hi there

This thing about cabin pressure is interesting; i bought a sixteen year old 4P and it was lousy on pressurization. my previous experience was with a Cessna 340 with 4.2 differential pressure. that gave me a sea level cabin at 10,500 feet after we found the leaks.

in the 4P, Matthew found that a seam/joint behind the rear seat had never, yea never been sealed. it leaked like a sieve for the last sixteen years, there was a leak around the door and several firewall leaks. so it may well be that the plane you fly may never has had full pressure differential. or it had it and over time leaks developed; and having maintained the twin to a high standard, i know from the experience that the pressure vessel and systems always need attention.

now
i get sea level at about 12,500 and at 17,500 i have about 4,700 foot cabin; i still think there is ground for improvement.

As to speed:

17,500 feet 3,450 gross weight, 17.5 gph, 32" and 2250 rpm gives 260 knots  (same conditions at 3,850 gross 250 knots)
6,500 feet  3,450 gross   17,5 gph  32" and 2250 rpm gives  a reliable 238 knots (that one seems to never vary by more than a knot)

i do not believe that my plane is leak free; it might be that we could do a little better.

what should be clear is that if you are not doing as well, it time to check for leaks in the pressurization vessel.

i love my 4p, put 62 hours in six months and will likely continue at that rate or higher. and still learning and still cautious as hell.

yours

peter williams