SpudFiles

If you have a chamber 4"x16" the volume is 200.9

I add air until the psi is 100psi.

If I add 4" to the chamber but don't add air what would the psi be?

The volume of the new chamber is now 251.2.

Boyle's law says that pressure x volume is constant.

100 psi x 200.9 = X psi x 251.2

X psi = (200.9 x 100)/251.2

= 80 psi