The Gipsy engine as fitted to the Chipmunk has 'warm' and 'cold' air positions. It is actually the 'warm' that is filtered, and 'cold' that is unfiltered, and normal practice is to run the engine continuosly in 'warm'. The 'warm' air is drawn from alongside the cylinders, and the 'cold' from an outside air scoop.
RAF Chippys were permanently wired in 'warm', but because we often operate from short strips we like to be able to use 'cold' for take off for those few extra bhp. Once established in the climb, it's into 'warm' and there it stays until the next take off. Go-arounds in 'warm' are not a problem.
SSD