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Originally Posted by BAN SUVS
Pressure != flow. People replace fuel pumps because, at peak boost, fuel flow volume can drop off at high duty cycles and peak fuel pressure. Hence STi owners always being encouraged to replace their fuel pumps feeding their stock ~550cc/min injectors, whereas WRX owners do okay with stock pumps feeding their stock ~420 cc/min injectors.
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Fuel flow is entirely determined by injector duty cycle and pressure at the rail. The amount of fuel flowing out of an injector is based on how long the injector is open, and how much pressure there is behind the injector.
I don't see how a larger pump on a stock car can change anything except how much fuel is cycled past the rails and back into the gas tank.
Apparently a stock STi that's tuned for more boost, and running its injectors at much higher rates to compensate, can run out of pressure in the rails. Since the stock mapping never runs the injectors at 100% DC, it's not a problem on a stock car. But if you're getting up there in DC, then you can run into the need for a bigger pump on an otherwise "stock" STi. i.e. the stock STi injectors are too big for the stock fuel pump if you tune it beyond stock power levels.