About volume and maintaining pressure, I can pull a -6 line off, put it in a jug and my pressure will stay where it's set as if no fuel were flowing. If your pressure is dropping going down the track you don't have enough pump because it can't keep up with the needles and seats being wide open for long. If the pressure is dropping the fuel level in the bowls is going down. It may not be enough, or long enough to run them dry, but it is going down. Splitting hairs on it means that with the bowl level going down you can be sucking some air into the mains with all the vibration, bouncing of the car, and the fuel spraying into the bowl, and the weight of fuel over the jet. I've seen cars slow down when they put enough pump on the car to keep the pressure because the car got enough fuel on the other end to go fat. What I'm saying is if a car with too small a fuel system get's tuned well it'll have too large a main jet to keep it from going lean on the other end. when you give it the pump it needs it'll go fat. Agreed most pumps are more than enough for the job, but that's so much better to tune with than the alternative, one less variable in your tuning equation.