For one thing you posted in the wrong category.This is why no responses.Try in the engine conversion forum.But right now i have a 16x8 apc on my 24cc echo.Dont remember rpm was at least 7000.I would say a 16x8 to a 17x6.The 18 inch props work better on 30cc.Some engines are getting 7-8000 on 18s but there not homlites.I have an 17x8 on my stihl 28cc and i know im putting out 7200 or better since i last tached it.Post over in the other forum.Im surprised no moderator caught this yet.