Actually, taking off with the cowl flaps closed is recommended in the pilot's notes for the Blenheim Mk.IV, they are supposed to be effective enough in terms of airflow getting through the engine once you start moving.
I'm sorry for pushing this back guys (i've been spending some time on other real life stuff lately and other aspects of the sim), i will try to finalize my copying of the relevant manual pieces and post it here sometime within the next couple of weeks
