@LOZZ: In your example, according to Robinson Safety Course this week, yes you could exceed the redline, because the hot air reduces the air density and therefore means you are pulling less BHP, the extra 1.5 in. allow for that, even if it means exceeding the redline. You are NOT exceeding the engine BHP limit. I think Dick explained this well.
Now if you are at operating high DA you may not be able to get there and may suffer a drop in engine/rotor RPM!
