Spark Plug Heat Range - M117 Motors...
 

Which Bosch spark plug will run hotter in an engine: W9DC or W7DC ?

Reason I ask: I remember "knowing" that higher numbers = hotter plug. Then I heard that the opposite is true, due to some method of spark plug classification.

I see from a Bosch catalog that early M117 motors take the W7DC plugs, but later M117 motors take the W9DC. Did EFI motors require a different combustion temp than the CIS ones? Is this a no-catalytic vs catalytic requirement?

All M117 motors take the 75C thermostat, AFAIK. So will an early motor run at "cooler" combustion temps? Hotter?

Who can set me straight on all this?

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.
;)

My 4.5 takes W7DC's while the newer 560 takes W9DC's, both are US spec.

Mike

Proves my point, we didn't have gridlock in 1973. :D