I guess I should have used different words.  In speaking to an engineer at Lancair, I was told the plane was tested to 7.5 psi, and that this person suggested a 5.5 psi max differential in operation was perfectly fine.

As for the pop-off valve, I discovered that mine needed to be adjusted.  I built a plenum out of an old candy container and pressurized it to calibrate the pop off valve to start releasing at about 5.8 psi.  During my initial testing, the Dukes wasn't operating correctly and was letting the pressurization climb infinitely.  I also discovered that the pop off valve, even when calibrated, did not let enough air bleed out to keep the pressurization from far exceeding 5.5 psi (I think it got as high as 6.5 psi before I manually released it with the mixer).  As a result of this, I added a 6 psi overpressure switch that lights an annunciator on my G900 (which already existed and was called Cabin Pressure) to warn me if this condition happens again.  I calibrated the pressure switch using the same plenum I used for the pop off valve.  Luckily, since getting my Dukes properly calibrated, I have not had an over-pressurization incident.

As for Brent's calculation, apparently the Dukes is designed to allow a specific volume of air to pass through the cabin at all pressurization settings so as to provide fresh air to the occupants.  To determine the minimum MAP needed, this flow rate (essentially a calibrated leakage) needs to be added to his numbers.  I forget how many CFM it was, but it is a significant number.

Pete

-----Original Message-----

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