what size injector would u guys suggest i go with for 550whp, 720cc, 900cc, or 1000cc? I'll be using a F.A.S.T system.

 

Either 720cc or 900cc. 720cc will max out around 450whp, but if you are running a fuel pressure riser you can squeeze around 600whp out of them. 900cc will support a bit over 500whp, and then you can always turn up the fuel pressure. If its a street car I wouldnt go over 900cc, but if its a race car than run the 1000cc idle isnt much of issue.