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What are Swirl Flaps? Swirl flaps are small butterfly valves fitted to the intake manifold just before the cylinder head inlet ports of many modern automotive engines, that unfortunately includes a lot of VAG and BMW Diesel Engines too. What do they do? The Swirl Flap position is adjusted by an electrical or vacuum-actuated servo mechanism, controlled by the ECU. In a typical implementation the flaps will be closed at idle speed, creating additional turbulence in the intake. As engine …