Hi Brian,
I was getting round to sending you a PM about this. Sorry I didn't do this sooner - I do value your input.
As I said I was looking for official documentation - I'm glad to have found it. I do find it interesting that Mercedes say about 19 degrees before TDC and you've observed 15 degrees before TDC.
(Today) I can think of a two reasons why this might be
1) The turbo OM617 (the data for which I have not found listed by Mercedes) has its injectors set at a different pressure from the non turbo ones - the turbo engine might be designed to deliver fuel later than the non turbo for some air fuel ratio reasons that I don't fully understand yet
2) The equipment used to measure the point of delivery might be working in a way that is influenced by the ignition of fuel in the cylinder rather than the point of delivery
For example if you were to look at the output of the transducer mounted to an injector line it MIGHT look a bit like this =>
(please excuse my wobbly mouse hand!)
Now this is pure conjecture - the point I'm trying to make is that the electronics in a box might be tuned to pick out the biggest spike within a certain period of time and it might be picking out the wrong one - it is always nice to see an output as a function of time of a transducer - this doesn't happen with these systems...