What is the purpose of the vacuum port on the regulator? I know that the mid 80s Jaguar V12 has a vacuum port that conencts through a temperature controlled valve to teh intake manifold. When the rail gets too hot, the valve shunts to atmospheric pressure, which cause a 14.7 PSA increase in rail pressure and reduces the tendancy for vapor pockets to for in the rail. But the ZX regulator connects directly to the manifold (or so I'm told) so I can figure out what it does. Increases fueling when the manifold pressure goes up obviously, but why?

TIA

Ed

By referencing the fuel pressure regulator to the intake manifold vacuum, this ensures the fuel pressure is always a constant pressure above the air in the manifold. This simplifies the fuel injection logic since they do not have to adjust the pulse width to account for a smaller pressure difference (and thus slightly less fuel injected) when the engine is under load.

Newer fuel injection systems accommodate this in the fuel map.