The stock 2.0TFSi can request as much as 129.99 bar from the high pressure fuel pump into the fuel rail. The fuel rail has a relief valve on it to prevent the rail pressure exceeding this. Most 2.0TFSi engines will see around 127bar of rail pressure max when observed.
When fitting upgraded Autotech HPFP internals and running an appropriate map, this rail pressure request can be increased to supply more fuel and make more power – typically around 133-135bar, but even as high as 140-150bar.
Sometimes, but not always the fuel return valve can open under these conditions and cause a drop in rail pressure, which is felt as a ‘flat spot’ in the mid range.
Fitment of the RS4 fuel return valve ensures fuel pressure generated by your upgraded HPFP internals is actually used, and not lost through the stock valve. Particularly useful on high power cars or cars with custom tunes that require high rail pressure demands.