The OS 1.60 FX will swing a 17x13 APC in the 8000 to 8300 range with a tuned pipe setup, so likely with a standard muffler, you would see it handle that load in the high 7000 range, probably around 7900. This with 15% nitro. It's a great running engine, which I have been using in 2M pattern airplanes for about a year now. I've run three of them and they are all rugged and 100% reliable. It won't have any problem hauling the freight with a 12 or 13 lb airframe. Use a type F plug for best results.