this is the size of box i want to build: 16tall x 40wide x 28deep, there is an internal volume amount of 8.11 cu ft. after you account for the entire top being double baffled... subs up, port back (in the middle).

i came up with a port, 13.75tall x 8wide x 23.5deep for a tuning of 32hz using this program:

http://www.carstereo.com/help/Articles.cfm?id=31

i used the "Calculate Your Vent Length" section to come up with that port. it asks what tuning frequency you desire but there is not a place to enter how many or what size drivers you are using.

is that port good for four 12"s?

The diameter isn't big enough.

You need more port area. That will increase the port length.

Port area falls within our recommendations. But looking at the length of the port (23.5") and the overall depth of the box (28") you might have some issues with how close the port terminates to the back of the box. It will effectively make it into 2 chambers with a common port. It might be ok... but Id plavce a ^ shape in the center of the port to help direct airflow from either side.

Thanks,

Scott