Swirl flap

Swirl flaps are small butterfly valves fitted to the intake manifold just before the cylinder head intake ports of many modern automotive diesel engines, including those from Audi, BMW, Vauxhall and Alfa Romeo. The flaps are smaller than the intake runners and therefore allow air to pass around them even when "closed". The photo shows a cleaned swirl flap removed from the intake manifold of a BMW M47TU 2-litre diesel engine. The flap itself is made from stainless steel and secured to a spindle by two small Torx screws. The sealing O-ring and external actuating lever can be seen below the flap itself.