The injectors simply push into the intake bosses and are held in place by the fuel rail. Lube the O-rings with a little white lithium. FAST recommended 42-lb/hr injectors (rated at 45 psi), which are good for up to about 650 horsepower. Injector size is fairly critical to overall performance. Too small, and you'll limit power and risk going lean. Too big gains you nothing on the top end but will likely cause idle and low speed issues. Jeff recommends that your injector duty-cycle at max power be no more than about 80 percent (our Hemi was at 70) and the duty cycle at idle be no lower than about 21/2 percent. That way you have room for the XFI system to protect against any lean conditions that might occur and you'll have fine enough control on the low end for good idle and low speed operation. Jeff has also found that a vacuum referenced fuel pressure regulator will help make a little oversized injector appear smaller, run better, and be easier to tune down low.