Actually, I don't think I'm "all over the map" with props. The 120SP is supposed to turn an APC 20x12 at around 5150 rpm. I wasn't able to get a 20x12 so I opted for an 18x12 (another size recommended for 120SP break-in). I've read on many sites that when going from a 2 blade to a 3 blade prop, you need to downsize diameter by 1-2" (depending on which site you look). So, if the 120SP is supposed to spin a 20x12 prop at 5150, then it makes sense to me that it should spin an 18x12 3 blade prop at a simular rpm. As I stated earlier, I expected to loose a few hundred rpm when going from an 18x12 2 blade to an 18x12 3 blade prop, but not 1100-1200. I wanted to get advice from Keith, since he invented the engine.