Hey - don't knock bushing engines! I won many Control line speed contests in the 50's with a Fox 29X bushing engine! first class B engine I had that exceeded 150MPH!

As for more speed with the OS61FX. use an 11-7 or 11-7 1/2 APC,
get a tuned pipe or muffler (Performance Plus), and Wildcat 15% nitro - 18% oil - or put in an extra head gasket and go 25% nitro!

Phil