Hi everybody,
I'm a happy owner of an Isomac Zaffiro.

Recentely I noticed one thing about brew pressure, which has also been partially discussed here in the past; here is the thing: as many of you, when I pull a shot, I get pressures of approx 11 bars, which seems to be normal for this model.. the problem is that if I activate the pump without the portafilter (same thing happens if i remove the dispersion screen/gasket of the E61 group), so basically if I just run the pump without any apparent resistance, I get a pressure of 4 bars on the pressure gauge.

Does this happen to you too?

The problem is: if I have 4 bars of resistance just to get water to the portafilter, then (having the max pressure valve on the pump set to 11, for example) I'm left with "just" 11-4=7 bar of "real" pressure on the coffee, before the pump stars to send water back to the reservoir.
This might be the reason for which I always find my shot a bit sour, maybe I can't build the right pressure.

I've just descaled my machine, so everything should be clean.
Any ideas?

The 3.50 to 4.00 pressure when simply flushing is due to the restriction imposed by the gicleur (~90%) and the summation of the other parts in the hydraulic system (~10%).

Adjust your OPV such that the gauge reads 9.0 bar with a blind filter and you will be in very good shape. You will then have a touch less than 9.0 bar presented to the properly ground and dosed coffee which is "just right". Your 11 - 4 = 7 bar is not correct but it would take a short thesis on fluid mechanics which I am not prepared to type.

I actually assumed it would have been more or less like a series circuit in an electric system.. but I read a bit about it and realized there are many other factors implied, thank you.
so you suggest to actually adjust the OPV to a lower pressure than 11 bar.. I'll try that.