[FPR] err yeah, another question..

[Kreator]

Ok, so i hooked up 2 fprs in a row (don't ask why, i'll explain later) one set at 20psi (my messed up afpr) and the other one bone stock set at 43.5psi. Now when i turn the car to on position, the needle on the fuel gauge jumps up to about 48psi. So my question is, is there some messup in the system (like pressure addup, alhtough i can't see why would they) or can i blame it on the crappiness of my gauge (summit gauge for $19.99) Thanx

[AceInHole]

It's very possible that having both FPR's in line with each other is causing the higher than normal fuel pressure. If the AFPR is restricting fuel flow at all, the stock FPR won't be able to operate as it normally would (as a throttling device it's job is to create a pressure differential). At least, I'm pretty sure that's how it is.

[Kreator]

well... see i'm just thinking... the afpr right now is set waaaay below the second fpr's setting. so in theory it shouldn't be affecting anything.... just acting as a straight flow through path as long as it's pressure setting is below the second fpr's setting....

So in theory, first fpr get's hit with fuel.... it holds till pressure reaches 20psi and starts leeking. the leaking fuel hits the second fpr and pressure rises to 43.5psi... now this pressure hits the first fpr with 20psi setting and just opens it up more....

where's the flaw in my thinking?

[AceInHole]

you're assuming the FPR works by measuring pressure in the rail, while it doesn't. The stock FPR and most AFPRs are simply throttling devices that create a pressure differential by blocking a certain amount of flow. Regardless of how much pressure is in the line, the FPR will open and close its "valve" based on the manifold vacuum/ pressure. It isn't closed untill it reaches a certain psi. Therefore, since you have a restriction at the 2nd FPR, pressure is higher after the 1st FPR than it should be, so the pressure differential that the 1st FPR is set to create causes higher pressure in the rail. This train of thought works for both setups, having the higher pressure FPR as 1st or 2nd.

I'm fairly sure that's how it goes.

[Kreator]

well the fprs are spring operated... so they actually do open up as the flow reaches a certain psi.... there is a ball (that locks the flow) a little spring (that pushes the ball out) and a big spring that pushes hte ball shut. They big spring is in the dry part and it pushes on the border between dry and wet. So as the fuel rises in the wet part, it pushes against the border, and compresses the big spring. As the big spring is getting compressed, the little spring is able to push the ball out, opening up the return passage. When the pressure reaches 43.5 the ball gets pushed out completely for the excess flow to go out and not raise the pressure any more..... im pretty damn sure about that cuz that's that's basic physics from what i've seen inside the fpr.... the manifold vacuum/pressure only adds pull/push on the border fomr the vacuum side. Meaning that at idle, the vacuum sux the border out, helping the fuel press push the border out, opening up the passage quicker resulting in the lower fuel psi.... when pressure is positive, we got a push against the border from the vacuum side, against the work of the fuel so the pressure in the rail rises.

So in theory from my thinking, when the 43.5 is at the second fpr, that same 43.5 pushes against the wall in the first fpr so it pushes it out even more, even though the passage is completely opened already. So it shouldn't be affected.

However now i'm realizing that the pressure must be higher... still can't really get to why it's higher, but if say instead of the afpr at 20psi there was just a gutted fpr (eg no ball and small spring, just open passage - eg 0psi) then the pressure would be 43.5 in the rail. If there is 20 psi, fuel has to do "work" to open it up and then hit the 43.5.... you know what i'm saying... those 2 things shouldn't be equal :-/ Still don't understand the actual physics way.

at any rate, from this stuff that i just said, does it imply that this is equivalent to a single fpr set to 48psi? or should i start searching for an fpr for "gutting"?

[AceInHole]

hmm... it seems you're right about the FPR's workings.... i was under the assumption it worked a bit like the FMU's i had taken apart....

best guess is your AFPR is messed up. heh.