G'day Bob and Rich,

I've got a question that I can't find an answer too. It's not an exam question per se, but I think this is my best avenue for an answer.
If you take a piper arrow power setting table (io-360, csu, non-turbo). To maintain a constant power setting (65% in this example) at 2400RPM, the required manifold pressure to maintain 65% power decreases. at SL it's 22.9" and at 8000ft it's 21". I'm wondering why the manifold pressure required decreases, with an increase in altitude.

The only thing I can think of is, that it's got to do with the butterfly valve position?

Thanks in advance for your answer!