Spark plugs
 

I want to change the plugs in my 1987 - 420SEL. Th book calls for W8 plugs, but I presently have W9 plugs installed. What is the difference between the two and what are the advantages of one over the other?

stick with the w8DC's the work fine.
m

The difference is in the heat range -- I don't remember which was Bosch numbers go, hotter or colder, but each heat range has a different length and diameter of the ceramic insulator. This results in the tip of the sparkplug being hotter (longer, thinner insulator) or colder (shorter fatter insulator) for any given operating condition.

Stick with the standard W8 unless you have evidence of overheating that isn't related to lean running (fix the mixture control if it runs lean), or cold fouling (very unlikely).

Peter