This is from a POH of a Cessna 421c

Normal is 0-5.0 psi
Max is 5.3 psi

Is there a way to get the full 5.3psi? At 25,000’ that .3 difference represents a difference of 800’ in cabin altitude.. about 10%

Comparing altitudes in percentages is misleading - it's the pressure that makes a difference to your lungs. The altitude is just a reference to our atmosphere, where 0 ft = 14.7 psi.

Here's an easier-to-understand example: 10,000 ft is "ten times higher" than 1,000 ft, but it still has 2/3 of the oxygen that's available at 1,000 ft.

5.0 psi at 25,000 ft gives you a 9100 ft cabin. 5.3 psi at 25,000 gives an 8400' cabin - about 700' lower. A 9100 ft cabin is 10.47 psi total, and an 8400 ft cabin is 10.75 psi total. There's about 2.5% more oxygen available at the lower cabin altitude. Even this is misleading, because there's a minimum pressure we need to move oxygen in and out of our bloodstream, and it's not zero - but that's another discussion.

But to answer the question: My understanding is no, you can't adjust it. You have two outflow valves. One is electrically controlled and will open at an adjustable pressure, but no higher than 5.0 psi. - that's the one that's used to control your pressure. I don't think you can adjust it and I wouldn't personally try it.

The other is a mechanical fail-safe only and will let go at 5.3 psi as a backup.

Your safety valve is probably set at or just over the max pressure, while the regulating valve tries to regulate at the 5.0psi level. It's not adjustable, but it can be defeated if you want to try to blow out your windows. There is a hose from the end of the outflow valve that goes through a fitting to open into the empennage. This is how the valve knows the outside pressure. Block that vent and defeat the pressurization controller. I think I recall someone debugging a pressurization problem and finding this setup.

I don't know what it would be like to try to ride the safety valve -- if it would pop your ears.

Don't try this at home.

I see.. 5.0.. if you want more, choose a different airplane

Right now I fly a p210n with 3.35 psi and regularly fly between 18-21k.. 2-3 hr legs with no problems ... Just dreaming and scheming about the future...

I love pressurized comfort.. wouldn’t have it any other way unless I had a plane dedicated to low altitude flying.. like a 182 bush plane or the like...