For the OS 55 in a T-34 I would run an APC 10X7 with a stock muffler and a 10X8 with a Jett muffler. The engine would handle up to 30% nitro without issue but there is little return on that investment. 15% to 20% works just as well and run at least 18% oil. The good old OS #8 plugs work well but I seem to get a little more life from an Enya #3