This family of plugs is available in heat ranges 6, 7, 8, and 9 ,and the higher the number, the "hotter" the plug. For a given set of operating conditions a "hot" plug should be used if average engine output demand is low and a "cold" plug if average output demand is high - such as racing or high speed autobahn driving.

As stated, Mercedes recommends heat range "9" for most USA M103s. If you do lots of high speed driving - say over 160 KPH - a colder plug than "9" would likely be more suitable. Also, as stated, check Mercedes heart range recommendation for your typical local driving conditions.

Dukec