A Bosch W 9 plug is hotter than a W 7.
Most sparkplug mfg's. go with higher numbers = hotter plugs.
The exception to this rule is NGK.
A # 7 plug would be hotter than a # 9 plug.
Heatrange has nothing to do with engine temperature.
M-B may have specified a W 7 plug for the european consumer ( highspeed autobahn driving ), but we are much less fortunate in this regard.
Stop & go driving ( that's us ), would call for a hotter plug, to minimize fouling.
Too cold a sparkplug also places greater demands on the ignition system.
