Well, since SVO 30#'s are rated at 31.6# at 43psi (f-bod stock fuel pressure) and given that my injector constants are at 31.6.

Should I set fuel pressure at 43psi at idle with the vaccum line disconnected?

 

Yes.

 

but when I re-connect the vaccum line doesn't fuel pressure go down.....should i set fuel pressure to 43psi at idle with vaccum line connected?

 

You set the correct fuel pressure with the vacuum line disconnected. Then you reconnect the vacuum line, and the RAIL pressure drops in proportion to intake manifold vacuum. Typically 43.5psi without the vacuum will drop to 36-38psi with the vacuum line connected.

But you need to understand the purpose of the vacuum connection. A fuel injector is rated at a specific pressure DIFFERENTIAL.... not at a specific RAIL pressure. When the injector opens, fuel at RAIL pressure flows through an orifice, and discharges into the intake manifold, which can be at any pressure from 21"Hg vaccum, to atmoshperic pressure, depending on throttle opening and air volume flow.

So... the purpose of the vacuum connection is to compensate for the fact that the manifold is under pressure, and hold the DIFFERENTIAL pressure (RAIL pressure minus intake manifold pressure) constant at 43.5psi.

Remember... vacuum is a "negative" pressure, so if the intake is at 15"Hg vacuum (minus 7.5psi), the fuel rail pressure needs to drop to 36psi to keep the DIFFERENTIAL pressure at 43.5psi.

(36psi RAIL pressure) minus (minus 7.5psi manifold pressure) =

36 + 7.5 = 43.5psi.

Remember also that "minus a minus" is a "plus".....