Explain FMU's

[Whistler]

I'm asking for myself and a few others that have been pondering FMU's lately. Here's the question... I know the FMU adds fuel pressure but how does it do so? I know the FMU is boost responsive but how does it ADD fuel pressure when the fuel regulator is already set at a certain level (say 40 psi)?? And if the FMU is adding fuel (say a 12:1 ratio) how do the fuel rails hold 80-100 psi? If the engine is seeing 2 psi (with a 12:1 disc in the FMU) the FMU will raise fuel pressure by 24 psi....but the regulator is already set at 40 psi....so how does the 24 psi stay in the rails in ADDITION to the 40 psi that the regulator is set for?

I don't use my FMU, actually I never installed it with my Novi 1k, so I haven't taken the time to learn exactly how it works.

[5150 LX]

Here's my take on it Dan, the FMU restricts the fuel that is returned back into the tank, in doing this it raises the pressure inside the rails. Something to that effect, lol.

[UNTCHBL]

Dan, boost forces the diaphram out in the fmu that in exchange pinches down on the return. It basically over rides what the stock regulator is doing.

does this make since?

[Whistler]

Dan, boost forces the diaphram out in the fmu that in exchange pinches down on the return. It basically over rides what the stock regulator is doing.

does this make since?

Yes, that makes sense. Dankeshen! :D

[mygt8a4re]

just to add, yes the factory fuel rails can handle the added fuel pressure built up under boost conditions. the main concern with the fuel rails would be the amount of fuel it can feed to the injectors under a severe duty cyle. the factory rails arent weak, just small when it comes to moving alot of fuel! so those trying to force a large amount of fuel to huge injectors may not get the needed fuel do to lack of stock fuel rail suffiency!

also, look at a FMU as a more productive fuel pressure regulator. the regulator diaphram is controlled by vacuum while the FMU diaphram is controlled by boost. same principal with the diaphram inside really. if you could add more/less vacuum at idle naturally you would change your idle rpm fuel pressure reading via your FPR ~ just as adding more/less boost changes your overall fuel pressure reading via your FMU.

though this was explained about the FMU restricting the return fuel back to the tank i just wanted to touch more on it. as boost applies pressure on the internal diaphram in the FMU it closes the area in wich the fuel can return to the tank. this builds pressure between the fuel pump & fuel rails so that when the injectors open a larger amount of fuel will then be forced through the pintle opening pushing more fuel into the combustion chambers. the pressure remains or increases as long as boost is applied to the FMU. of course injector size plays a large roll as too small of an injector will only feed a certain measure of fuel regardless of fuel pressure! some claim that lean spots are more noticeable while using FMU's so larger injectors are better. but i personally have had nothing but great success to date with my FMU, but then again i have full adjustability with my FMU to create a perfect ratio, not a 2:1 chosen/preset ratio wich i could see having limitations.......
......not sure why i got so deep into that, just like FMU's personally. i hope i enlightened anyone who was curious or may find this in a search down the road, lol!

-justin

[5150 LX]

Damn Justin

Good info though :D

[mygt8a4re]

hehe, was in one of those moods :D