whatever reason, NGK's heat rating numbers decrease as the plug gets "hotter". The "5" burns a little hotter than the "6".
You generally increase the heat range on a car which; tends to burn a bit of oil, runs consistently rich, has a weak ignition spark system or is used in mainly low speed applications. No significant performance issues, it's just a "work around" to address a specific problem.