Pete Writes:
<<During a flight yesterday I tested the minimum MAP required to maintain pressurization and came up with the following (@2400 rpm): 25" MAP = 5.5 psi 23.5" MAP = 5.0 psi 25" @ 2400 rpm equates to 65% power. Thus, when descending with full pressurization, I keep the power setting at or above 65% power. The inner and outer rings on my Dukes controller seem to be calibrated for at 5.0 psi differential. To reach the 5.5 psi design spec of the plane, I set the Dukes 2500 feet under my target altitude. This consistently gives me 5.5 psi. >>

Pete's statement is mostly true but indicates some misconceptions about pressurization.

FIrst, the design operating point is 5.0 PSI differential (Inside Pressure - Outside Pressure). 5.5 PSI is when the over Pressure Relief valve should start to open. The design flow through the cabin is specified by the FAA at 10 SCFM per passenger, 40 SCFM for a IV-P.

Second, planes leak air. The pressurization system utilizes a Sonic Venturi to provide air from the Deck (after the turbo but before the throttle) to the cabin. A Sonic Venturi is a little like a Black Hole in that what happens downstream doesn't much effect what is happening upstream so releasing the door seal does not significantly change the Deck Pressure. Deck Pressure is above Manifold Pressure by an amount determined by the Waste Gate Controller, but usually runs around 2 InHg.  Regulation of cabin pressure is done by the Outflow Valve which controls how much air leaves the cabin. Air can also leave the cabin via air leaks. All planes leak and the amount of the leakage varies from one aircraft to another. A perfectly sealed cabin could be pressurized with a bicycle pump, but this would require the smallest pinhole be plugged. In practice, a well sealed IV-P can be pressurised on the ground with a 10Hp air compressor. I performed a static ground proof test on my plane to 7.5 PSI (150% of operating pressure). It took me two days of chasing leaks to achieve that pressure.

Third, Cabin Altitude is the absolute pressure altitude of the cabin while Cabin Pressure is the differential between inside pressure and outside pressure.

From all this we can deduce that the manifold pressure needed to maintain a specified Cabin Pressure is a function of Altitude and Leakage. At FL280 the ambient pressure is 9.75 InHg. 5 PSI is equal to 10.15 InHg so if we assume the condition where the deck pressure is 2 InHg over manifold and the Sonic Venture pressure drop is also 2 InHg, the minimum needed manifold pressure is about 20 InHg, assuming low leakage. As the leakage increases the outflow valve must restrict its flow until at some leakage rate the outflow valve is completely closed and can no longer control the cabin pressure. As the leakage rate increases above this point the manifold pressure

In conclusion, every aircraft's leakage rate is different, so the relationship between Cabin Pressure, Manifold Pressure and Altitude will be different for every aircraft.

Get to know your aircraft with flight testing. You will gain valuable knowledge, skills and become a better pilot.

Regards
Brent Regan