My understanding on this is like "stick 40" said, with more than two blades the air is still "dirty" when the next blade gets to it, cutting down the efficiency of it. The other factor is our model engines run at a much higher RPM than a full scale engine. Our 2 stroke engines can turn at 12,000 plus RPM and a full scale piston engine is likely all done at less that 4,000 RPM. So at 1/2 throttle cruise our 2 stroke model engine is turning at say 6,000 RPM and a full scale is doing maybe 2,000-2,400 RPM. A 4 stroke model engine will be turning less that the 2 stroke but still way more than the full scale. No science on this, just my understanding of it.