This is a BMW m51 #1 injector:
http://www.brit-car.co.uk/shopimages/products/allmakes/normal/S/STC/STC4218.jpeg

It has feedback to tell the computer that the needle has lifted - start of injection.

If the upper body housing the sensor fits a Mercedes bottom you could use the signal to get:

Input for strobe light and real btdc injection angle.

Input for rev counter.

What do you think?