[Sluttypatton]

Filling the bottle with CO2 will not effect the engine. Injecting CO2 into the engine will. If only for purging go wild. Injecting it into the engine, most likely, wont damage it. The only effect I see happening is the engine stalling out, as the CO2 will interfere with combustion. If there is still CO2 left in the bottle when you refill it, it shouldn't be a HUGE problem, but you if there is any significant amount left it may stall your engine. Also, you won't make as much power if there is still CO2 left in the bottle, as you won't be injecting as much nitrous. A small amount of CO2 may actually help prevent pinging, as it will help control combustion speed (Don't hold me to this though, I have never tried it).