The TDI injectors have two stages of injection, a small quantity at the start, called the pilot injection, and the larger, main injection. This is done to primarily to reduce noise and NOx emissions.

The two pressure figures simply refer to the pressure at which the pilot- and main injections begin to occur, respectively. 190/300 means that pilot injection occurs when the pressure reaches a threshold of 190 bar, and 300 is the threshold for the main injection.

A larger number indicates that the threshold for the beginning of injection occurs at a higher pressure; it follows that atomization should be better, but that's a secondary effect. Primarily, the relationship of the two pressures will also determine how far apart the pilot- and main injections occur.

For example, having both pressure close to each other may give an injection profile like this:
_/\_/\_

Whereas if the pressures are wide apart, it would look something more like this:
_/\___/\_ (not to scale and exaggerated for illustration).

The first peak is pilot, the second peak is main. The pilot injection quantity will usually be very small, in the order of 1 mg/stroke, while the main injects the rest of the quantity required for the corresponding load.


http://www.myturbodiesel.com/forum/f9/tdi-injector-pressure-testing-tools-794/


http://forums.tdiclub.com/showthread.php?t=220755