My thought with the injection timing is in regards to flame travel in the cylinder. CNG's rate of combustion is faster than that of diesel fuel. Once the diesel fuel ignites, the CNG will spontaneously ignite as well but it will burn faster, possibly creating a pressure spike large enough to cause damage.
Think of it this way. In some instances, the injection event is commanded 30+ degrees BTDC. It doesn't always happen there and ignition doesn't always occur instantaneously, but there is some mad advance there. If you are running straight LPG or CNG in a gasoline vehicle, the ignition timing will be set near stock levels of 8-12 degrees (maybe just a little more advanced). If you're igniting the CNG at 20+ degrees BTDC, it's going to combust, expand, and be done burning by the time the piston reaches TDC.
Now the actual concentration of CNG per unit of ambient air is much lower than that of what a dedicated CNG-fueled vehicle will see so the detrimental effects of CNG will probably be realized only when a whole boat load of the gas is used.
These are just a couple thoughts I have.
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Tuning, flashing, burning chips, and repairing all aspects of 7.3L Powerstrokes.
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