sushibug

we all know they are 12:1 which means 12 pounds of fuel per pound of boost right? now the way it is installed, after the fpr, where basically leftover fuel goes back to the gas tank. so basically the fmu is pinching that line, increasing the pressure in the fuel rail which forces the injectors to stay open longer? i dont see how the stock fuel pump is actually working any harder if the fmu is only denying return of the excess fuel. someone care to set me straight?

sushibug

what... no one knows if this is true or not?

falcongsr

no actually it cannot affect injector duration. raising fuel pressure just forces more fuel thru the injector in the same amount of time.

the higher the pressure, the harder the fuel pump must work to maintain flow. actually the flow diminishes as you raise pressure. and at some point the fuel pump wont be able to maintain pressure at the injectors and you lean out.

another problem is that the injectors will eventually stop dumping fuel even as you raise fuel pressure. mechanical engineers refer to this as when an orifice goes 'static'. i think stock injectors experience this somewhere above 120psi. but this doesnt mean that your stock injectors are good up to that point cause the additional pressure over stock makes them open and close slower - and thats not good.

overall, raising fuel pressure significantly over stock taxes the hell out of the entire system and is not a good idea.

Smashback

Think of it like this...you are blowing thru a straw. As you blow harder, the other end of the straw will get smaller. Now you want to flow the same amount of air, but the end of the straw keeps getting smaller.

Not a perfect analogy, but it works.

sushibug

okay but umm... why is the fuel pump working any harder? isnt the pressure just being kept in the fuel rail that was there to begin with (that is supposed to return to the fuel tank) already?

falcongsr

the FMU is pressurizing everything from the fuel rail all the way back to the pump. its having to push fuel into a hose and its resisting more, so it has to work harder.

go play with a garden hose.

the fuel pump is supplying MUCH more fuel than what the injectors need. it has to so that the system reaches an equilirium and the fuel pressure can be held relatively constant. i think the point you're missing is that the stock system has a lot of pressure at 40psi. and the injectors opening doesnt hardly lower that at all.

sushibug

you think its okay to run the 12:1 fmu up to 8psi? with an inline fuel pump of course.

schwett

8*12 = 96... 96 + 40 = 136. i would say................. probably not. few if any fuel pumps are going to manage that kind of pressure, and even if they did it would be a pretty horrible idea. keep the pressure under 100psi if at all possible. get some bigger injectors and a lower rate fmu.

sushibug

if i get bigger injectors, doesnt that mean i need a standalone to idle with them? or is it possible to get 310cc injectors, an aem fpr... lower the idle fuel psi, and a 8:1 fmu?

falcongsr

dont forget the additional 8psi from the stock 1:1 fpr : 136+8 = 144 psi