Had a wierd thought. I was reading about doing a turbo 2.8, and it mentioned something about how if you have 50 Lbs (more like 42-47) of fuel pressure at WOT, if you had 10 lbs of boost, you'd have a net of 40 lbs of effective fuel pressure (50 PSI - 10 psi of boost pushing back).

It said WOT for a reason though. The fuel pressure drops when manifold vacuum is high (idle). The stock fuel pressure regulator as I understand it uses a diaphram that's connected to the spring keeping the valve controlling fuel pressure shut. When vacuum is high, the vacuum pulls on the diaphram, which counteracts the force of the spring, allowing the valve to open with less pressure than normal. When vacuum is low (WOT) the fuel pressure increases, as the full force of the spring closing the reg valve is unopposed.

So my question is this - when pressure goes higher than atmospheric (boost), does having "negative vacuum" assist the spring, increasing the fuel pressure higher than it ordinarily would go as well? The obvious question, is can you use a fuel pressure regulator (modified or otherwise) as a rising rate fuel pressure regulator for boosted applications? Does it already perform this function to an extent? If it does, then perhaps the need to increase fuel pressure to overcome the boost may not be nessicary (bigger injectors would still be needed, of course). This might end up being an interesting idea for turbo and blower people out there.

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Chris

Blue 1987 GT Getrag 5 speed, 1" lowering springs, 225 50R16 rims and tires, Fiero Store 9 3/4" HD clutch
Silver 1986 2m4 THM 125c automatic
Black 1985 2m4 SE Isuzu 5 speed (no paperwork)

Not sure if the stock Fiero fuel pressure regulator works the same way but on my SC3800 the regulator increases fuel pressure under boost. The manifold pressure pushes on the spring raising fuel perssure and at vac it pulls on the spring decreasing pressure. That is the reason that you disconnect the vac line when setting the baseline pressure. The reason for the fuel pressure variing with the manifold pressure is to keep a constant pressure differential across the injector. You could check the Fiero regulator by changing the vac from pos to neg and see what pressure readings you get.

Yeah, the stock pressure regulator will keep constant pressure difference across the injectors, even with boost.

The point of a rising rate regulator is that fuel pressure rises faster than manifold pressure. This has the effect of richening the engine under boost.

So the common opinion that fuel pressure has to be raised in order to overcome the boost may not be true then?

You either need more fuel pressure or you need to richen up the injectors in the chip.

The fuel pressure should rise with the boost

quote Originally posted by Blue Shift: So the common opinion that fuel pressure has to be raised in order to overcome the boost may not be true then?

It's not a case of "overcoming boost". The flow rate of fuel across the injector tip has to be precisely controlled in order for the ECM to control fuel delivery properly. Since differences in pressure inside the manifold affect fuel flow, the regulator is set up so as to keep the relative difference between fuel pressure and manifold pressure constant. The fuel pressure is more than high enough to overcome manifold pressure on any turbo'd motor.

JazzMan

I thought so, that way the fuel flow rate will remain constant. Wonder how linear it is? Wonder if it'd be linear all the way to boost or if it'd mechanically limit out?

A conventional FPR keeps a constant pressure across the injectors even with boost in the manifold.

A rising rate FPR increases pressure difference across the injectors when it sees boost.