My use of the 9X6, 10X6 and 11X6 were to keep the standards of testing the same for all engines tested by limiting the variables. Like using the same fuel. Apples to apples you see. All used the same plug, fuel, spinner etc. My choice of propeller will be based on further testing. I could put a 9X8 on my lightest plane and get fantastic speed but the thrust would not be enough for my heavy trainer. I would like to add that polishing the throat of the carb netted A few hundred rpms on the smallest props turning the higher rpms and almost nothing with the larger ones. As I would expect. There is stil work that can be done inside the crank I believe but the flow to obtain the torque at the lower rpms is probably just about maxed out.. This engine shows no signs of stress and I do not think that operating it at the lower rpms with the bigger props will hurt it one bit. Internal stresses increase at the square to rpm. So getting a bigger prop to do the job means less stress on the reciprocating assemblies generally speaking. Like comparing an over the road diesel truck to a ferrari. They may both make 400 hp but they do it in completely different ways. The diesel only turns about 3000 rpms doing it. I also am a firm believer in the use of castor to protect my engines and thus far have no complaints. So it's a little messy. I have plenty of grease rags.