I agree with the respondent that recommended the K&B. They are running a discount on the twist head .61 that makes it very reasonable. It is very light an it quite handily out powers the OSFX.61. It does fine with variations in Nitro both high and low.
My son just flew identical planes with the FX & the K&B at the SAE Aerodesign competetion. There was no comparison in power. Both these engines were legal in the competition last year. Last years winner used the K&B and my son wished it was still legal.