I was browsing online for a fuel filter and saw many different ones claiming to perform better than the stock fuel filter because it increases fuel flow. Can they actually increase the fuel flow in the fuel lines? doesn't the fuel pressure regulator eliminate such claim?
tl dr: I don't see how they can make that specific claim.
I'm not sure how they can claim it increases fuel flow. What it can do is allow more fuel to flow by reducing a restriction in the fuel system. I realize there may be a fine line here between the two lines of thought. The claim it "increases" to me means it does something which actively puts more fuel through the lines. The thought of "allows more fuel to flow" is of the idea that by removing the restrictive stock filter, you allow the fuel to flow as it has the ability to flow. It may end up being the same in the end, but to me there is a difference. To me the difference is like saying (for instance), putting an electric water pump on a car creates horsepower. It doesn't do that, it just frees up horsepower which was already there, but was getting utilized to run the belt driven pump.
As for the fuel pressure regulator, remember what the FPR does ... it controls the pressure and not the flow. It does create a large obstruction in the fuel line which creates the pressure, but that's it's job. It cannot speed up or slow down the flow of the fuel going from the pump into the engine in and of itself.
There are two main parts which control the flow of the fuel. First is the fuel pump. It provides the main output which equates to the rate of flow. Secondly is the size of the lines taking it from the pump to the engine (and return line as well). If you want the ability to flow more fuel, you have to have a large enough pipe to handle it. You can effectively increase the flow by increasing the pressure. (NOTE: This may seem to contradict what I was saying earlier about the FPR, but again, the FPR doesn't in and of itself increase the rate of flow, but rather controls the amount of pressure. These are two separate things.) At the fuel injector, the more pressure equates to more fuel being dropped during a given injector cycle time. At a given pressure, you can increase the flow by increasing the size of the line carrying the fuel as long as the pump can deliver it.
How would you get any benefit from the increased fuel flow? The fuel that actually goes into the engine is precisely metered by injectors to have a precise air-fuel ratio, and the whole system including the fuel filter, fuel pump and injectors is sized so that the limiting factor is the air going into the engine and the desired air-fuel ratio.
If you have somehow modified the engine by e.g. compressing more air into the engine by using a supercharger/turbocharger, I can certainly see the benefit of a better fuel filter. But typically people don't add superchargers/turbochargers to their engines, and in this typical case, the limiting factor is indeed the air and not the fuel.
In theory, a less restricting fuel filter could cause the fuel pump to use less electricity, and therefore reduce the drag on the alternator. This is the only performance benefit I can see a better fuel filter could have on an otherwise unmodified vehicle. However, this effect is extremely minimal. The amount of electricity consumed by the fuel pump is practically nothing. Consider this: the fuel pump is probably behind a 10 ampere fuse, meaning it can consume no more than 120 W on a 12-volt system, and a typical engine produces about 80 000 W. This 120 W is 0.15% of the total engine power, and even that is the absolute maximum the pump can consume. I would say a better fuel filter probably improves performance by 0.01% or so due to reduced alternator load.