BlaCkPlaGUE

Does it open up more with increased or decreased vacuum from the manifold? Just curious. I know a selenoid activates it, but what does the air pressure do to the actuall FPR, does it open it more or what?

thx

Kenteth

I used to know this :P

Solenoid activated - (hot start) manifold vacuum - FPR reduces PSI
Solenoid deactivated - (nromal) atmousphere pressure - FPR constant PSI

HAILERS

99.9999 percent of the time the solenoid is not activated and lets the fpr see intake manifold pressure/vaccum. At idle the fpr puts out approx 28-32 psi.

During HOT Start, the ECU puts a ground on the solenoid and lets the fpr see atmospheric pressure which results in a fuel pressure of approx 37psi....for about 50seconds, then it de-energizes and lets the fpr see whatever the manifold pressure/vacuum is during the rest of your drive.

Why higher pressure at startup. Got me. See the 87fsm, page 4B-84.

At higher atmospheric pressure the fpr opens more to raise the fuel rail pressure. On a turbo car you'll see approx 28psi give or take and at full boost the fuel rail pressure will be around 48 psi, give or take.

Kenteth

whoops, looks like I had it backwards

NZConvertible

The more vaccum is applied to the FPR, the more it opens. This allows fuel to flow back to the tank faster, and so lowers the pressure in the rails. At manifold pressure falls (i.e. more vacuum), fuel pressure falls. As manifold pressure rises (i.e. less vacuum), fuel pressure rises. The FPR keeps the pressure difference between the fuel rail and the manifold constant, so that the flow rate through the injectors is constant no matter what the engine load.

The FPR is not "operated" by the solenoid as such. Most of the time the solenoid does nothing. Under hot-start conditions the solenoid opens and allows the vacuum to the FPR to escape, which increases fuel pressure to it's highest to eliminate vapour lock.

BlaCkPlaGUE

So if you capped off the FPR you would run lean at higher rpms, cause there is not as much pressure to feed the injectors?

Im guessing the FPR for the turbo and na are totally different, as one sees positive pressure and the other see's vacuum for functionality?

thx guys, great info.

*EDIT*

I remember someone told me before that you always want fuel to be pressurized at something like 45psi, and that is the best way cause it atomizes better with the air. Now with boosted cars, if your running 10lbs of boost, then you want your FPR to regulate 55psi of fuel pressure, so that when the fuel injects into the manifold, its injected at 45psi and the 10lbs of boost is pushing back.
This is all fine and dandy for the turbo car, but why does the NA need an FPR?

NZConvertible

No, the FPR reduces fuel pressure as manifold pressure drops (i.e more vac), so if you disconnected the vac line the FPR would see atmospheric pressure and act as if the throttle was wide open (no vac). You'd get full pump pressure all the time. Mixtures would be normal at WOT only, and very richer the further closed the throttle was.

No, they're esentially the same. Forget about vac and boost for a sec and just think of absolute manifold pressure. An NA sees manifold pressure of a bit above zero on decel up to just below atmospheric (14.7psi) at WOT. A turbo is simply able to see manifold pressures above atmospheric as well as below.

Correct, and this is why all EFI cars have a FPR.

For the reason I stated in my first post. It's desireable to keep a constant pressure differential between the fuel rail and the manifold. In the FC this is about 37psi. The fuel pressure is always 37psi higher than the manifold pressure. This keeps the injector flow rate constant. When open, a TII injector is flowing 550cc/min whether the engine is idling or under full boost (as long as that pressure differential remains constant). Varying the total amount of fuel injected is done by the ECU altering the injector open time (pulsewidth).

If your NA's vac gauge reads 16inHg at idle, thats ~8psi below atmospheric. Add 37psi to that and you have a fuel pressure of -8 + 37 = 29psi. As you open the throttle the manifold pressure increases up to near atmosphric at WOT, so fuel pressure then is ~37psi. Now exactly the same holds for a Turbo, but as manifold pressure keeps rising (i.e. boost), fuel pressure keeps rising too. So at 10psi boost we have 10 + 37 = 47psi fuel pressure.

Note that is you disconnect a Turbo's FPR like you sugggested above, fuel pressure would not fall with vac or rise with boost, so you'd run rich under vac and lean under boost. Don't ever do this...