Had this discussion in another brisket thread. My belief is that just like with other meats, a brisket's cook time at any given chamber temp is determined by it's thickness, NOT by it's weight.

Say that an 8lb flat takes 8 hours to cook. IF you cut a 4.5 chunk off of the flat and cook it by itself, it will still take the same 8 hours to cook even though it's half the weight. Here's my reasoning. When cooking, heat travels from all surfaces of the meat towards the center. If I piece of meant is 14 inches long, the heat will have to travel 7 inches in from each end to reach the center. If that cut is 8 inches wide, the heat would have to travel 4 inches from each side. If that brisket is 2 inches thick, the heat only has to travel 1 inch to the center.

Therefore, the heat coming from the top and bottom in this case will bring the meat to temp well before the heat coming from either end or either side could possibly reach it.

If you were to cut the brisket in half from side to side, the dimensions would be 7 inches long, 8 inches wide and still 2 inches thick. If you cut it in half from front to back, the piece would be 14 inches long, 4 inches wide, but still 2 inches thick.

We are accustomed to X mins per pound. If an 8lb brisket flat takes 8 hours, that's 1 hour "per pound". IF you cut the brisket in half, it should still take 8 hours but that brings the "cook rate" up to 2 hours per pound. Say that you cut another pound off that brisket and the smallest dimension is still 2 inches. You would then have a 3 lb brisket that will also take 8 hours to cook, making it almost 3 hours per pound.

"Hours per pound" really only works when you are talking about full pieces of meat, where the difference in weight affects how thick the cut is. Look at a full packer for example. A 16lb packer is going to be longer, wider and more importantly, THICKER than a 14lb packer or a 12lb packer. Similarly, a 12lb Boston Butt is also going to be longer, wider and most importantly, thicker than a 10lb or an 8lb butt. In these cases, the difference in weight is accompanied by a difference in thickeness, which would make "X hrs per pound" pretty accurate.

Look at this sirloin as an example:

Say that the sirloin is 1 1/2 inches thick and weighs 3lbs. What determines how long it will take to cook at any given temp ? It's length, width or thickness ? Say that you cut right along the black line and reduce it's weight by roughly 1/3'rd, making it a 2 lb sirloin ? Will it take the same amount of time to cook as the whole 3 lb'er ? What about the 1 lb chunk on the right ? Will it take just as long to cook as the 2lb'er on the left ? Or the whole 3lb'er ?"

That answer to all the questions above is yes. Cutting the sirloin along that line won't change the cook times. Both pieces will take the same amount of time to cook even though the piece on the left weighs twice as much as the piece on the right. Also, the piece on the right would take the same amount of time to cook as the entire uncut piece, even though the whole piece weighs 3 times as much. Why ? Because the thickness of this sirloin is what governs it's cook time at any given temp.