I am not sure that putting the 51 on your 4 star is going to gain you anything. I have an OS 40 FP on mine and it is a perfect match. I might consider an OS 46, however I think that exceeds the suggested size range and once again, I do not think that you would gain much. I am assuming that you are looking for more speed? Super Tiger does make good engines, however I am an OS engine man myself. Good Luck, Dave