Well I never ran an engine without a prop but like my last post my spinner came loose as did the prop at full throttle while running some fuel through it and the Rpms sky rocketed in seconds which caused my engine to smoke, I hit the engine kill within seconds after I heard the rpms scream and the temp was around 280 or so which is ok on my 55ax but I killed the engine before it could overheat. I am fairly new to the hobby but I would assume not running it without a prop because the engine is not turning over anything and the rpms will get very very high, I could be wrong but with my last issue I would assume that is not a good idea. Jason