All our UK Daks had adjustable gills, and I agree with all before, but being able to fully-close a well-adjusted set of gills on a failed engine was reassuring if not immediately noticeable, drag-wise!
As an amusing aside, we fitted a brand new pair of cowlings, (with gills), to an aircraft on check, and on it's single-engine climb tests afterwards, the rate of climb would start off well enough, but slowly decrease to the extent that the dead engine would have to be restarted to maintain height, and this happened on EITHER engine, at low weights!
Needless to say, as test-pilot I was much perplexed, as were the rest of the maint.personnel. No end of engine checking and fettling would improve matters; it failed the tests dismally.
I called down our Chief Pilot, and he found the same. During one of these attempts, I sat in the passenger cabin looking out at the labouring engine giving it's all, and I had an eureka moment. The new cowlings had come with nice tight gill linkages, but the space at the top immediately below our short air intakes was just an open space, about 17cm x 9cm. As the engine warmed up during the clinb, all the hot air was being fed up into this space to be sucked straight into the air intake, thus diminishing the engine's performance. A dash forward to ask the C/P to land asap, a piece of ally, some tin-snips and some rivets, with the new panel feeding right in flush below the intakes, saw the next tests done with climb to spare! We had been struggling with that aircraft for five wretched days!
You never stopped learning with a Dak. Happy days!