"The Drawbar horsepower needed to pull an implement is given by the weight of the implement ,including the average pounds of force applied per disc in the soil, (F) and the speed that the implement is pulled through the soil (S). The equation to find the necessary horsepower to pull an implement is FS/375."
Each disc is about 38 pounds of weight load into soil (@4-6mph) and most 8' disc-harrows have 20 or more discs. So weight load will be 38 x disks + weight of implement. If the disk weighs 1000lbs and you add say 24 disks, that's 1912lbs for F. Multiply that by a speed of 6mph and you get 11,472. Divide that by 375 and the necessary horsepower comes out to 30.592hp. If you reduce the speed to say 4mph, needed horsepower drops to 20.395hp. So you should be able to pull an 8' disk but at reduced speed.
This is, of course, dependant on your soil conditions. YMMV.