1 Roast in a preheated 500 degree (F) oven for 20 minutes. 2  Reduce heat to 300 degrees and roast for an additional 30 minutes per pound. 3  Total cooking time for an 8lb roast should be about 4hrs and 20 minutes.4  Remove the pork roast from the oven and let rest for 30 minutes. Roast in a preheated 500 degree (F) oven for 20 minutes. 2  Reduce heat to 300 degrees and roast for an additional 30 minutes per pound. 3  Total cooking time for an 8lb roast should be about 4hrs and 20 minutes.