P51 Exaust Shrouds

Is there a reason why some Merlin-powered P-51s have shrouded exaust stacks and others have unshrouded exaust stacks? I have seen photos showing both
and I am trying to figure out why. Was there a belief that one arrangement offered better performance or engine reliability than the other?
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On Monday, November 13, 2017 at 9:08:26 AM UTC-5, Mike wrote:

ks and others have unshrouded exaust stacks? I have seen photos showing bot h and I am trying to figure out why. Was there a belief that one arrangemen t offered better performance or engine reliability than the other?
An answer from someone with more knowledge than me.
http://cs.finescale.com/fsm/modeling_subjects/f/2/t/129752.aspx
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