R44 MAP Gauge over-reading?
I am puzzled by the following behaviour of the manifold pressure (MAP) gauge in our Robinson R44 Raven II: At our elevations and temperatures (hot & high, did I mention this?) the 5-min take-off rating is always higher than what the engine can deliver. At least according to the limitations chart in the Pilot's Operating Handbook (POH).
That is to say that - while the 5-min take-off rating for the balance of the aircraft (gearbox, rotor head, blades, etc.) would allow higher power from the engine (e.g., equivalent to 24.8 in Hg of MAP), as per limitations placard in the POH, available engine power is supposedly reached before that (namely when the throttle is fully open, e.g., at a MAP of 23.5 in. Hg). We would say that the max avail engine power at this moment is "throttle limited".
In the before said example, the ambient pressure would be around 25.0 to 25.2 in. Hg, and this is also what the MAP gauge shows with engine off (both before, and after the flight, somebody asked me this, so it might be worth mentioning).
Now, in actual flight I seem to consistently be able to pull more MAP than what the limitations placard says: Using again above example, I can without exception pull 24.5, even 24.8 in. MAP without any droop in RRPM. It feels as if I could still pull more, but don't dare trying as at > 24.8 in. Hg MAP one would exceed the take-off power rating of the R44.
Is this normal? How is this possible? I thought that the full-throttle MAP reading in a normally aspirated engine is mostly dictated by ambient pressure, and not subject to the variation between individual engines that there may be.
Could the MAP gauge be over-reading? But how could one then explain that the MAP reading with engine off (which must be equal to ambient pressure) is again correct?