I bought one of the Costco Prime Briskets. It's 20 Lb. trimmed. Should I still allow 1.5 hours / pound cooking time?
You need to go by internal temp. rather than time per pound. Buy a good remote read thermometer. and smoke it until the temp reads 190 degrees. Once you hit 190 to 195 degrees, You have to by feel. Take a toothpick, and probe the brisket. When the toothpick goes into the briskets and it feels like warm butter, it is basically done. If you want really good brisket, wrap it the brisket up, and place it in a warm ice chest for a couple of hours.
The founder of this site was champion cook. He said: Good BBQ is done when it is done" . This doesn't have anything to do with 1.5 hours/pound.
What Mike said. To elaborate, wrapping in double heavy foil, then an old towel, then in the (at least room temp) cooler - what has been referred to as FTC (foil, towel, cooler) can keep it getting tender as it rests and hot enough to serve for a couple of hours at least. BTW, just for planning whether you'll be able to go to bed, at 225 degrees I'd plan on at least 24 hours. The "stall" where it sits at 165 or so for a long time, will take several hours so don't get discouraged or tempted to turn the heat up. PS - make sure you put your probe on the fattest part of the flat, not the point. Sorry if I'm telling you stuff you already know. If your question was specific to what to do with prime brisket, as opposed to choice or CAB, I'd probably look for slighty less overall time.
I get my packers from Costco as well. Pure prime. Something not mentioned is whether to smoke/cook fat side up or fat side down. I have the fat down as an insulator from the heat, sort of. What is everyone else's method?
I do mine fat side up so that as the fat melts, it bastes the meats.
Thanks Mike - I had a brisket that was nearly devoid of fat on one side. So I trimmed it and placed it fat side down and used the trimmings on the now top side for some moisture and basting. Turned out well and I just kept doing it that way. I also inject and on occasion Jacquard the meat.