After modifying an engine for more power it's necessary to increase the capacity of the fuel system: usually bigger injectors and a bigger (or additional) fuel pump.

What is the real purpose of more pumping capacity? More flow? Or more flow AND more pressure?

I'm asking because I just had to remove a damaged fuel pump supplied with a Vortech supercharger kit. This pump is supposed to be installed in series after the stock pump located in the fuel tank.

With only the stock pump the fuel trims are now around 2% leaner than before (I was surprised to see only that little difference), so I have to assume that the purpose of the additional pump was just to ensure enough flow but not necessary more pressure. Am I right?

This is a 98 Trans Am, the injectors are Bosch 36 Lbs/h.
There isn't a real pressure regulator, just an orifice in the return of the fuel line.

The supercharger is no longer installed and the car is tuned for NA.


Thanks

Stefano