MSefi.com read only forum (Jan.15,2018)

Hey guys, I understood why a pressure reg is boost referenced and that it raises the fuel pressure 1:1 with boost pressure, But I noticed on a car I was working on today at idle (high vacuume) with the boost/vac line connected it actually pulls fuel pressure down to 35psi, once you open the throttle it raises to 44psi.

So just wondering why it drops pressure under vac? would not keeping the higher pressure at idle help atomization and give a better quality idle?

The idea of a vacuum controlled fuel pressure reg dates back to the early days of injection. It counteracts the 'suck' on the injector by reducing the fuel pressure. Hence at idle - high vacuum - you have low pressure. Wide throttle - low vacuum, so maximum pressure. These days it is likely better done by electronics.

But not sure what happens with a boosted unit. I thought it simply stayed at max pressure (same as low vacuum) under boost.

The idea is to maintain the pressure differential across the injector orifice. Since flow through an orifice varies with the square root of pressure differential (to double the flow you need 4x the pressure), it's easier on the tuner and the processor to maintain a steady pressure. Assuming a nominal 3 bar / 43.5 psi / 300 kPa differential, at idle (40kPaA MAP) the rail pressure would be 340 kPaA. At wide open on a NA engine (100 kPaA), the rail pressure is 400 kPaA. Under 140 kPa of boost (~20.3 psi), the manifold pressure is 240 kPaA, so the rail pressure would be 540 kPaA, or 78.3 psia / 65.6 psig.