what does the different ratios mean on FMUs? i was looking at them and some said 8:1, 10:1, 12:1. what do those numbers mean? i want to run 5 to 6 psi... which one should i get?

The FMUs like that require you to put in a spacer (disc) inside the FMU to make it produce a certain ratio to your application hence, 4:1, 6:1, 8:1, 10:1 etc; What that means is every PSI of air that the FMU see's its gonna produce 4, 6, 8, 10, PSI of fuel pressure through your fuel line. There for to make your injectors force more fuel into your engine. Cartect makes an Adjustable FMU that you dont need dics to install or take out for certain apps. Which is nice! It has a turn knob where it can adjust the ratio in real time.