Manifold pressure - Altitude effects
This is not the standard question about how altitude affects manifold pressure.
I am wondering what happens to the required manifold pressure (and why), as you increase in altitude, if you want to maintain the same power? So, if at sea level, with 65% power set at 23"/2400RPM, and 20GPH -- what do I need to set the manifold to if I wanted to keep 65% power (and 20GPH) with 2400RPM at 8,000'? Assume standard atmospheric conditions.
I would assume that as long as you are moving the same amount (density) of air into the cylinders, at roughly the same temp, then you should require the same manifold setting. But, I do know that static pressure and atmospheric density don't decrease the same with an increase in altitude so I'm assuming there would be a slight difference. At 10,000' the density ratio is .7385 and the pressure ratio is .6877. The lower pressure ratio, compared to the density ratio, I believe might indicated that you require a lower manifold pressure at altitude to create the same power.
I noticed the cruise performance charts on a certain piston airplane also showed that the manifold pressure was generally lower, with an increase in altitude, to maintain the same % power. Is my hypothesis correct?
I'm looking for a detailed technical answer as to what and why, if anyone has some info on this that would be fantastic!