I do not believe that your Bosch valve will open with vacuum. It will just change the blow off pressure based on manifold pressure.

A by-pass valve will totally open an air passage when manifold vacuum reaches the designed level, regardless of boost pressure.

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Jim Villers
190SL, 230SL 5-speed, 95 E320 Wagon, 01 E320 Wagon, MGB, Boxster 'S', 190SL "Barn Find"