Originally Posted by creeper
What Laramie said about the oil level from the top of the tube... that's all that matters.
You can vary the "air chamber" capacity by changing the oil level. A higher oil level, say 110mm from the top of the tube (the minimum air chamber volume for a 4860 MXMA WP fork) will make the fork firmer, especially in the last 1/2-2/3 of travel... inversely, a lower level such as 150mm (the maximum air chamber volume for a 4860 MXMA WP fork) will make the suspension softer, again in the last 1/2-2/3 of travel.
You always start out with atmospheric pressure in the air chamber of 14.7 psi (at sea level), but changing the volume of the chamber will determine how quickly the rate of pressure rises.
As an example...
Suspension ‘A’ has a 100mm air chamber with atmospheric pressure. We compress the suspension 75mm, reducing the air chamber to 25mm. The pressure of the air is now 58.8 PSI
Suspension ‘B’ has a 150mm air chamber with atmospheric pressure. We compress the suspension 75mm, reducing the air chamber to 75mm. The pressure of the air is now 29.4 PSI
For a given amount of travel, suspension ‘A’ pressure numbers demonstrate that, although it starts out with the same pressure as suspension ‘B’, the rate that it increases will be faster and the end pressure higher due to the smaller starting volume.
After looking at creeper's post (again) I wanted to see the influence of air chamber through the fork's travel so I made a graph:
Mid-stroke there is about 50% more pressure, but at full compression there is 100% more - not linear - interesting but probably only useful to a tuner...