My setup will not allow running without the fan. I'm spinning a 24x10 WW2 Falcon prop in the low 6K range on the ground with a hand held tach.
Until this discussion I never considered the fan was costing any loss of RPM but I guess there is some.
I can't seem to get the RPM readout to work on my telemetry otherwise I could tell you the inflight RPM. I do have a temp dielectric wrapped around the base of the right spark plug and it is in the 250F range which validates my cooling system. Since the Spektrum will only give one temp read out next time I have the cowl off. (And i remember) I'll move the temp loop to the left spark plug just to make sure everything is Kosher. I don't expect any surprises as I have over an hour of flying without a hint of engine issues.
I do know it will twist the prop to supersonic during flyby's!
Sparky