First, define. Reading from time to time how it is an evil trait, causes problems. Then I read about how plenum cooling is better. (which seems like the ultimate in a 'close cowl') So, is having a cowling immediately adjacent the hot bits better or not? Seems like you'd get faster flow (which could be good), if less volume (which could be bad). Chapter 1293 in my aviation training is somehow blank, on 'Closely Cowled".