[Supra Bob]

I know fuel pressure plays a role in determining the amount of fuel an injector will flow...

however...

In reguards to forced induction... Does the relative pressure in the manifold reduce the amount of fuel the injectors will flow? (i.e. If the F.P. in the rail is 36psi, and there is 15 psi in the runner, does the injector only see 21 PSI of fuel pressure and only deliver fuel as if it had only 21 PSI?)

Also, someone once mentioned somewhere to me, that the stock FPR's reference the pressure in the intake, and increases it at a 1:1 ratio... Is this true?

 

[Norbie]

Yes it's true, all FPR's have a reference line to the inlet manifold which ensures the fuel pressure always remains a fixed value above manifold pressure.

 

[Supra Bob]

Where does that reference begin? Is "zero" based at idle vacuum, or is it at some point in operation range?

For example, if it's based at idle, and the car idles at 20 in. HG. and operates at 36 psi of fuel pressure, then at 0 in. HG., would the injectors be seeing 56 PSI of fuel? And at 10 PSI boost it would be seeing 66?

 

[Norbie]

What? It's simple: fuel pressure is always X above manifold pressure. I don't understand what you mean when you ask where "zero" is...

 

[Supra Bob]

Where does the fuel presure begin to rise in relation to the intake pressure? while still under vaccuum, just off idle, or at the onset of positive pressure.

 

[Norbie]

Um... it doesn't, unless you have installed a rising-rate fuel pressure regulator (RRFPR).

As far as I'm aware, a RRFPR will start to increase the fuel pressure (relative to the manifold pressure) when the engine starts to come on boost, ie when the manifold pressure is greater than atmospheric pressure.

 

[Supra Bob]

Ok...now you have me lost...


These numbers are just for example...

Manifold pressure is 20 in. HG. (which is essentially the same as -20 psi)
Fuel pressure is 36 PSI.

Now, the difference in pressure is 56 PSI. You said that that difference in presure will always remain the same. If thats true, then the Fuel pressure would have to increase as manifold pressure increases. Right?

 

[Norbie]

Yes, that's correct!

I have a fuel pressure gauge installed on my Celica, and I can watch the fuel pressure move up and down as the engine load (hence manifold vacuum) increases and decreases.

 

[Supra Bob]

Ok..so it increases 1:1 with manifold pressure, and as soon as the manifold pressure starts to increase, the fuel pressure does the same.

Is there a point where it stops?

 

[Norbie]

No. If the 1:1 ratio ever changes, the effective flow rate of the injectors will also change - and that will screw up the fuel mixtures.

 

[Supra Bob]

Phew... ok

Finally got the answer I was looking for


Just couldn't figure out how to ask the question right I guess..