I built mine using a Walbro WT644 and am using Ignition. So far, it has turned a Master Airscrew 18X10 wood prop at 7900. I will be using a 20x8 MA, wood Schimitar prop, to fly I think. The muffler makes a big difference. I am using one from Wacker engines. Check out my 1/4 scale Loening build at the RCSB web site, forum under "other designers".