BlackNite said:
The issue with 550's and 9psi isn't that they 550s are big enough.. just that their spray pattern at a certain cam angle/ignion point/RPM ect. causes the car to go lean in some peoples car. I had the issue so I sold my 550's and went 650's. Dont regret it one bit either cuz now I can go to 11psi with no worries.
You can pretty much figure out boost for any pulley combination by using my pulley calculator and also the rule-of-thumb numbers I posted on top of this tread.
As for injectors, I feel it's time for an injector primer in a stickie somewhere. We usually go by hearsay and posted experiences. Fact is that there are lots of things to consider with injectors, and some have a direct influence on drivability and performance.
I've now extensively tested five different sets of injectors in my SC car, and each behaved a bit differently. I saw each set being run in Comptech's flow tester machine, and I studied their heads and jets carefully.
The spray difference in the flow tester is dramatic. Some injectors spray almost like a garden hose on full blast, all turbulent. Others are more like a hose set on very, very fine mist. You can direct them at a flower and the flower won't get hurt. This means excellent atomization. In my experience, that leads to somewhat better gas mileage and lower boost to get the same power (lowest possible boost to reach the same power is easiest on your motor).
In addition to all that are the fitment issues, the resistance issues, general operation issues (peak&hold vs. saturated) and the connector compatibility issues.
In direct answer to the 9psi question--the 550s are enough, but, as you pointed out, their spray pattern is suboptimal.