Originally Posted by streborx

The Wikipedia page on Gasoline Direct Injection (https://en.wikipedia.org/wiki/Gasoline_direct_injection) deserves a read (actually 2 or 3 reads to comprehend it all), and while spark plug indexing is not addressed, the methods of direct injection described therein provide some insight into the benefit/necessity of indexing.

Direct injection can implement either a homogenous charge mode or a stratified charge mode. The homogenous mode injects fuel just prior to the intake stroke. Fuel and air are mixed during intake and compression. Ignition then occurs with a homogenized air/fuel mixture, and the location of the ignition spark within this mix is irrelevant (how would the spark plug know where the fuel/air mix originated?). In contrast, stratified injection occurs just prior to ignition (at the end of compression), and because the air/fuel mix is not homogenized, the location of the spark becomes critical (this seems to be the implication of the 2014 MB "technical" paper). I would agree spark plug indexing in an engine designed for stratified fuel injection charge would be critical and imperative.

Indexing spark plugs on an engine that uses homogenous charge mode doesn't hurt anything, but does it help?

Originally Posted by JettaRed

And I believe our engines use stratified fuel injection.