To find the displacement, multiply by the same units. The xxx18" Xmax is 54 mm, 5.4 cm.
5.4 cm x 1210 cm2 = 6534 cm3 which is 6.534 Liters.
The PR is 3.8 cm x 1210 cm2 = 4598 cm3 which is 4.598 liters.
A PR being able to move 2x to 4x the displacement of a sub is a general rule, the actual results are dependent on the cabinet size, tuning frequency and the amount of power applied to the sub which modeling will show.
No. 2 CSS 18" PR's are not enough to keep up with a XXX 18 IMO. Believe me I looked at this stuff. Try modeling it and looking at the PR excursion around tune when you are getting the XXX to do it's thing. I like to have at-least 3 times the displacement available for the PR's over the driver. 4 CSS 18" PR's is about right. :coocoo:
Mike is right it's 6.54L going off of one way displacement, or if you want to play the #'s game and multiply that by 2 like a lot of people do for subs... Just call it 13 liters.