To elaborate on Ian's point, refer to this graph:
The short explanation: The more volume a vibratory pump displaces, the lower the pressure. The less volume a vibratory pump displaces, the higher the pressure, up to its maximum rated capacity (usually around 15 bar). Around double espresso flow rates, the actual brew pressure works out pretty close to 9 bar. For ristretto flow rates, see I still don't get it: Why adjust the OPV?
srossnz wrote:...something seems off with my pressure, the shots are disgusting (although I am still wrestling with other variables).
At brew pressures well above 9 bar, the taste profile of some coffees will become acrid and the roast notes exaggerated (e.g., burnt cedar and peat flavors). Based on numerous reports, the factory setting for the Rancilio Silvia is 11 bar. If pulled as a tight ristretto at that pressure, the shots could be harsh/bitter tasting. For most owners, I suspect the positive outcome of dropping the pressure is the reduction in channeling due to the increased "forgiveness factor", not an inherent issue with espresso at higher pressures.
That said, if you really want to know the brew pressure, it's not difficult or expensive to build a portafilter gauge and there's ample online hints:From Rancilio Silvia Pressure Gauge Test; also see Building a Portafilter Pressure Gauge
The DIY portafilter gauge above has a needle valve to simulate the flow rate of espresso. It's more accurate, but you can get a "good enough" measurement by simply not applying thread sealant so the fitting drips a bit.