When I get 18 hours to spend on BBQ, I am going to find the definitive answer to your question. Until then, this is what I understand.
The lower the temperature you cook a brisket, the lower the internal temperature it needs to be tender. The lower the temp under 185 F that you can bring a brisket to tenderness with, the more juicy a brisket will be.
If you are cooking hot and fast, the higher the internal temp needs to be to be tender and foil helps. If you are cooking a brisket at a lower temp, around 225 F, the longer it will take, but it can be ready at around 185 F internal temp. 185 F is the "magic" number. After 180 F - 185 F, brisket begins to dry up, unless you are cooking hot and fast (around 275 F - 290 F) and using foil just as it reaches its plateau.
Hot and fast (which is how I cook brisket) calls for cooking at 275 F - 290 F until internal temp reaches 165. Then foil until internal temp reaches 205 F. Remove from smoker and put in a warming cooler for 2 hours. De-fat the au jus and slice it using the au jus as a sauce. Add whatever sauce you want after that. Using that method, I have NEVER had a dry brisket. Kreuz in Lockhart, TX cooks hot and fast (no foil) and they have some juicy brisket and really juicy boneless shoulder clods, before 2 PM especially.
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