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You can't really use the stock ECU with 555cc injectors... 555cc injectors will flow 555cc of fuel every minute its open, while stock injectors only flow ~380cc of fuel every minute.  The ECU can only tell the injectors to open/close, so in order for the ECU to figure out how long to open the injector in order to deliver x amount of fuel, it has to know what the injector size is.  The stock ECU will assume its 380cc, but with 555cc injectors, the same opening time will deliver 46% more fuel... that's why you need the ECU to be reprogrammed for the 555cc injectors. Check out my prices on Nismo injectors... 
 
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