Take a look at the MAP gauge before starting the engine.
When the engine isn't running, the gauge should read ambient air pressure. (You do check this before every flight, don't you ? ). The reduction from this ambient value is a measure of the airflow through the inlet manifold, and hence a measure of power that the engine is producing.
If the ambient pressure is lower, then you are starting from a lower "non-engine-running" reading, and you will end up with a lower MAP reading for a given power.