Here in the uk the standard recomended plug is NGK BCP5ES. However I use NGK BCPREIX-11, which together with SHELL V-POWER fuel gives a better idle and a slight improvment on engine rpm pickup. (scientivicly tested by my right foot!)

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87 w124 zender 3.6 - 3.0 kompressor in progress!
96 w124 e300 diesel - slowly being modded
98 w208 230 kompressor - m103 testbed SOLD.
96 s124 om606 superturbo diesel 508bhp