I was just curious how much of a pressure drop is usually normal across a fuel filter on a typical consumer passenger car?
This will all depend on the type of fuel pump that your vehicle uses. The stronger the pump, the larger the pressure difference it can manage. If you wanted to calculate a theoretical limit, I guess you could estimate it by calculating the fluid power based on the flow rate your engine requires and the pressure upstream of the fuel filter:
Power = Pressure * volumetric flow rate
Make sure you're careful with the units in your equation, but this will tell you the maximum pressure you can have upstream of your fuel filter before your fuel pump craps out. As an example, if you have a 0.05 watt fuel pump and your engine uses 0.1 gal/hr of fuel, then:
Pressure = Power / volumetric flow rate = 0.05 watt / 0.1 gal/hr = (0.05 watt * 0.73756 ft-lb/s * 1/watt) * (1 hr / 0.1 gal * 3600 sec/hr * 1 gal / 0.133681 ft^3 * 1 ft^2 / 144 in^2) = 68.9 PSI
I dunno if 0.05 Watts is typical for a fuel pump. I do think 0.1 gal/hr is realistic for fuel rate at idle. So, based on this analysis, our pump can handle 68 PSI. If the fuel line pressure is 60 PSI, then that means we can handle an 8 PSI increase in pressure due to a restriction in the fuel filter.
I should note that this analysis is very very basic, and does not take into account pump efficiency or pressure drop due to the pipe length and other obstructions.
I decided to buy a new fuel filter, and measure after the old filter, and before and after the new filter. I let it idle two minutes before taking each reading. After the old filter, which had been in there for ages, it read 35 psi. Before and after the new filter were both also 35 psi, although the needle on the prefilter reading was jittering a little bit. I think when I installed the new filter a little bit of filter material might have gotten blown out and stuck in the fuel regulator or return line ( or something ) cause I was getting 43 psi for a little while right after installing the new filter.
Anyways, this seems to validate what JPhil1618 and X-tech2 were saying, that there should be zero measurable pressure drop across the filter.
While this doesn't answer the general question, it does give one point of empirical data.