On Tower's web site there is a number of different OS 4 stroke lines - FS ultimate, FS surpass, FL... Can someone clarify on the difference between all these? Is it similar to the difference between the AX and the LA?

Also, most of the guys at my field fly Magnum 4 strokes - which OS engine is in the same category as the Magnum XL 70 RFS?

Last question is about understanding the pros and cons of using either a 55AX or a 70 four stroke in a Hangar 9 Pulse 40. From what I remember I'd get more RPM's out of the 55AX, and more low end power out of the four stroke? Anything else?

Thanks.