Since we're all so bent on going with inline pumps with FMU setups because of the high pressures, why not just lower FMU rates to be able to use high-flo units like Walbro's? Say a 4:1 or 6:1 FMU at 8psi and larger injectors, that'd keep pressures in the 80ish psi rate which would allow for use of non high pressure units like the avg 255lph Walbro correct? I'm sure there's a good reason why NOT to use this route, I guess I just haven't thought of it since i'm still kind of in the learning phase with the whole fuel pressure/FMU rate/injector size stuff (but thanks to te good people at Honda-Tech, I sure as hell learned alot in the past 2-3 weeks!)

 

The simple answer is that you would have to increase injector size accordingly. The pump would flow plenty but the whole reason for the FMU is to raise pressure and force additional fuel through undersized injectors. Injectors are expensive, and the ECU has to be corrected in some way to idle with anything larger than 310cc injectors, which is presumably why everyone wants to use FMUs in the first place. So if you're going to stick with stock or up to 310cc injectors, you need to raise fuel pressure at least 8 psi or more for every psi of boost you run; which is why we're so completely screwed trying to find a pump that can handle it.

Note: this is why I decided to run 450s and a VAFC and now I run stock fuel pump and still run rich at 7psi.

 

Interesting point dbman, now this raises a new question for me.. since large enough injectors like 440's flow so much fuel without an insane amount of pressure, do I really need an FMU for my setup? Would 440 injectors and the stock H22 pump with my VAFC suffice and provide me with enough fuel under boost , or should I stick with a low FMU rate of 6:1 and an inline pump? Kinda high pressure for such large injectors right?

 

Thats a very good analagy. I hope someone has an explanation?

 

Yes, with a VAFC and 440s, stock pressure is sufficient; however since the VAFC is not boost-dependent to begin with, it's tough to tune... which is why I am trying my little VAFC mod. But I'm running 450s with no FMU and no problems.

 

Dude thanks!! I think for the first time since i've been here I really got a good grasp on fuel pressure/injector size/boost etc... If anything, I think I kinda like your style of doing it. Reason being, with larger injectors, fuel pressure remains relativley low, closer to stock levels than with small injectors and large FMU rates. The way I see it, the closer to pressure to stock, the less the strain on the system altogether (pump, injectors, rail, FPR) therefore the less likely a major failure at the wrong time! True large injectors can be a pain to tune via a VAFC, but once you've got that down, its smooth sailing. Now to mention the large boost potential once can have with a high-flow pump + big *** injectors. As boost goes up all you have to do it loosen up a little on the injectors or even go with a small 2:1 FMU...wow! Now there's potential! Thanks again dude you really set a light off in my head