The problem with the OEM returnless system is that the factory fuel pressure regulator is installed in the fuel tank. The factory fuel pressure regulator does not allow fuel pressure adjustments and it can't hold consistant fuel pressure with high HP levels. The factory fuel pressure regulator is also unable to add fuel pressure under boost. With a return setup it removes the factory regulator's control of fuel pressure and replaces the control with a vacuum/boost sensitive adjustable fuel pressure regulator (under the hood). This allows you to adjust both the level of fuel pressure and increase fuel pressure under boost.
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