I think Ed hit the nail on the head. Some 40 size engines are ported in such a way that an 11x6 will work OK. It has been my experience (which spans almost 40 yrs) that SuperTigres run better turning more RPM's. Others may have different results but I have 3 ST40's one ST45 and one ST51. I also have an extensive collection of other egines including an old OS40 FSR, a Tower 46 ABC, an RJL 60, and various Saito four strokes. One thing I would like to make a point of is that no two brands have the same running characteristics. As with most threads you can get as many opinions as you like and these are mine. I am always amazed at how many experts there are giving their two cents and those opinions vary widely most of the time. I know there are lots of different ways to get the same job done. Good luck.[8D]