I frequent several car forums besides 2gnt and alot of people are against using an fmu and say they're an unreliable way to tune, even at 1-12 psi in most I-4's that weren't given factory boost.

Why is that. I've heard some people not being able to raise enough fuel pressure for meager amounts of boost. The unit wasn't broken. Alot of people do a temp solution by crimping the return line to keep it up there. Not my cup of tea but people do it.

I haven't heard of a problem with this in our cars so what's the big deal and why do fmu's effect similar engines on nearly identical boost with around the same size injectors differently? Why are some tuners runing 6-15 psi and larger fuel pumps not have a tollerable afr?