wyatt, you're right. RC does rate their injectors at 43 psi. so that 234cc is most likely at 43 psi. but so are all their other injectors. the 320's, 440's, 500's, whatever. they're all rated for that flow at 43 psi. there is a simple calculation that will tell you what the flow would be at a higher pressure. increasing fuel pressure from 43 psi to 49 psi would only make the 234's flow 249cc's. that's not much of a difference. of course there is more of a difference if you were to increase fuel pressure with a boost-sensitive FPR.

550's are some seriously BIG injectors. i didn't run the calculations on RC's site, but i'd imagine that they're overkill unless you're planning on running MAD boost. all you have to do is know what kind of boost you plan on running, estimate the HP output with that boost, and calculate the right size injector. if that calculation tells you to get 550cc injectors, keep in mind that it's calling for 550's running at 43 psi!

if you have the fuel pressure cranked up under boost, you might only need 440's to flow 550cc/min. that's where you have to play with the numbers and see what injector flows what at what psi. sharpen your pencils and get some scrap paper. it's alot of fun! ;)