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When tuning your BMW N55, S55, S63TU it is recommended to go with 1-step colder spark plugs. We've found the NGK SILZKBR8D8S (97506) in the picture attached working great when gapped to appropriate spec. If you'll be running less than 25psi 30psi of boost we suggest a 0.020022-0.023" gap, or . We've gone to 0.018" if planning to go higheron some cars with 30+psi but in most cases that isn't needed and will cause rough idle in some cases. In most cases though we just go with the 0.022-0.018023" gap and call it a day.
One great source is from: http://www.sparkplugauto.com/
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