Thank you for the responses received so far!
Both potential explanations offered (a leak in the vacuum line to the MAP gauge, and different degrees of inefficiencies in the induction system) explain why a MAP gauge reading station (ambient) pressure correctly while then higher MAP values with the engine running.
- In the case of a leak in the vacuum lines, the MAP gauge would indeed be over-reading (actually under-reading the vacuum in the manifold), thereby artificially limiting power allowed.
- In the case of differing levels of inefficiencies of the induction system (variation from one engine to the other), the gauge would read correctly. Effectively that means that with lesser induction loss you have a stronger engine, and the pilot would need to adhere to the limits shown in the MAP chart, in order not to overstress drivetrain or engine.
Would it be plausible though that the induction losses vary by such a large degree from one R44 installation to another?