How long should I cook 2 three pound bipork sirloin roasts for? I tried cooking one before for 3 hours at 400° but it came out real tough.
As in all meat cookery, it is not possible to give an exact time, as that depends on too many idiosyncratic variables: the size of your roast, the temperature it starts at, the temperature of your oven, and so on.
Instead, you want to cook it to a particular temperature as measured with an instant read thermometer in the thickest part of the roast.
A lean roast like this should be cooked approximately to mediumish, which is 150 F or 66 C. This will probably take about an hour, to an hour and quarter at 350 F / 180 C.
When you suspect the roast is close to being done, insert your instant read thermometer in the thickest part of the roast, as near to the center as you can. When the temperature stop rising, that is your reading. You should check several spots, and assume the lowest of them is the temperature to watch.
When the roast is uniformly cooked to the target temperature, it is done.
Pork roasts should cook for about 20 minutes per pound. However, a pork shoulder may need to cook for 30 minutes per pound depending on its size. This means for a pork roast that is 5 pounds, cooking time will be around 2 1/2 hours, as opposed to the normal cooking time of 90 minutes for other cuts of pork. Bones in a roast will speed up the cooking time. A bone acts as a heat conductor which helps to cook the meat from the inside.