Depends...you gonna run the stock muffler on the K&B? Ken is the Rossi really lighter than the K&B? If so, that is kinda scary...

I love both of those powerplants. I test everything on a 9x8 apc just to get some figures in my Excel spreadsheet and the K&B with the Ultrathrust muffler performs better (600rpm's more) than the Rossi-45 and mini-pipe. Although the UT is like $70 bucks.

I do not think the Rossi will out-turn the K&B with a decent tuned muffler. Again it all comes down to muffler/pipe combos. Dave's Rossi screams since he found the perfect pipe combo for himself. But I disregard his RPM figures since he uses MA props. It is impressive that is for sure but I can never seem to translate his figures to the APC equivalent.

In the end that F-20 is kinda draggy so I would use something like a 10x7 and these two engines will be purty close with that prop.
James