One pumps more volume of fuel than the other. The real question is fuel pressure. If the 120 unit produces higher than normal fuel pressure (say 60psi vs the stock 40-45psi) then engine will run rich and the ECM will try to remove fuel from engine by reducing injector pulse timing. But you can only reduce the pulse so much.
The 120 unit may pump so much volume that the FPR cannot bypass enough back to tank causing the increase in fuel pressure.
High fuel presure can damage the injectors and supply/return lines to fuel rail.
Wednesday, December 31st, 2008 AT 9:30 AM